Cobra 2010 loses Tx audio
 

Another eBay special that "worked perfectly". It doesn't matter if I put in
an external audio source from an audio oscillator or a microphone. I've
tried three good microphones, and have the same problem. When I first key
the mic, Tx audio is fine, but it quickly, almost abruptly fades after about
2 seconds. Has anybody seen this problem?

Pete

Cobra 2010 loses Tx audio
 

On Sat, 17 Feb 2007 01:59:59 -0600, "Pete KE9OA"
wrote in
:

Another eBay special that "worked perfectly". It doesn't matter if I put in
an external audio source from an audio oscillator or a microphone. I've
tried three good microphones, and have the same problem. When I first key
the mic, Tx audio is fine, but it quickly, almost abruptly fades after about
2 seconds. Has anybody seen this problem?

Pete



Assuming your ground plane is at least nine square feet.....

Someone screwed with the audio AGC. Whoever did it probably screwed up
other stuff, too. You should take it to a shop to have the whole radio
re-aligned.

Cobra 2010 loses Tx audio
 

Sounds like a RESISTOR is changing value causing the bias to change on the
input preamp.

"Pete KE9OA" wrote in message
. ..
Another eBay special that "worked perfectly". It doesn't matter if I put
in an external audio source from an audio oscillator or a microphone. I've
tried three good microphones, and have the same problem. When I first key
the mic, Tx audio is fine, but it quickly, almost abruptly fades after
about 2 seconds. Has anybody seen this problem?

Pete

Cobra 2010 loses Tx audio
 

Thanks for the tip. The problem is definitely in the modulation stage. It
appears that the operating point of one of the preamplifier transistors is
changing into a nonlinear region. I didn't see any new soldering that had
been done, and all of the coils still have the wax intact. The receiver
sensitivity measures at , 0.1uV across the whole band, so that section is
working fine. This unit has one of the hottest receivers I have seen in a
long time; this is the reason I want to restore it. I do have the service
manual, but I have never seen this kind of problem.
For testing purposes, I am using a CT Systems Model 3000B Communications
Test Set. For the brief time that Tx audio is present, the modulation
envelope looks fine. This unit has a built in 100W dummy load.
RF output is flat across the band, at 4.5W, so it doesn't look like somebody
tried to align the exciter/output stages.
It looks like I will have to do some signal tracing with an o'scope.
Hopefully, I will be able to find that 5-pin mic plug so I can make up a
test cable. Thanks again for the advice! I think I remember you as also
working in the RF industry.

Pete

"Frank Gilliland" wrote in message
...
On Sat, 17 Feb 2007 01:59:59 -0600, "Pete KE9OA"
wrote in
:

Another eBay special that "worked perfectly". It doesn't matter if I put
in
an external audio source from an audio oscillator or a microphone. I've
tried three good microphones, and have the same problem. When I first key
the mic, Tx audio is fine, but it quickly, almost abruptly fades after
about
2 seconds. Has anybody seen this problem?

Pete



Assuming your ground plane is at least nine square feet.....

Someone screwed with the audio AGC. Whoever did it probably screwed up
other stuff, too. You should take it to a shop to have the whole radio
re-aligned.

Cobra 2010 loses Tx audio
 

YES - for a MSS and as a RF bench engineer years ago.

If u have the schematic it should be easy to trace the input and output
stages of each section of the TX audio sections.. Just run a constant 1K
tone to the mic input line - I would tend to think that its in the TX line
only, as most of these radios used the same transistors for both RX/TX in
the audio output stage and you are not reporting any distortion in the RX
audio.
Best 73's

"Pete KE9OA" wrote in message
...
Thanks for the tip. The problem is definitely in the modulation stage. It
appears that the operating point of one of the preamplifier transistors is
changing into a nonlinear region. I didn't see any new soldering that had
been done, and all of the coils still have the wax intact. The receiver
sensitivity measures at , 0.1uV across the whole band, so that section
is working fine. This unit has one of the hottest receivers I have seen in
a long time; this is the reason I want to restore it. I do have the
service manual, but I have never seen this kind of problem.
For testing purposes, I am using a CT Systems Model 3000B Communications
Test Set. For the brief time that Tx audio is present, the modulation
envelope looks fine. This unit has a built in 100W dummy load.
RF output is flat across the band, at 4.5W, so it doesn't look like
somebody tried to align the exciter/output stages.
It looks like I will have to do some signal tracing with an o'scope.
Hopefully, I will be able to find that 5-pin mic plug so I can make up a
test cable. Thanks again for the advice! I think I remember you as also
working in the RF industry.

Pete

"Frank Gilliland" wrote in message
...
On Sat, 17 Feb 2007 01:59:59 -0600, "Pete KE9OA"
wrote in
:

Another eBay special that "worked perfectly". It doesn't matter if I put
in
an external audio source from an audio oscillator or a microphone. I've
tried three good microphones, and have the same problem. When I first key
the mic, Tx audio is fine, but it quickly, almost abruptly fades after
about
2 seconds. Has anybody seen this problem?

Pete



Assuming your ground plane is at least nine square feet.....

Someone screwed with the audio AGC. Whoever did it probably screwed up
other stuff, too. You should take it to a shop to have the whole radio
re-aligned.

Cobra 2010 loses Tx audio
 

I wondered about that myself. Looking at the schematic, I do see a couple of
electrolytic coupling caps that can be causing the problem. Since Tx audio
is there for the first few seconds, this tells me that it is a Tx keyed
stage that has the problem. I am going to have to trace the Tx enable line
to see which stages are being keyed in Tx mode only.
I am looking at the block diagram, and it appears that either TR27, the PA
switch or TR21, the AF switch might be causing the problem. There is also a
feedback network that starts back from TR34, the ALC amp. This feeds a
couple of other stages caled AMC amps, that appear to go through a summing
network into TR23, the Mike amp.
Since the Tx audio problem affects both AM and SSB modes, it has to be in a
keyed stage that is common to both modes.
The nice thing about this radio is that no mods appear tohave been
done.............even the clarifier control has not been "married" to the Tx
mode. This one is worth fixing........the only problem is getting at all of
the wiring. Much of it is hidden behind a shield plate at the front panel.
It could be something as simple as a cold solder joint, but that might just
be wishfull thinking!

Pete

"John Doe" wrote in message
...
Sounds like a RESISTOR is changing value causing the bias to change on the
input preamp.

"Pete KE9OA" wrote in message
. ..
Another eBay special that "worked perfectly". It doesn't matter if I put
in an external audio source from an audio oscillator or a microphone.
I've tried three good microphones, and have the same problem. When I
first key the mic, Tx audio is fine, but it quickly, almost abruptly
fades after about 2 seconds. Has anybody seen this problem?

Pete

Cobra 2010 loses Tx audio
 

Thanks, I appreciate the help. I am going to fire up the test bench and give
it a go. I will let you and the folks know what I fine.
Ok on that RF engineering experience...........that is also what I am doing
right now. I worked at Collins Radio in the 2nd half of the 90s, switching
to Motorola when we moved back to chicago. For the last couple of years, I
have been working as a contractor/consultant at Motorola. I think that's
what happens when you get to be a semi-old guy like me. They make you a
consultant!
Thanks again to you guys for the help!

Pete

"John Doe" wrote in message
...
YES - for a MSS and as a RF bench engineer years ago.

If u have the schematic it should be easy to trace the input and output
stages of each section of the TX audio sections.. Just run a constant 1K
tone to the mic input line - I would tend to think that its in the TX line
only, as most of these radios used the same transistors for both RX/TX in
the audio output stage and you are not reporting any distortion in the RX
audio.
Best 73's

"Pete KE9OA" wrote in message
...
Thanks for the tip. The problem is definitely in the modulation stage. It
appears that the operating point of one of the preamplifier transistors
is changing into a nonlinear region. I didn't see any new soldering that
had been done, and all of the coils still have the wax intact. The
receiver sensitivity measures at , 0.1uV across the whole band, so that
section is working fine. This unit has one of the hottest receivers I
have seen in a long time; this is the reason I want to restore it. I do
have the service manual, but I have never seen this kind of problem.
For testing purposes, I am using a CT Systems Model 3000B Communications
Test Set. For the brief time that Tx audio is present, the modulation
envelope looks fine. This unit has a built in 100W dummy load.
RF output is flat across the band, at 4.5W, so it doesn't look like
somebody tried to align the exciter/output stages.
It looks like I will have to do some signal tracing with an o'scope.
Hopefully, I will be able to find that 5-pin mic plug so I can make up a
test cable. Thanks again for the advice! I think I remember you as also
working in the RF industry.

Pete

"Frank Gilliland" wrote in message
...
On Sat, 17 Feb 2007 01:59:59 -0600, "Pete KE9OA"
wrote in
:

Another eBay special that "worked perfectly". It doesn't matter if I put
in
an external audio source from an audio oscillator or a microphone. I've
tried three good microphones, and have the same problem. When I first
key
the mic, Tx audio is fine, but it quickly, almost abruptly fades after
about
2 seconds. Has anybody seen this problem?

Pete



Assuming your ground plane is at least nine square feet.....

Someone screwed with the audio AGC. Whoever did it probably screwed up
other stuff, too. You should take it to a shop to have the whole radio
re-aligned.

Cobra 2010 loses Tx audio
 

On Sat, 17 Feb 2007 13:15:27 -0600, "Pete KE9OA"
wrote in
:

Thanks for the tip. The problem is definitely in the modulation stage. It
appears that the operating point of one of the preamplifier transistors is
changing into a nonlinear region. I didn't see any new soldering that had
been done, and all of the coils still have the wax intact. The receiver
sensitivity measures at , 0.1uV across the whole band, so that section is
working fine. This unit has one of the hottest receivers I have seen in a
long time; this is the reason I want to restore it.


Sounds like it might be a little too hot for the band.....


I do have the service
manual, but I have never seen this kind of problem.


Never assume the manual is correct. Establish a baseline by injecting
a single tone post-AGC, run it up to 100% and measure. Then re-align
the AGC to your mic according to that baseline.


For testing purposes, I am using a CT Systems Model 3000B Communications
Test Set. For the brief time that Tx audio is present, the modulation
envelope looks fine. This unit has a built in 100W dummy load.
RF output is flat across the band, at 4.5W, so it doesn't look like somebody
tried to align the exciter/output stages.


Sounds like it may have an RF limiter. If you have a pulse generator,
hit the audio with 0.1ms spikes. That will also tell you if the audio
limiter has been disabled.


It looks like I will have to do some signal tracing with an o'scope.
Hopefully, I will be able to find that 5-pin mic plug so I can make up a
test cable. Thanks again for the advice!


No problem. If you ever get around to scanning the manual I wouldn't
mind having a copy.


I think I remember you as also
working in the RF industry.


On occasion.

Cobra 2010 loses Tx audio
 

Sounds like it might be a little too hot for the band.....

I am not sure what you mean.


I do have the service
manual, but I have never seen this kind of problem.


Never assume the manual is correct. Establish a baseline by injecting
a single tone post-AGC, run it up to 100% and measure. Then re-align
the AGC to your mic according to that baseline.

I might inject the signal right at the audio amplifier and work backwards.


For testing purposes, I am using a CT Systems Model 3000B Communications
Test Set. For the brief time that Tx audio is present, the modulation
envelope looks fine. This unit has a built in 100W dummy load.
RF output is flat across the band, at 4.5W, so it doesn't look like
somebody
tried to align the exciter/output stages.


Sounds like it may have an RF limiter. If you have a pulse generator,
hit the audio with 0.1ms spikes. That will also tell you if the audio
limiter has been disabled.

I'll give that at ry.


It looks like I will have to do some signal tracing with an o'scope.
Hopefully, I will be able to find that 5-pin mic plug so I can make up a
test cable. Thanks again for the advice!


No problem. If you ever get around to scanning the manual I wouldn't
mind having a copy.

I will do that. I only can scan up to 11 X 14 size, so the schematics will
be broken pages,but that should be fine.


I think I remember you as also
working in the RF industry.


On occasion.

Cobra 2010 loses Tx audio
 

On Sat, 17 Feb 2007 01:59:59 -0600, "Pete KE9OA" wrote:

Another eBay special that "worked perfectly". It doesn't matter if I put in
an external audio source from an audio oscillator or a microphone. I've
tried three good microphones, and have the same problem. When I first key
the mic, Tx audio is fine, but it quickly, almost abruptly fades after about
2 seconds. Has anybody seen this problem?

Pete



I had this problem on my 2510. Ended up finding out that my power supply wasn't
cutting it. I went to a larger power supply. Since yours in internal, I am not
sure that is the problem. But the damn symptom iwas the exact same.

Vinnie S.

Cobra 2010 loses Tx audio
 

On Sat, 17 Feb 2007 15:49:51 -0600, "Pete KE9OA"
wrote in
:


Sounds like it might be a little too hot for the band.....

I am not sure what you mean.


I have found that it's sometimes better to DE-sensitize the receiver
front-end on a noisy band. Tends to improve S/N.


I do have the service
manual, but I have never seen this kind of problem.


Never assume the manual is correct. Establish a baseline by injecting
a single tone post-AGC, run it up to 100% and measure. Then re-align
the AGC to your mic according to that baseline.

I might inject the signal right at the audio amplifier and work backwards.


Same difference.


For testing purposes, I am using a CT Systems Model 3000B Communications
Test Set. For the brief time that Tx audio is present, the modulation
envelope looks fine. This unit has a built in 100W dummy load.
RF output is flat across the band, at 4.5W, so it doesn't look like
somebody
tried to align the exciter/output stages.


Sounds like it may have an RF limiter. If you have a pulse generator,
hit the audio with 0.1ms spikes. That will also tell you if the audio
limiter has been disabled.

I'll give that at ry.


It looks like I will have to do some signal tracing with an o'scope.
Hopefully, I will be able to find that 5-pin mic plug so I can make up a
test cable. Thanks again for the advice!


No problem. If you ever get around to scanning the manual I wouldn't
mind having a copy.

I will do that. I only can scan up to 11 X 14 size, so the schematics will
be broken pages,but that should be fine.


No hurry. I only get a couple CB radios a year, so it's hard to tell
if or when I'll run across this model. There really should be a public
CB manual repository like BAMA..... maybe some day.

Cobra 2010 loses Tx audio
 

That sounds good.............I will probably scan the service info in the
next couple of weeks. now, about the receiver desensing.....I don't have any
problems with IMD in the receiver, and a diode attenuator is used in the
front end. I have found that IMD performance can be improved by changing
that general purpose switching diode in the front to a 1N5767 PIN diode. I
wanted to get started on this project this afternoon, but a friend stopped
over. I'll give it a whirl tonight. It will be quite a nice radio, once I
get through with it.

"Frank Gilliland" wrote in message
...
On Sat, 17 Feb 2007 15:49:51 -0600, "Pete KE9OA"
wrote in
:


Sounds like it might be a little too hot for the band.....

I am not sure what you mean.


I have found that it's sometimes better to DE-sensitize the receiver
front-end on a noisy band. Tends to improve S/N.


I do have the service
manual, but I have never seen this kind of problem.


Never assume the manual is correct. Establish a baseline by injecting
a single tone post-AGC, run it up to 100% and measure. Then re-align
the AGC to your mic according to that baseline.

I might inject the signal right at the audio amplifier and work backwards.


Same difference.


For testing purposes, I am using a CT Systems Model 3000B Communications
Test Set. For the brief time that Tx audio is present, the modulation
envelope looks fine. This unit has a built in 100W dummy load.
RF output is flat across the band, at 4.5W, so it doesn't look like
somebody
tried to align the exciter/output stages.


Sounds like it may have an RF limiter. If you have a pulse generator,
hit the audio with 0.1ms spikes. That will also tell you if the audio
limiter has been disabled.

I'll give that at ry.


It looks like I will have to do some signal tracing with an o'scope.
Hopefully, I will be able to find that 5-pin mic plug so I can make up a
test cable. Thanks again for the advice!


No problem. If you ever get around to scanning the manual I wouldn't
mind having a copy.

I will do that. I only can scan up to 11 X 14 size, so the schematics will
be broken pages,but that should be fine.


No hurry. I only get a couple CB radios a year, so it's hard to tell
if or when I'll run across this model. There really should be a public
CB manual repository like BAMA..... maybe some day.

Cobra 2010 loses Tx audio
 

Thanks Vinny. I'll check the power supply and see if droop is occuring on
Tx. I need to take a close look at the schematic and check the low current
line. I think it is 8VDC. The seller was nice enough to tell me that he
removed the battery from the microphone so that damage wouldn't occur. He
probably figured that it would take me a few days before tested the radio.
Whenever I get ahold of an eBay item, the first thing I do is open up the
unit and make sure that somebody didn't try to align it. Next, I connect up
an RF generator and measure the sensitivity, after which I check the RF
output and modulation index of the transmitter on a service moniter. Looks
like I picked up another "tech special".
This unit isn't as bad as the Drake TR7 I picked up from a fellow in Ohio
last year. He conveniently didn't include a microphone or a power cord, but
he was nice enough to include a Cinch Jones connector for the 12V input.
I had a PS7 power supply on hand, and I noticed that power output was low on
10M. I found a cold solder joint on one of the exciter boards. In an attempt
to cover up the problem, he defeated the ALC circuit, so after I repaired
the unit and checked the power output it was putting out over 275 watts. I
readjusted the ALC and everything was fine.
When I contacted the seller and shared my findings with him, he said "Oh, I
was wondering why I had to crank up the mic gain with this one".

Pete

"Vinnie S." wrote in message
...
On Sat, 17 Feb 2007 01:59:59 -0600, "Pete KE9OA"
wrote:

Another eBay special that "worked perfectly". It doesn't matter if I put
in
an external audio source from an audio oscillator or a microphone. I've
tried three good microphones, and have the same problem. When I first key
the mic, Tx audio is fine, but it quickly, almost abruptly fades after
about
2 seconds. Has anybody seen this problem?

Pete



I had this problem on my 2510. Ended up finding out that my power supply
wasn't
cutting it. I went to a larger power supply. Since yours in internal, I am
not
sure that is the problem. But the damn symptom iwas the exact same.

Vinnie S.

Cobra 2010 loses Tx audio
 

On Sat, 17 Feb 2007 22:26:50 -0600, "Pete KE9OA"
wrote in
:

....... about the receiver desensing.....I don't have any
problems with IMD in the receiver......


IMD is minor compared to the negative effects of high input impedance
of the RF preamp, which is almost always too high. Reducing the 1st RF
gain (A = 5 to 10) will reduce the input impedance and make it less
vulnerable to noise -- especially the white background noise. You can
compensate by increasing gain in the IF stages. Then realign your
input matching network and =blammo=, weak signals are solid with
much less noise.


......., and a diode attenuator is used in the
front end. I have found that IMD performance can be improved by changing
that general purpose switching diode in the front to a 1N5767 PIN diode.


As long as the original isn't used for overload protection, I don't
see any problem with that. But then I don't see any advantage, either.
The band is just too noisy for it to make any significant improvement.
Still, I've got a crusty old Diamond 60 laying around somewhere, so
I'll give it a try anyway.

Cobra 2010 loses Tx audio
 

I understand what you are saying, but the RF amplifier should be conjugate
matched to 50 ohms anyway, in order to have uncondisional stability. I am
not sure what the noise figure of this system is, but it seems that the gain
distribution is such that most of the gain is in the 2nd I.F. strip anyway.
Even so, under 30MHz, in most areas the excess environmental noise is in the
15dB region so a receiver with a 12dB noise figure does just fine.
I remember the old Allied Model 2568 CB radio. This thing had quite a bit of
RF gain and relatively low I.F. gain. As soon as you connected an antenna,
it sounded like an FM unit. The problem with that design is that the AGC
voltage was derived from the RF stage with its relatively low selectivity,
in addition to the I.F. strip. Strong off channel signals would capture the
AGC loop and desense the whole system. Remember the old term "bleed over"?
You do have a good point about keeping the RF gain ahead of the mixer as low
as possible, since any gain ahead of the 1st mixer degrades the dynamic
range by that same amount. In my last contract with Motorola, we were using
mixers that had an IP3 of +40dBm so we were able to get away with having
some gain ahead of that mixer.


IMD is minor compared to the negative effects of high input impedance
of the RF preamp, which is almost always too high. Reducing the 1st RF
gain (A = 5 to 10) will reduce the input impedance and make it less
vulnerable to noise -- especially the white background noise. You can
compensate by increasing gain in the IF stages. Then realign your
input matching network and =blammo=, weak signals are solid with
much less noise.




......., and a diode attenuator is used in the
front end. I have found that IMD performance can be improved by changing
that general purpose switching diode in the front to a 1N5767 PIN diode.


As long as the original isn't used for overload protection, I don't
see any problem with that. But then I don't see any advantage, either.
The band is just too noisy for it to make any significant improvement.
Still, I've got a crusty old Diamond 60 laying around somewhere, so
I'll give it a try anyway.

Cobra 2010 loses Tx audio
 

On Sun, 18 Feb 2007 09:21:02 -0600, "Pete KE9OA"
wrote:

+++I understand what you are saying, but the RF amplifier should be conjugate
+++matched to 50 ohms anyway, in order to have uncondisional stability. I am
+++not sure what the noise figure of this system is, but it seems that the gain
+++distribution is such that most of the gain is in the 2nd I.F. strip anyway.
+++Even so, under 30MHz, in most areas the excess environmental noise is in the
+++15dB region so a receiver with a 12dB noise figure does just fine.
+++I remember the old Allied Model 2568 CB radio. This thing had quite a bit of
+++RF gain and relatively low I.F. gain. As soon as you connected an antenna,
+++it sounded like an FM unit. The problem with that design is that the AGC
+++voltage was derived from the RF stage with its relatively low selectivity,
+++in addition to the I.F. strip. Strong off channel signals would capture the
+++AGC loop and desense the whole system. Remember the old term "bleed over"?
+++You do have a good point about keeping the RF gain ahead of the mixer as low
+++as possible, since any gain ahead of the 1st mixer degrades the dynamic
+++range by that same amount. In my last contract with Motorola, we were using
+++mixers that had an IP3 of +40dBm so we were able to get away with having
+++some gain ahead of that mixer.
+++
***********

Conjugate match is needed for maximum power transfer. Nuetralization
helps extend stability over various mismatch condistions.

In a receiver RF front end it is preferable to match for best noise
figure and accept the gain. The less gain before the mixer the better.
The RF front end sets the noise figure for the whole receiver. The
gain of the RF Front end need only be high enough to overcome the
noise figure of the next stage and any losses it may present if any.

More often than not,CB calibur radios places far to much gain in the
first stages so that more simple IF stages can be used. Thereby
lowering costs.

james

Cobra 2010 loses Tx audio
 

On Sun, 18 Feb 2007 09:21:02 -0600, "Pete KE9OA"
wrote in
:

I understand what you are saying, but the RF amplifier should be conjugate
matched to 50 ohms anyway, in order to have uncondisional stability.


I don't have the schematic for your radio in front of me, but if that
1st RF stage is like most CB radios it's common emitter. So the input
impedance is a lot higher than 50 ohms, and is matched to the antenna
with a transformer or LC network. Not exactly ideal.


I am
not sure what the noise figure of this system is, but it seems that the gain
distribution is such that most of the gain is in the 2nd I.F. strip anyway.
Even so, under 30MHz, in most areas the excess environmental noise is in the
15dB region.......


Are we talking 11m here?


so a receiver with a 12dB noise figure does just fine.
I remember the old Allied Model 2568 CB radio. This thing had quite a bit of
RF gain and relatively low I.F. gain. As soon as you connected an antenna,
it sounded like an FM unit. The problem with that design is that the AGC
voltage was derived from the RF stage with its relatively low selectivity,
in addition to the I.F. strip. Strong off channel signals would capture the
AGC loop and desense the whole system. Remember the old term "bleed over"?
You do have a good point about keeping the RF gain ahead of the mixer as low
as possible, since any gain ahead of the 1st mixer degrades the dynamic
range by that same amount.


The objective is not low gain but low input impedance. Closer to the
impedance of the feed, to keep the first impedance transformation as
small as possible. With a common emitter, the only way to do that is
by reducing the gain. And just at the first RF stage, not necessarily
everything else in front of the first mixer.


In my last contract with Motorola, we were using
mixers that had an IP3 of +40dBm so we were able to get away with having
some gain ahead of that mixer.


Most of the problems I've had with mixers came not from the mixers but
from unbuffered oscillators. Anywayz.....

I guess the question is if the radio works well enough as it sits. If
you can hear a signal buried in the band noise then that's about as
good as it gets. The only way I know to improve it is by matching the
impedance of the first RF to the antenna. Beyond that you'll need to
get a directional antenna.

Cobra 2010 loses Tx audio
 

On Mon, 19 Feb 2007 01:09:52 GMT, james wrote
in :

On Sun, 18 Feb 2007 09:21:02 -0600, "Pete KE9OA"
wrote:

+++I understand what you are saying, but the RF amplifier should be conjugate
+++matched to 50 ohms anyway, in order to have uncondisional stability. I am
+++not sure what the noise figure of this system is, but it seems that the gain
+++distribution is such that most of the gain is in the 2nd I.F. strip anyway.
+++Even so, under 30MHz, in most areas the excess environmental noise is in the
+++15dB region so a receiver with a 12dB noise figure does just fine.
+++I remember the old Allied Model 2568 CB radio. This thing had quite a bit of
+++RF gain and relatively low I.F. gain. As soon as you connected an antenna,
+++it sounded like an FM unit. The problem with that design is that the AGC
+++voltage was derived from the RF stage with its relatively low selectivity,
+++in addition to the I.F. strip. Strong off channel signals would capture the
+++AGC loop and desense the whole system. Remember the old term "bleed over"?
+++You do have a good point about keeping the RF gain ahead of the mixer as low
+++as possible, since any gain ahead of the 1st mixer degrades the dynamic
+++range by that same amount. In my last contract with Motorola, we were using
+++mixers that had an IP3 of +40dBm so we were able to get away with having
+++some gain ahead of that mixer.
+++
***********

Conjugate match is needed for maximum power transfer.


IMPEDANCE match... for maximum power transfer. A 'conjugate' match is
when the impedances are complex, which isn't always the case.


Nuetralization
helps extend stability over various mismatch condistions.


Lots of things can improve stability, but unless the amp/mixer is
oscillating the point is moot.


In a receiver RF front end it is preferable to match for best noise
figure and accept the gain. The less gain before the mixer the better.
The RF front end sets the noise figure for the whole receiver. The
gain of the RF Front end need only be high enough to overcome the
noise figure of the next stage and any losses it may present if any.


All true. But the point I was trying to make (which I incorrectly
assumed was already understood) is that any impedance matching device
or network between the antenna and the 1st RF can cause more noise
from IMD than the noise from the 1st RF or mixer, -especially- if that
matching device/circuit uses a ferrite core or solid dielectric, which
includes almost all CB radios. That's why strong signals can sometimes
be heard even when the source is several MHz away (often mistaken for
receiver overload).

The concept here is to reduce (or, ideally, eliminate) that impedance
transformation stage. A long time ago I built a common base (voltage
follower) RF preamp using eight transistors in parallel followed by
the impedance transformation stage (transformer). The input impedance
directly to the transistors is about 100 ohms, but I feed it directly
anyway. The difference is like night and day. I use it for lowfer work
these days.


More often than not,CB calibur radios places far to much gain in the
first stages so that more simple IF stages can be used. Thereby
lowering costs.


Like you said before, the first RF is all that matters.

Cobra 2010 loses Tx audio
 

On Sun, 18 Feb 2007 18:24:33 -0800, Frank Gilliland
wrote:

+++Conjugate match is needed for maximum power transfer.
+++
+++
+++IMPEDANCE match... for maximum power transfer. A 'conjugate' match is
+++when the impedances are complex, which isn't always the case.
***********

I have found that it is rare in the real world that impeadances are
not complex. Outside transimission lines, there is little that is not
complex. Then again when you conjugate match, the imaginary part of
the complex impedances is nulified and you are then left with the real
part.

james

Cobra 2010 loses Tx audio
 

On Mon, 19 Feb 2007 19:12:33 GMT, james wrote
in :

On Sun, 18 Feb 2007 18:24:33 -0800, Frank Gilliland
wrote:

+++Conjugate match is needed for maximum power transfer.
+++
+++
+++IMPEDANCE match... for maximum power transfer. A 'conjugate' match is
+++when the impedances are complex, which isn't always the case.
***********

I have found that it is rare in the real world that impeadances are
not complex. Outside transimission lines, there is little that is not
complex.


You just said that resistors have complex impedance and transmission
lines are flat.


Then again when you conjugate match, the imaginary part of
the complex impedances is nulified and you are then left with the real
part.


Reactances don't just disappear. They create a current between the
source and load that must be assessed to see if it is going to cause
any problems. Sometimes it doesn't and sometimes it does.

Cobra 2010 loses Tx audio
 

On Mon, 19 Feb 2007 12:43:12 -0800, Frank Gilliland
wrote:

+++On Mon, 19 Feb 2007 19:12:33 GMT, james wrote
+++in :
+++
+++On Sun, 18 Feb 2007 18:24:33 -0800, Frank Gilliland
wrote:
+++
++++++Conjugate match is needed for maximum power transfer.
++++++
++++++
++++++IMPEDANCE match... for maximum power transfer. A 'conjugate' match is
++++++when the impedances are complex, which isn't always the case.
+++***********
+++
+++I have found that it is rare in the real world that impeadances are
+++not complex. Outside transimission lines, there is little that is not
+++complex.
+++
+++
+++You just said that resistors have complex impedance and transmission
+++lines are flat.
+++
************

No I did not. Besides Resistors can have complex impedances depending
upon constrtuctinand frequency in which they are used.

+++
+++ Then again when you conjugate match, the imaginary part of
+++the complex impedances is nulified and you are then left with the real
+++part.
+++
+++
+++Reactances don't just disappear. They create a current between the
+++source and load that must be assessed to see if it is going to cause
+++any problems. Sometimes it doesn't and sometimes it does.
+++
+++
***********

I did not say they disappeared. At resonance the conjugate match
causes the net reactance to be zero. Thus nulify. The reactance are
always there.


james

Cobra 2010 loses Tx audio
 

On Tue, 20 Feb 2007 03:13:52 GMT, james wrote
in :

On Mon, 19 Feb 2007 12:43:12 -0800, Frank Gilliland
wrote:

+++On Mon, 19 Feb 2007 19:12:33 GMT, james wrote
+++in :
+++
+++On Sun, 18 Feb 2007 18:24:33 -0800, Frank Gilliland
wrote:
+++
++++++Conjugate match is needed for maximum power transfer.
++++++
++++++
++++++IMPEDANCE match... for maximum power transfer. A 'conjugate' match is
++++++when the impedances are complex, which isn't always the case.
+++***********
+++
+++I have found that it is rare in the real world that impeadances are
+++not complex. Outside transimission lines, there is little that is not
+++complex.
+++
+++
+++You just said that resistors have complex impedance and transmission
+++lines are flat.
+++
************

No I did not.


Go back and read your own words again.


Besides Resistors can have complex impedances depending
upon constrtuctinand frequency in which they are used.


When a resistor is used at its intended frequency, any reactance is
insignificant. If it wasn't then it would be called an 'inductor' or
'capacitor'.


+++
+++ Then again when you conjugate match, the imaginary part of
+++the complex impedances is nulified and you are then left with the real
+++part.
+++
+++
+++Reactances don't just disappear. They create a current between the
+++source and load that must be assessed to see if it is going to cause
+++any problems. Sometimes it doesn't and sometimes it does.
+++
+++
***********

I did not say they disappeared. At resonance the conjugate match
causes the net reactance to be zero. Thus nulify. The reactance are
always there.


Notwithstanding the fact that the non-reactive component of impedance
changes at or near resonance, maximum power transfer (due to matched
impedances) occurs regardless of whether those impedances are reactive
or not. Hence "impedance match" instead of the more limited "conjugate
match". As for your assertion that non-reactive impedances are rare in
the "real world", maybe you should describe -your- "real world" and
how it differs from the rest of reality.

Cobra 2010 loses Tx audio
 

On Mon, 19 Feb 2007 22:52:40 -0800, Frank Gilliland
wrote:

+++Notwithstanding the fact that the non-reactive component of impedance
+++changes at or near resonance, maximum power transfer (due to matched
+++impedances) occurs regardless of whether those impedances are reactive
+++or not. Hence "impedance match" instead of the more limited "conjugate
+++match". As for your assertion that non-reactive impedances are rare in
+++the "real world", maybe you should describe -your- "real world" and
+++how it differs from the rest of reality.

**********
I am not saying that the real portion of impendances are rare. I am
saying that pure resistance is but a subset of complex impedance. Pure
resistance is where the reactive part of the complex impedance is
zero. In the real world no component has a "zero" reactive component
as does it not have zero resistive part.

In conjugate matching, the nodal point where the output of the
transform network terminates with the load will have a net reactance
of zero. The real part is still there. It does not go away. The net
real part should be half that of the real part of the load.

All components have complex impedances. In cases where frequency of
operation is well below the self resonance frequency, discrete passive
components can be thought of as purely resistive or purely reactive
dending on construction of the passive part. That be whether it is a
resistor or a capacitor or inductor. Non passive components have
complex impedances.

All the above is valid only when you are dealing with time varying
signals. Complex impedance has no definition when dealing with a non
time varying signal(ie. DC).

james

Cobra 2010 loses Tx audio
 

On Feb 17, 1:59 am, "Pete KE9OA" wrote:
Another eBay special that "worked perfectly". It doesn't matter if I put in
an external audio source from an audio oscillator or a microphone. I've
tried three good microphones, and have the same problem. When I first key
the mic, Tx audio is fine, but it quickly, almost abruptly fades after about
2 seconds. Has anybody seen this problem?

Pete

It certainly could be many things... but what you describe would lead
me to replace the high power audio IC. I'm assuming that it has one...
and uses a high-level modulation scheme. I have seen these fail in the
way you have described. I believe what happens there is that the
internal die has somehow lost contact with the heat sink... and
therefor heats rapidly... sending semiconductors within the IC all
over the bias map. This IC is normally failrly cheap, available, and
is usually easy to replce. Hope that helps...

www.telstar-electronics.com

Cobra 2010 loses Tx audio
 

"Telstar Electronics" wrote in message
oups.com...
On Feb 17, 1:59 am, "Pete KE9OA" wrote:
Another eBay special that "worked perfectly". It doesn't matter if I put
in
an external audio source from an audio oscillator or a microphone. I've
tried three good microphones, and have the same problem. When I first key
the mic, Tx audio is fine, but it quickly, almost abruptly fades after
about
2 seconds. Has anybody seen this problem?

Pete

It certainly could be many things... but what you describe would lead
me to replace the high power audio IC. I'm assuming that it has one...
and uses a high-level modulation scheme. I have seen these fail in the
way you have described. I believe what happens there is that the
internal die has somehow lost contact with the heat sink... and
therefor heats rapidly... sending semiconductors within the IC all
over the bias map. This IC is normally failrly cheap, available, and
is usually easy to replce. Hope that helps...

You don't think it might be caused by him not having at least 9 square feet
of ground plane?



Posted Via Usenet.com Premium Usenet Newsgroup Services
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Cobra 2010 loses Tx audio
 

On Tue, 20 Feb 2007 18:52:07 GMT, james wrote
in :

On Mon, 19 Feb 2007 22:52:40 -0800, Frank Gilliland
wrote:

+++Notwithstanding the fact that the non-reactive component of impedance
+++changes at or near resonance, maximum power transfer (due to matched
+++impedances) occurs regardless of whether those impedances are reactive
+++or not. Hence "impedance match" instead of the more limited "conjugate
+++match". As for your assertion that non-reactive impedances are rare in
+++the "real world", maybe you should describe -your- "real world" and
+++how it differs from the rest of reality.

**********
I am not saying that the real portion of impendances are rare. I am
saying that pure resistance is but a subset of complex impedance. Pure
resistance is where the reactive part of the complex impedance is
zero. In the real world no component has a "zero" reactive component
as does it not have zero resistive part.


Well, in my "real world" there are many components with reactances so
small as to be insignificant and are therefore ignored.


In conjugate matching, the nodal point where the output of the
transform network terminates with the load will have a net reactance
of zero. The real part is still there. It does not go away.


Okay....


The net
real part should be half that of the real part of the load.


Huh?


All components have complex impedances. In cases where frequency of
operation is well below the self resonance frequency, discrete passive
components can be thought of as purely resistive or purely reactive
dending on construction of the passive part. That be whether it is a
resistor or a capacitor or inductor.


Thank you. And I should add that it is more often the case where an
intended reactive component is measured for resistive impedance than
an intended resistive component is measured for reactive impedance.


Non passive components have
complex impedances.


Not necessarily, for the very same reasons mentioned above.


All the above is valid only when you are dealing with time varying
signals. Complex impedance has no definition when dealing with a non
time varying signal(ie. DC).


For all practical purposes, true.

But you are still ignoring the fact that a conjugate match is nothing
more than an impedance match using a conjugate impedance, which is
often not necessary. Just because some comp resistors -- or even the
wires or PCB traces -- in an audio amp or power supply may have a very
slight inductive reactance doesn't mean you waste your time trying to
load them all with sub-pF caps. That's why, here in the real world,
the term "impedance match" is used to include any necessary conjugate
match that may (or may not) be necessary, and why you don't hear the
term "resistance match" used very often (i.e, never).

Cobra 2010 loses Tx audio
 

On Tue, 20 Feb 2007 21:46:57 -0800, Frank Gilliland
wrote:

+++But you are still ignoring the fact that a conjugate match is nothing
+++more than an impedance match using a conjugate impedance, which is
+++often not necessary.
**********

True

Sometimes it is the easiest and cheapest method though

james

Cobra 2010 loses Tx audio
 

On Wed, 21 Feb 2007 20:03:02 GMT, james wrote
in :

On Tue, 20 Feb 2007 21:46:57 -0800, Frank Gilliland
wrote:

+++But you are still ignoring the fact that a conjugate match is nothing
+++more than an impedance match using a conjugate impedance, which is
+++often not necessary.
**********

True

Sometimes it is the easiest and cheapest method though


I tend to disagree with that, too, but I'll drop it because it could
revive the great coax length debate..... AARGGHHH!!!

Cobra 2010 loses Tx audio
 

Frank Gilliland wrote:
I'll drop it because it could
revive the great coax length debate..... AARGGHHH!!!

NOoooo!

Cobra 2010 loses Tx audio
 

On Wed, 21 Feb 2007 14:38:38 -0800, Frank Gilliland
wrote:

+++On Wed, 21 Feb 2007 20:03:02 GMT, james wrote
+++in :
+++
+++On Tue, 20 Feb 2007 21:46:57 -0800, Frank Gilliland
wrote:
+++
++++++But you are still ignoring the fact that a conjugate match is nothing
++++++more than an impedance match using a conjugate impedance, which is
++++++often not necessary.
+++**********
+++
+++True
+++
+++Sometimes it is the easiest and cheapest method though
+++
+++
+++I tend to disagree with that, too, but I'll drop it because it could
+++revive the great coax length debate..... AARGGHHH!!!
+++
************

I use conjugate matches with active devices and crystal filters. In
that aspect they are easier and cheaper.

james

Cobra 2010 loses Tx audio
 

"Frank Gilliland" wrote in message
...
On Wed, 21 Feb 2007 20:03:02 GMT, james wrote
in :

On Tue, 20 Feb 2007 21:46:57 -0800, Frank Gilliland
wrote:

+++But you are still ignoring the fact that a conjugate match is nothing
+++more than an impedance match using a conjugate impedance, which is
+++often not necessary.
**********

True

Sometimes it is the easiest and cheapest method though


I tend to disagree with that, too, but I'll drop it because it could
revive the great coax length debate..... AARGGHHH!!!




Everyone knows you have to use multiples of 9 square feet! Doh!



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bang re Cobra 2010 loses Tx audio
 

"U-Know-Who" wrote:
Everyone knows you have to use multiples of 9 square feet! Doh!

Had to bang her 36 times. sheesh

Cobra 2010 loses Tx audio
 

I think we got off the original subject of this post, but that's ok. There
are NO problems with the Rx section. No IMD problems, no desense of the Rx
section. Although it is a cheap design, it isn't too bad. At least a
monolithic crystal filter is used as a roofing filter at the 1st I.F. to
protect the 2nd mixer from off-frequency signals. Most of the CB units use a
10.7MHz ceramic filter with a bandwidth of anywhere from 150 to 230kHz,
which is only going to reduce 2nd image response.
Below 30MHz, you don't really need to worry so much about
NF....................a 12dB NF is fine. As long as you are conjucate
matched, stability would be good. The main thing that can cause problems is
if you S11 parameters (input return loss) aren't high enough. If you have at
least a 10dB return loss for both your S11 and S22 parameters, you shouldn't
have any problems. Most RF amplifiers are going to be unilateral, so our S12
(input / output isolation) is going to be at least 25dB. It if isn't, the
designer of the equipment didn't know what he was doing, and doesn't have
any business doing that job in the first place.

Pete

Conjugate match is needed for maximum power transfer. Nuetralization
helps extend stability over various mismatch condistions.

In a receiver RF front end it is preferable to match for best noise
figure and accept the gain. The less gain before the mixer the better.
The RF front end sets the noise figure for the whole receiver. The
gain of the RF Front end need only be high enough to overcome the
noise figure of the next stage and any losses it may present if any.

More often than not,CB calibur radios places far to much gain in the
first stages so that more simple IF stages can be used. Thereby
lowering costs.

james

Cobra 2010 loses Tx audio
 

Resistors can have complex impedances, especially film resistors. Carbon
film resistors can get by up to 30MHz or so, and metal film resistors
shouln't be used above 10MHz. The problem with these devices is that they
consist of a sprial etched resistance material that has a fair amount of
reactance as you go up in frequency.
Carbon composition resistors are preferable in RF applications, but even
their lead length becomes too reactive at higher frequencies.
Nowadays, we use 0603 or smaller size components at higher frequencies. 0402
geometry is presently being used at higher frequencies, with 0201 size soon
to become the norm. This is what I have been working with for the last
couple of years.

Pete

"Frank Gilliland" wrote in message
...
On Mon, 19 Feb 2007 19:12:33 GMT, james wrote
in :

On Sun, 18 Feb 2007 18:24:33 -0800, Frank Gilliland
wrote:

+++Conjugate match is needed for maximum power transfer.
+++
+++
+++IMPEDANCE match... for maximum power transfer. A 'conjugate' match is
+++when the impedances are complex, which isn't always the case.
***********

I have found that it is rare in the real world that impeadances are
not complex. Outside transimission lines, there is little that is not
complex.


You just said that resistors have complex impedance and transmission
lines are flat.


Then again when you conjugate match, the imaginary part of
the complex impedances is nulified and you are then left with the real
part.


Reactances don't just disappear. They create a current between the
source and load that must be assessed to see if it is going to cause
any problems. Sometimes it doesn't and sometimes it does.

Cobra 2010 loses Tx audio
 

How about the real world above 1GHz? It is very easy to model these
"insignificant" reactances in a program such as ADS and see the effects on a
real world circuit design.

Pete

"Frank Gilliland" wrote in message
...
On Tue, 20 Feb 2007 03:13:52 GMT, james wrote
in :

On Mon, 19 Feb 2007 12:43:12 -0800, Frank Gilliland
wrote:

+++On Mon, 19 Feb 2007 19:12:33 GMT, james wrote
+++in :
+++
+++On Sun, 18 Feb 2007 18:24:33 -0800, Frank Gilliland
wrote:
+++
++++++Conjugate match is needed for maximum power transfer.
++++++
++++++
++++++IMPEDANCE match... for maximum power transfer. A 'conjugate'
match is
++++++when the impedances are complex, which isn't always the case.
+++***********
+++
+++I have found that it is rare in the real world that impeadances are
+++not complex. Outside transimission lines, there is little that is not
+++complex.
+++
+++
+++You just said that resistors have complex impedance and transmission
+++lines are flat.
+++
************

No I did not.


Go back and read your own words again.


Besides Resistors can have complex impedances depending
upon constrtuctinand frequency in which they are used.


When a resistor is used at its intended frequency, any reactance is
insignificant. If it wasn't then it would be called an 'inductor' or
'capacitor'.


+++
+++ Then again when you conjugate match, the imaginary part of
+++the complex impedances is nulified and you are then left with the
real
+++part.
+++
+++
+++Reactances don't just disappear. They create a current between the
+++source and load that must be assessed to see if it is going to cause
+++any problems. Sometimes it doesn't and sometimes it does.
+++
+++
***********

I did not say they disappeared. At resonance the conjugate match
causes the net reactance to be zero. Thus nulify. The reactance are
always there.


Notwithstanding the fact that the non-reactive component of impedance
changes at or near resonance, maximum power transfer (due to matched
impedances) occurs regardless of whether those impedances are reactive
or not. Hence "impedance match" instead of the more limited "conjugate
match". As for your assertion that non-reactive impedances are rare in
the "real world", maybe you should describe -your- "real world" and
how it differs from the rest of reality.

Cobra 2010 loses Tx audio
 

I don't have the schematic for your radio in front of me, but if that
1st RF stage is like most CB radios it's common emitter. So the input
impedance is a lot higher than 50 ohms, and is matched to the antenna
with a transformer or LC network. Not exactly ideal.

This method has been used in the real world for many years, and it is still
being used. Better ways?


I am
not sure what the noise figure of this system is, but it seems that the
gain
distribution is such that most of the gain is in the 2nd I.F. strip
anyway.
Even so, under 30MHz, in most areas the excess environmental noise is in
the
15dB region.......


Are we talking 11m here?

Of course!



The objective is not low gain but low input impedance. Closer to the
impedance of the feed, to keep the first impedance transformation as
small as possible. With a common emitter, the only way to do that is
by reducing the gain. And just at the first RF stage, not necessarily
everything else in front of the first mixer.

As long as we are on that subject, an RF stage isn't even needed at
frequencies below 30MHz. As an example, you can use a Mini-Circuits SRA-3
doubly balanced diode ring mixer, that has only 4.77dB conversion loss at
11M. You also have approximately 35dB of port to port isolation. The only
advantage that an RF amplifier would provide in this situation is minimizing
1st LO radiation through the antenna port of the radio.


In my last contract with Motorola, we were using
mixers that had an IP3 of +40dBm so we were able to get away with having
some gain ahead of that mixer.


Most of the problems I've had with mixers came not from the mixers but
from unbuffered oscillators. Anywayz.....

I guess the question is if the radio works well enough as it sits. If
you can hear a signal buried in the band noise then that's about as
good as it gets. The only way I know to improve it is by matching the
impedance of the first RF to the antenna. Beyond that you'll need to
get a directional antenna.

Agreed.

Cobra 2010 loses Tx audio
 

Thanks...........that sounds super. I have the radio apart on my bench
downstairs. Gosh........what a brick! Isn't is great, when you have to undo
everything that the "technician" did to upgrade the radio?
There were several other problems that I have found. Why the manufacturer
chose to use hot glue on all of the solder lugs of the switches escapes me.
This caused several solder connections to fatigue and break off.
It looks like this repair will be an hour here and there. I might get it
fixed in a few months. The one redeeming quality is the great Rx section.

Pete

"Telstar Electronics" wrote in message
oups.com...
On Feb 17, 1:59 am, "Pete KE9OA" wrote:
Another eBay special that "worked perfectly". It doesn't matter if I put
in
an external audio source from an audio oscillator or a microphone. I've
tried three good microphones, and have the same problem. When I first key
the mic, Tx audio is fine, but it quickly, almost abruptly fades after
about
2 seconds. Has anybody seen this problem?

Pete

It certainly could be many things... but what you describe would lead
me to replace the high power audio IC. I'm assuming that it has one...
and uses a high-level modulation scheme. I have seen these fail in the
way you have described. I believe what happens there is that the
internal die has somehow lost contact with the heat sink... and
therefor heats rapidly... sending semiconductors within the IC all
over the bias map. This IC is normally failrly cheap, available, and
is usually easy to replce. Hope that helps...

www.telstar-electronics.com

Cobra 2010 loses Tx audio
 

On Sat, 24 Feb 2007 18:12:49 -0600, "Pete KE9OA"
wrote in
:

I don't have the schematic for your radio in front of me, but if that
1st RF stage is like most CB radios it's common emitter. So the input
impedance is a lot higher than 50 ohms, and is matched to the antenna
with a transformer or LC network. Not exactly ideal.

This method has been used in the real world for many years, and it is still
being used. Better ways?


Several.

Long story short, the power-to-voltage ratio of a signal is always
higher than the power-to-voltage ratio of noise. Most RF front ends
are voltage amps. But a -power- amp on the left can dig the signal out
of the noise on the order of 2-4dB, sometimes more. I like using a
common-base for the 1st RF, but you can re-bias a common emitter and
make pretty good improvements. And, as I stated before, a low input
impedance will reduce or eliminate the impedance transformation prior
to amplification.


I am
not sure what the noise figure of this system is, but it seems that the
gain
distribution is such that most of the gain is in the 2nd I.F. strip
anyway.
Even so, under 30MHz, in most areas the excess environmental noise is in
the
15dB region.......


Are we talking 11m here?

Of course!



The objective is not low gain but low input impedance. Closer to the
impedance of the feed, to keep the first impedance transformation as
small as possible. With a common emitter, the only way to do that is
by reducing the gain. And just at the first RF stage, not necessarily
everything else in front of the first mixer.

As long as we are on that subject, an RF stage isn't even needed at
frequencies below 30MHz. As an example, you can use a Mini-Circuits SRA-3
doubly balanced diode ring mixer, that has only 4.77dB conversion loss at
11M. You also have approximately 35dB of port to port isolation.


You can do better with discretes from Radio Shaft, which is really sad
when you realize that those are their lab numbers. The only advantage
I've seen to Mini-Circuits is the size. For performance, their stuff
sucks.


The only
advantage that an RF amplifier would provide in this situation is minimizing
1st LO radiation through the antenna port of the radio.


It also serves as a buffer to the mixer, which is essential for
reducing mixer IMD. The RF amp is generally a good idea.

Cobra 2010 loses Tx audio
 

On Sat, 24 Feb 2007 18:03:18 -0600, "Pete KE9OA"
wrote in
:

How about the real world above 1GHz? It is very easy to model these
"insignificant" reactances in a program such as ADS and see the effects on a
real world circuit design.


Did you miss this part?


When a resistor is used at its intended frequency.....

Cobra 2010 loses Tx audio
 

There is no such thing as an intended frequency. Now, if you said that if a
resistor is used, taking into account its limitations. Must we continue this
silly bantering?

"Frank Gilliland" wrote in message
...
On Sat, 24 Feb 2007 18:03:18 -0600, "Pete KE9OA"
wrote in
:

How about the real world above 1GHz? It is very easy to model these
"insignificant" reactances in a program such as ADS and see the effects on
a
real world circuit design.


Did you miss this part?


When a resistor is used at its intended frequency.....