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

Had to bang her 36 times. sheesh

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

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.

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.

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.

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

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.

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.....

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.....