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.

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.

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.

"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.


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.

From the above statement, I can tell that you have very little experience
with doubly balanced mixers, especially the ones from Mini-Circuits. The
LAVI-XXX series of mixers have IP3s in the +33 to +40dBm range. The only
type of discrete mixer that can even come near this type of performance is
something that uses either a quad JFET ring, a quad CATV bipolar ring, or a
dual power FET type that uses something like the Siliconix VN66. Your
typical balanced dual JFET mixer, as used in some of the Yaesu and Icom
transceivers will achieve IP3s in the +10 to +15dBm range, which isn't bad.
This is without having the preamp switched in.
Now, to even be able to measure that type of performance, you need to have
all of your RF sources very clean. This means at least -65dBc for all RF
signals. Special attention must be paid to the 6th and 9th harmonics of the
LO, as these artifacts can cause poor return loss of the I.F. port and also,
2nd order IMD measurements can be degraded.
The test setup must have an intermodulation free dynamic range of at least
10dB better than the device you will be testing. This includes connectors,
attenuators used for isolation, etc. Attenuators with transverse heat sink
fins have the best IMD characteristics.

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.

The RF amp will not reduce IMD..........it will actually degrade the IMD
performance of the mixer by the amount of gain that the RF amp provides. It
is very easy to see this if you are making IP3 measurements on a mixer. Add
10dB of gain ahead of that mixer, and IP3 degrades by 10dB.

On Sun, 25 Feb 2007 16:40:37 -0600, "Pete KE9OA"
wrote in
:


"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.


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.

From the above statement, I can tell that you have very little experience
with doubly balanced mixers, especially the ones from Mini-Circuits.


You're right. I ran some of their stuff through the bench many years
ago and was disappointed, so I never used it. As for size, Analog
Devices has been making some remarkable stuff in the last few years.


The
LAVI-XXX series of mixers have IP3s in the +33 to +40dBm range.


You used dB before, which I assumed was carrier attenuation. Still,
I'm not impressed.


The only
type of discrete mixer that can even come near this type of performance is
something that uses either a quad JFET ring, a quad CATV bipolar ring, or a
dual power FET type that uses something like the Siliconix VN66. Your
typical balanced dual JFET mixer, as used in some of the Yaesu and Icom
transceivers will achieve IP3s in the +10 to +15dBm range, which isn't bad.
This is without having the preamp switched in.
Now, to even be able to measure that type of performance, you need to have
all of your RF sources very clean.


Exactly! That's why I pointed out those numbers are "lab numbers". If
you want to get some realistic numbers you have to test it under
realistic conditions, which isn't that hard. The only drawback is that
the numbers will be relative; i.e, it's a comparison test against
other circuits. But if you do you will find that what I'm saying is
true -- discretes perform much better. And yes, you have to carefully
match the curves. This added labor, along with higher assembly costs
and parts counts, are the primary reasons why discretes are rejected
over mini-bricks; it rarely has anything to do with performance.


This means at least -65dBc for all RF
signals. Special attention must be paid to the 6th and 9th harmonics of the
LO, as these artifacts can cause poor return loss of the I.F. port and also,
2nd order IMD measurements can be degraded.
The test setup must have an intermodulation free dynamic range of at least
10dB better than the device you will be testing. This includes connectors,
attenuators used for isolation, etc. Attenuators with transverse heat sink
fins have the best IMD characteristics.

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.

The RF amp will not reduce IMD..........it will actually degrade the IMD
performance of the mixer by the amount of gain that the RF amp provides. It
is very easy to see this if you are making IP3 measurements on a mixer. Add
10dB of gain ahead of that mixer, and IP3 degrades by 10dB.


I can see that you are locked into a voltage-only mode. Feed your
mixer under test with signals of varying impedance. I think you will
be suprised, if not shocked.

From the above statement, I can tell that you have very little experience
with doubly balanced mixers, especially the ones from Mini-Circuits.


You're right. I ran some of their stuff through the bench many years
ago and was disappointed, so I never used it. As for size, Analog
Devices has been making some remarkable stuff in the last few years.

I have worked with some of their newer stuff, and it has been very good. AD
got their act together pretty well, in the RF arena.
The Analog Devices AD831 isn't a bad design; it does have a good IP3, but in
order to reach the NF of a Mini-Circuits SRA-3 however, you need to have a
preamplifier ahead of it. With its 12dB NF, it isn't a bad mixer for HF use
up to 30MHz. I had started a receiver design using the 831, but things got
so busy at work that I shelved that project for awhile.


The
LAVI-XXX series of mixers have IP3s in the +33 to +40dBm range.


You used dB before, which I assumed was carrier attenuation. Still,
I'm not impressed.

I thought the only reference to dB was port to port isolation and SSB
conversion loss.


The only
type of discrete mixer that can even come near this type of performance is
something that uses either a quad JFET ring, a quad CATV bipolar ring, or
a
dual power FET type that uses something like the Siliconix VN66. Your
typical balanced dual JFET mixer, as used in some of the Yaesu and Icom
transceivers will achieve IP3s in the +10 to +15dBm range, which isn't
bad.
This is without having the preamp switched in.
Now, to even be able to measure that type of performance, you need to have
all of your RF sources very clean.


Exactly! That's why I pointed out those numbers are "lab numbers". If
you want to get some realistic numbers you have to test it under
realistic conditions, which isn't that hard. The only drawback is that
the numbers will be relative; i.e, it's a comparison test against
other circuits. But if you do you will find that what I'm saying is
true -- discretes perform much better. And yes, you have to carefully
match the curves. This added labor, along with higher assembly costs
and parts counts, are the primary reasons why discretes are rejected
over mini-bricks; it rarely has anything to do with performance.

I agree on those points. Unless the LO in the actually is actually filtered
to the point where all higher terms are at least -65dBc, that performance
won't be realized. The manufacturers I worked for over the years were quite
happy with -25dBc for the 2nd harmonic of the LO.


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

The RF amp will not reduce IMD..........it will actually degrade the IMD
performance of the mixer by the amount of gain that the RF amp provides.
It
is very easy to see this if you are making IP3 measurements on a mixer.
Add
10dB of gain ahead of that mixer, and IP3 degrades by 10dB.


I can see that you are locked into a voltage-only mode. Feed your
mixer under test with signals of varying impedance. I think you will
be suprised, if not shocked.

You do make a good point; an unconditionally stable low gain RF amplifier
will satisfy this requirement. I have done the measurements that you
mention, and I have noted some level of disparity between real world
conditions and manufacturers' specs. I know................too many
manufacturers play the "numbers game". As long as they stick to the same
standards, one can use these numbers to initially select a product but the
devices still need to be characterized before those numbers are actually
believed.

Pete

On Sun, 4 Mar 2007 11:23:03 -0600, "Pete KE9OA"
wrote in
:


From the above statement, I can tell that you have very little experience
with doubly balanced mixers, especially the ones from Mini-Circuits.


You're right. I ran some of their stuff through the bench many years
ago and was disappointed, so I never used it. As for size, Analog
Devices has been making some remarkable stuff in the last few years.

I have worked with some of their newer stuff, and it has been very good.


I'll have to run some of the new stuff across the bench.


AD
got their act together pretty well, in the RF arena.
The Analog Devices AD831 isn't a bad design; it does have a good IP3, but in
order to reach the NF of a Mini-Circuits SRA-3 however, you need to have a
preamplifier ahead of it. With its 12dB NF, it isn't a bad mixer for HF use
up to 30MHz. I had started a receiver design using the 831, but things got
so busy at work that I shelved that project for awhile.


Call me old-fashioned but I still prefer discretes.


The
LAVI-XXX series of mixers have IP3s in the +33 to +40dBm range.


You used dB before, which I assumed was carrier attenuation. Still,
I'm not impressed.

I thought the only reference to dB was port to port isolation and SSB
conversion loss.


Port to port isolation or carrier rejection, whatever you want to call
it..... you can easily get 60 dB or better using discretes. Heck, some
of the old DSB-SC tube rigs were even designed to mix in a -power-
stage!

Anyway, you used dBm in one post and dB in another; not the same
thing.


The only
type of discrete mixer that can even come near this type of performance is
something that uses either a quad JFET ring, a quad CATV bipolar ring, or
a
dual power FET type that uses something like the Siliconix VN66. Your
typical balanced dual JFET mixer, as used in some of the Yaesu and Icom
transceivers will achieve IP3s in the +10 to +15dBm range, which isn't
bad.
This is without having the preamp switched in.
Now, to even be able to measure that type of performance, you need to have
all of your RF sources very clean.


Exactly! That's why I pointed out those numbers are "lab numbers". If
you want to get some realistic numbers you have to test it under
realistic conditions, which isn't that hard. The only drawback is that
the numbers will be relative; i.e, it's a comparison test against
other circuits. But if you do you will find that what I'm saying is
true -- discretes perform much better. And yes, you have to carefully
match the curves. This added labor, along with higher assembly costs
and parts counts, are the primary reasons why discretes are rejected
over mini-bricks; it rarely has anything to do with performance.

I agree on those points. Unless the LO in the actually is actually filtered
to the point where all higher terms are at least -65dBc, that performance
won't be realized. The manufacturers I worked for over the years were quite
happy with -25dBc for the 2nd harmonic of the LO.


And then they moved on to designing CB amps?


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

The RF amp will not reduce IMD..........it will actually degrade the IMD
performance of the mixer by the amount of gain that the RF amp provides.
It
is very easy to see this if you are making IP3 measurements on a mixer.
Add
10dB of gain ahead of that mixer, and IP3 degrades by 10dB.


I can see that you are locked into a voltage-only mode. Feed your
mixer under test with signals of varying impedance. I think you will
be suprised, if not shocked.

You do make a good point; an unconditionally stable low gain RF amplifier
will satisfy this requirement.


Hence my recommendation to use a low-impedance front end.


I have done the measurements that you
mention, and I have noted some level of disparity between real world
conditions and manufacturers' specs. I know................too many
manufacturers play the "numbers game". As long as they stick to the same
standards, one can use these numbers to initially select a product but the
devices still need to be characterized before those numbers are actually
believed.


I won't even use a 2-cent resistor until I destroy it on the bench
first. A lot of the manufacturer specs look really good on paper but
don't mean squat beyond the ideal conditions of a lab test. Even a
supposedly identical component made by different manufacturers will
behave differently in the actual circuit, especially under failure
analysis (which can be a very expensive lesson if not learned before
designing or repairing power equipment..... don't ask!).

On Sat, 24 Feb 2007 22:22:56 -0800, Frank Gilliland
wrote:

+++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.
************

That is true in most cases. Most of my RF work in the front end dealt
around using small loop antenna( less than 1/8 wave) for paging
recievers and those puppies have very low radiation resistance. You
need some impedance transformation even if you do use common base.

james

On Mon, 26 Feb 2007 02:03:09 GMT, james wrote
in :

On Sat, 24 Feb 2007 22:22:56 -0800, Frank Gilliland
wrote:

+++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.
************

That is true in most cases. Most of my RF work in the front end dealt
around using small loop antenna( less than 1/8 wave) for paging
recievers and those puppies have very low radiation resistance. You
need some impedance transformation even if you do use common base.


Well, yeah, with a 1/8 wave loop? LOL!

Anyway, a common base with a single transistor can get you in the
neighborhood of 100 to 500 ohms, depending on the transistor. With a
50 ohm input that leaves you with a transformation ratio from 2:1 to
10:1, which is -way- better than the typical 1000:1 to 10000:1 range
needed for a bipolar voltage amp (I won't even mention FET's). The
lower the ratio the better. Put two or three transistors in parallel
and you can divide that ratio down even further.

Take a half-hour or so and sift through your pile of schematics. I'm
sure you'll find a few radios that do this. Even some HF tube radios
used a grounded-grid triode on the front end -- not for stability as
might be assumed, but for performance.

On Sun, 25 Feb 2007 18:55:29 -0800, Frank Gilliland
wrote:

+++On Mon, 26 Feb 2007 02:03:09 GMT, james wrote
+++in :
+++
+++On Sat, 24 Feb 2007 22:22:56 -0800, Frank Gilliland
wrote:
+++
++++++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.
+++************
+++
+++That is true in most cases. Most of my RF work in the front end dealt
+++around using small loop antenna( less than 1/8 wave) for paging
+++recievers and those puppies have very low radiation resistance. You
+++need some impedance transformation even if you do use common base.
+++
+++
+++Well, yeah, with a 1/8 wave loop? LOL!
+++
*********

Actually not that difficult. Definitely the frontend transistor were
bipolar. Often configured in cascode and operating at 0.95VDC and
narrow band operation (5 MHz wide) anywhere between 30 and 1000 MHz.


+++Anyway, a common base with a single transistor can get you in the
+++neighborhood of 100 to 500 ohms, depending on the transistor. With a
+++50 ohm input that leaves you with a transformation ratio from 2:1 to
+++10:1, which is -way- better than the typical 1000:1 to 10000:1 range
+++needed for a bipolar voltage amp (I won't even mention FET's). The
+++lower the ratio the better. Put two or three transistors in parallel
+++and you can divide that ratio down even further.
+++
+++Take a half-hour or so and sift through your pile of schematics. I'm
+++sure you'll find a few radios that do this. Even some HF tube radios
+++used a grounded-grid triode on the front end -- not for stability as
+++might be assumed, but for performance.
+++
***********

true.

I still like depletion mode MOSFETs as they operate more like vaccum
tubes than bipolar transistor do. Ever tried a common gate depletion
mode MOSFET amp in any RF AMP?

james