"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.

Well, I have replaced a few of the electrolytic caps in the modulator
section, and I have SSB audio. Now, on to the AM section.............

"Pete KE9OA" wrote in message
. ..
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 22:24:56 -0800, Frank Gilliland
wrote:

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

How about used below its self resonant frequency?

james

On Sat, 24 Feb 2007 18:01:09 -0600, "Pete KE9OA"
wrote:

+++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
+++
*************

And carbon composition should be avoided. They absorb moisture and
change resistance with time. I have seen to many 100K carbon comps
measue around 60K with time. I would aviod them like the plague.

I have worked with chip components for over 20yrs. I stay away from
the samllest one unless the board density constraints or the design
dictates it.

james
james




+++"Frank Gilliland" wrote in message
m...
+++ 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.
+++
+++
+++
+++

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 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.

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 16:23:27 -0600, "Pete KE9OA"
wrote in
:

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?


My apologies. I'll rephrase:

When a resistor is used within it's intended operating frequency
range.....

When a resistor is used below the frequency where it no longer behaves
like a resistor.....

When a resistor is used in the frequency range for which it was
designed.....

Better?

And if you don't like "silly bantering", why are you in this group?

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

On Mon, 26 Feb 2007 21:38:24 GMT, james wrote
in :


snip
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?


With a JFET, not a MOSFET. I'll bet it's as sensitive as a gay
attorney watching a Snicker's commercial. It sounds a bit scary
though..... maybe if it's DC coupled to a bipolar 2nd stage? However
it's used it would definately need some sort of protection that
doesn't contribute to noise. And imagine the possibilities using a
dual-gate MOSFET (too bad they don't make them anymore).