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Old June 14th 04, 01:45 AM
John Byrns
 
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In article , (Steven
Swift) wrote:

Patrick Turner writes:

3. Diode detectors are quite low distortion detectors even with
very low voltages of 100 mV if there is a constant current trickeled
through the crystal diode to keep them turned on with their
forward conducting voltage.
I gave details yesterday in another post of a detector which will change your
views about
diode detectors.
Diodes can be used with DC shunt feedback around an RF opamp,
and thd is negligible.


I agree that this can be made mostly true using active filters and such, but a
perfect diode, with perfect modulation has lots of distortion. I am willing
to take a look at your analysis, but if you use Volterra series expansion,
you simply can't prove that you'll get better than a few percent distortion.
Somewhere in my old grad school notes, I have a derivation done by Prof.
Meyer (of Gray and Meyer, UC Berkeley) which shows the limits. I'll look for
your other post. Better than a few percent is NOT possible with just an RC
load (diagonal clipping) except for low modulation percentages.


It would be interesting to see the derivation you speak of! It was my
impression that if we had a "perfect diode" it could be used make a
perfect envelope detector, with the exception of the "tangential clipping"
problem that you mentioned. "Tangential clipping" is not just a function
of the modulation level, but is also a function of the modulating
frequency. As Patrick mentioned using a higher IF frequency will allow
using a smaller peak hold capacitor which will reduce "tangential
clipping". Also doubling the IF frequency by using a full wave detector
will reduce the "tangential clipping".

It is hard to believe that the distortion of a reasonably designed diode
detector is anywhere near "a few percent", as simple diode detectors were
used in the modulation monitors used by AM broadcast stations in times
gone by, and they had to have distortion low enough to measure the system
distortion at modulation percentages up to 100%. I will have to look up
the specifications of a few. Of course there is the issue that negative
peak clipping that is visible on a scope may represent only a small
fraction of a percent distortion on an RMS basis.


Regards,

John Byrns


Surf my web pages at,
http://users.rcn.com/jbyrns/
  #23   Report Post  
Old June 14th 04, 02:00 AM
Patrick Turner
 
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Steven Swift wrote:

Patrick Turner writes:

Let me say a few words.


1. Many pages of RDH4 are devoted to AVC.
The time constant for AVC application is very long,
comprising of 1M and 0.047 uF, and measurement
of bass distortions resulting from well applied AVC is low
enough to be negligible.


To work properly for fading, the AVC needs to be about 100ms. This causes
significant distortion at bass audio. If a longer time constant is used
since only local (non-fading) stations will be tuned, then you are right.


You don't get fading on locals.
1M plus 0.047 uF typically used in AVC circuits has a TC = 47 ms
signifcant bass distortions do not occur.
The pole is at 3.37 Hz.



2. Nothing wrong with cathode bias, especially nowdays when cheap
large value elcaps are plentiful, and we have better plastic caps.
RDH4 speds a lot of time on cathode bias.


Cathode bias is great, but do not bypass the resistor. The degenerative
feedback will improve audio performance, but you lose gain.


If the Gm of the tube is like that of 6AU6, or 6BX6, the gain is high enough
to throw 6 dB away on current feedback from an unbypassed Rk.



3. Diode detectors are quite low distortion detectors even with
very low voltages of 100 mV if there is a constant current trickeled
through the crystal diode to keep them turned on with their
forward conducting voltage.
I gave details yesterday in another post of a detector which will change your
views about
diode detectors.
Diodes can be used with DC shunt feedback around an RF opamp,
and thd is negligible.


I agree that this can be made mostly true using active filters and such, but a
perfect diode, with perfect modulation has lots of distortion. I am willing
to take a look at your analysis, but if you use Volterra series expansion,
you simply can't prove that you'll get better than a few percent distortion.


Its obvious from my tests of my detector that ths is minimal,
and below the 1% level.



Somewhere in my old grad school notes, I have a derivation done by Prof.
Meyer (of Gray and Meyer, UC Berkeley) which shows the limits. I'll look for
your other post. Better than a few percent is NOT possible with just an RC
load (diagonal clipping) except for low modulation percentages.


There is no diagonal clipping in a well set up diode detector used
with a virtual CCS current flow from the C used to collect RF pulses.
If the ripple voltage does not vary with signal strength, the detection is linear.


4. AC coupling is fine from an RC load fed by a diode.
The impedance fed by the audio + RF ripple voltage should be high,
like a cathode follower grid.


5. I have tried my radio with various speakers, and no trouble
making full amplitude signals at 20 Hz.
The LF pole is determined by the audio amp in the radio, but at the detector, the


There is a discussion of speaker/cabinet resonances in RDH4 somewhere.
Lots of distortion when you approach resonance.


But that distortion isn't a problem due to the tuner.


On which pages are RDH4 and Terman "dont's" spelled out?


Each chapter has "crumbs" of knowledge in it. I have yet to find a nice
do/don't do list anywhere. I may also be "integrating" Terman and Henney.

I think the guy who started the thread should build his idea and try them
on a few "beta" testers. He'll be able to sell enough to pay off his costs.
Maybe pick a few channels in a few big markets (LA, New York, Chicago) to
keep the work load down.


Mr Noring has a huge backlog of R&D work ahead of him to
come up with his prototype tuner.

I'd rather build my own gear than wait for his solution.

Patrick Turner.



Steve.
--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA


  #24   Report Post  
Old June 14th 04, 02:10 AM
Patrick Turner
 
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John Byrns wrote:

In article , Patrick Turner
wrote:

Steven Swift wrote:

If you are willing to live with about 5-10% THD, then you can use more
common circuits.


True, but onje doesn't have to live with 5-10%.
linearize the IF amp and detector, and thd plummets.

However, there are dozens of "Hi Fi" AM circuits published

by the hobby magazines, tube vendors and kit makers. Have a look at them.

The RC-19 circuit uses a 6BA6 as an RF amp, followed by a 12AU7 used as a
detector and audio amplifier.


RDH4 has the circuit for the Selsted and Smith "infinite impedance "
detector, where a 12AU7 performs as credible detector, and as a diode,
but I think I'll stick with a germanium diode fet by a 12AU7 CF.


The "Selsted and Smith" detector is not the same thing as the so called
"infinite impedance" detector. The "infinite impedance" or "reflex"
detector was designed by RCA, while the "Selsted and Smith" detector was
designed by, well "Selsted and Smith", or at least "Selsted" who is still
around, or was a year or two ago.

The "Selsted and Smith" detector differs from the "infinite impedance"
detector in that it has a diode in series with the grid, and also a diode
load resistor.


Yes, you are right, and I should have checked my RDH4.
But the S&S detector pulls much less power from the tuned circuit....

There is no peak detection capacitor across the diode
load, so the diode does not act as an ordinary diode peak detector, and
the triode doesn't act as a cathode follower. The triode is the actual
detector operating in a fashion similar to the "infinite impedance"
detector, with the diode apparently serving to linearize the "infinite
impedance" detector. The input impedance of the "Selsted and Smith"
detector is not infinite due to the presence of the diode load resistor.


True, but its higher than most other detectors.


For that matter the input impedance of the so called "infinite impedance"
detector is also not infinite, and can even have a negative resistance
component which can cause stability problems. The negative resistance
effect can occur when circuit conditions are right, similar to the
conditions that can cause oscillation in cathode and emitter follower
circuits if you aren't careful.


I am happier with direct feed of the RF/IF signal to a CF, with following
crystal diode and C, with nearly constant current discharge from the C.



But does the RC-19 have enough tuned circuits to give over 70 dB rejection of
signals which are 50 kHz away from the wanted station at any place on

the band?

The 6BA6 is a variable U tube, with a non linear
transfer curve.


There is nothing wrong with the 6BA6, it was specifically designed for
this service and has very low odd order distortion which is all that
matters since the even order distortion products can't get through the
IFT. I hope I got that the right way around, if not it is explained in
some detail in some of the old texts, I think "Radio Receiver Design" by
Sturley is one that explains it. You only get in trouble if you try to
run the tube at a very high signal level, simultaneously with a high AGC
voltage applied for a large gain reduction. This is mainly a problem in
the stage driving the diode detector, so it is best to avoid AGC on that
stage, but in a minimal radio that is of course problematic. This is one
of the many topics that the RDH4 gives short shrift. The 6BA6 is even
usable as a gain control element in audio circuits where even order
distortion does matter. IIRC the peak limiter at a radio station where I
once worked used four 6BA6s in the audio path, where they were connected
in push pull, presumably to cancel the even order nonlinearities which are
inherent in the design of the tube.


Indeed the PP connection of two 6BA6 would lead to cancelation of 2H in the thd.
Not a bad idea for an RF/IF amp either.
All distortion is bad.

Patrick Turner.



Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/


  #26   Report Post  
Old June 14th 04, 02:15 AM
Patrick Turner
 
Posts: n/a
Default



John Byrns wrote:

In article , (Steven
Swift) wrote:

Patrick Turner writes:

3. Diode detectors are quite low distortion detectors even with
very low voltages of 100 mV if there is a constant current trickeled
through the crystal diode to keep them turned on with their
forward conducting voltage.
I gave details yesterday in another post of a detector which will change your
views about
diode detectors.
Diodes can be used with DC shunt feedback around an RF opamp,
and thd is negligible.


I agree that this can be made mostly true using active filters and such, but a
perfect diode, with perfect modulation has lots of distortion. I am willing
to take a look at your analysis, but if you use Volterra series expansion,
you simply can't prove that you'll get better than a few percent distortion.
Somewhere in my old grad school notes, I have a derivation done by Prof.
Meyer (of Gray and Meyer, UC Berkeley) which shows the limits. I'll look for
your other post. Better than a few percent is NOT possible with just an RC
load (diagonal clipping) except for low modulation percentages.


It would be interesting to see the derivation you speak of! It was my
impression that if we had a "perfect diode" it could be used make a
perfect envelope detector, with the exception of the "tangential clipping"
problem that you mentioned. "Tangential clipping" is not just a function
of the modulation level, but is also a function of the modulating
frequency. As Patrick mentioned using a higher IF frequency will allow
using a smaller peak hold capacitor which will reduce "tangential
clipping". Also doubling the IF frequency by using a full wave detector
will reduce the "tangential clipping".

It is hard to believe that the distortion of a reasonably designed diode
detector is anywhere near "a few percent", as simple diode detectors were
used in the modulation monitors used by AM broadcast stations in times
gone by, and they had to have distortion low enough to measure the system
distortion at modulation percentages up to 100%. I will have to look up
the specifications of a few. Of course there is the issue that negative
peak clipping that is visible on a scope may represent only a small
fraction of a percent distortion on an RMS basis.


The "tangential clipping or diagonal clipping ocurs if the C value in
the RC circuit after the diode is of too large a value for the
voltage and frequency generated.
You see the phenomena best when the AF modulation F = 10 kHz or more,
and output voltage is a few volts.
So it becomes more likely as F of audio rises.
But the amount of audio declines with rising F.

Patrick Turner.


Regards,

John Byrns

Surf my web pages at,
http://users.rcn.com/jbyrns/

  #29   Report Post  
Old June 14th 04, 02:36 AM
Frank Dresser
 
Posts: n/a
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"Telamon" wrote in message
...

I have sent a dozen responses to the other two newsgroups asking them to
stop cross posting.

I don't like playing netcop but I will report them if it continues.

--
Telamon
Ventura, California


Here's an excerpt from the rec.radio.shortwave charter:

"This group is intended to be a place where ANY radio monitoring topic can
and
should be discussed. We are happy to hear from posters who listen to any
part
of the radio or microwave spectrum, from DC to daylight. We discuss topics
of
almost any kind, ranging from (but not limited to) shortwave broadcasting,
DXing small or distant shortwave stations, utility and teletype monitoring,
military eavesdropping, station schedules, QSLing sw broadcasters, spectrum
usage, equipment design and modifications, antennas, receiver reviews and
recommendations, and many more."

"Despite what the newsgroup name might imply, we definitely DO NOT limit
discussions to shortwave only. Any radio-related topics are welcome."

http://www.faqs.org/faqs/radio/monitoring/introduction/

Frank Dresser


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