John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

Patrick, before I get into a point by point rebuttal to your comments, let
me briefly summarize my current understanding of the difference between
the traditional RC diode peak detector load network with its exponential
decay, and the current source load you use which has a constant linear
sawtooth decay.

The following discussion does not consider problems due to the effects of
a non ideal diode, or those due to an AC/DC load ratio that isn't unity.

It appears that the current source load is the best from the point of view
of tracking the modulation wave form, however with a fixed current source
operation is only optimal at one fixed carrier level, if a wide dynamic
range is to be achieved for the detector, then means must be provided to
cause the current source to track the carrier level.

No, this isn't so. The current source is used precisely because it does
provide the same good performance regardless carrier or modulation level.

Patrick, this is a joke right? This is the really simple stuff, this
isn't one of the more complex and subtle points that we can all get
wrong.

I am not joking when I said that the best performance under all conditions is
possible with a CCS, or a large value R with a fairly large 55v across it
to approximate a CCS.

Using CCS is new age stuff, and one never saw any CCS anyplace in domestic
electronics because there was always a cheaper crummier way to get the job done.

I am going to try and keep things simple by just sticking to this
one point. It is easily demonstrated that your claim is wrong with
respect to increasing carrier levels. Your circuit discharges the peak
hold capacitor with what is a reasonable approximation of a current
source, which means that the discharge is at a fixed rate of volts/sec.

Thank goodness the rate of discharge is fairly constant, regardless of AM%
and for the lower Audio F.


Assuming a given fixed modulating frequency, and depth of modulation, the
maximum slope of the modulation that must be tracked by the voltage on the
peak hold capacitor is proportional to the average carrier amplitude.
That implies that if the carrier level is increased by say 6 dB, then the
slope in volts/sec that must be tracked increases by a factor of two,
while the discharge slope of your constant current circuit remains fixed,
ultimately leading to tangential clipping at some carrier amplitude.

The innitial part of the discharge slope from the 270 pF and 1M that I use is
quite steep,
but quite fast enough to allow a few volts of 10 kHz audio AM, without tangential
distortion, where the discharge slope cuts off part of the negative going sine
wave.

Try my circuit on a breadboard, and you will see that all's well!!!!!!

How many times must I suggest you try something new for a change!!

On
the other hand, while the traditional RC circuit has its problems, it is
not affected by the average level of the carrier that is feed to it. If
the average carrier increases by 6 dB, the peak modulation slope that must
be tracked increases by a factor of two as before, but since the discharge
current is not fixed, and varies in proportion to the carrier level, the
discharge slope also increases by a factor of two, and there will be no
additional tangential clipping with the traditional circuit when the
average carrier level is increased.

The bottom line is that the traditional circuit can handle any carrier
level no matter how large, without an increase in tangential clipping,
while the tangential clipping in your circuit, with a fixed discharge
rate, increases as the carrier level increases above the design point,
hence a poor dynamic range.

I found the typical traditional circuit suffered from tangential distortion just
like any other.

Whatever the the diode detector circuit is, it should be set up
to be able to produce a large enough voltage without tangential or other
distortions
and I believe my circuit produces the least compared to the traditional.

When you try my circuit instead of wasting an enormous amount of time on
discussions,
you will see the superiority of the circuit I have posted.



Now specifically what is wrong with what I have just said, where is my
error? This is the simple part of the problem, it is not even the complex
stuff where we all go wrong from time to time, yet you don't seem to be
able to grasp it.

I am able to grasp it, but I have limited time to discuss something so trivial,
and and you won't even give the idea of mine a try, so WTF do you know about
my idea if you have not tried it?

I have tried yours, and a pile of other variations.



There was never any intention to have superlative AGC control in this radio
of mine.

I never assumed that was your intention, but since your detector is
sensitive to the average carrier level, it is relevant, but that's not the
real reason I wondered out loud about your AGC circuit. The real reason
was that I was simply curious about the performance of an AGC system with
a single controlled stage, given that most radios use a minimum of two
controlled stages.

There is adequate AGC for local stations.
The mixer and IF amps are working in their linear regions,
and the main purpose of the AGC is to prevent IF overload.

The detector is quite linear regardless of whatever level of IF is present
up to about 10vrm of output, but I ask only 3vrm of audio output.
There is SFA distortions from my detector.





Some of the convenience benefits of the traditional AGC philosofy were
traded away for a more linear performance of the mixer and IF tubes, which
work with fixed bias.

That's fine as far as it goes, but it creates a problem for a detector
with a fixed discharge current source, and the consequent sensitivity to
overload that implies.

I dson't think so.

BTW, I tried shunting the secondary of an IFT while monitoring the signal at the
primary.
As I predicted, the primary signal reduced about 1 dB.

You have said the primary signal will increase when the seconday is shunted, ie
gain will increase, but I saw no sign of that.

Patrick Turner.





Regards,

John Byrns

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