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 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.
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. 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.

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.

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.

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.


Regards,

John Byrns


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