"John Byrns" wrote in message
...
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.
John,
You seem to be limiting your considerations to the 'tangential clipping' and
not to other distortions that will occur. With the simple RC circuit the
decay of the signal differs from positive to negative peaks in the
modulation. This imparts an assymetry to the recovered signal. You are
trading one type of distortion for another. Using a constant current to
drain the capacitor provides a more linear output and one where slew rate
limiting can be easily computed as a function of frequency and amplitude.
Using a resistor to drain the capacitor provides an output where slew rate
limiting is more a function of frequency and less of amplitude.
However, if you know the maximum amplitude and modulating frequency of the
signal you are trying to detect, then for either detector one can determine
the proper component values for the desired result. These are the tradeoffs
that go into every design.
I suggest that those who are interested and/or following this discussion
would be well served by doing some modeling of the two proposals and
consider the results and how they are affected by changes in the input
signal. For a simple approach you could consider an ideal diode and signal
source, a capacitor and either a current source or a resistor. Try various
amplitudes and modulation levels.
Both circuit approaches work within their limitations. The question is 'what
are the limitations?'.
Suggestion: Consider a triangle wave for the modulation source. The math is
a lot easier.
Hint: Linear is good.
Once you understand the limitations of each circuit topology, then you can
understand how it interacts with the rest of the radio, or what requirements
each places on the rest of the radio.
Have fun, I'll be watching,
craigm
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/