RadioBanter - View Single Post - Channel-based AM tube tuner (was Designs for a single frequency high performance AM-MW receiver?)

June 14th 04, 01:45 AM

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/