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.
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.
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.
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.
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.
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