[John Byrns]

In article ,
John Byrns wrote:

In article
,
Patrick Turner wrote:

On Jul 12, 12:42*pm, John Byrns wrote:
In article
,
*Patrick Turner wrote:



I already know that a well-designed diode detector works best for me;
at
least
since I regressed to tubes, in my IC days I would have answered
differently.

With ICs you get a dc coupled amp and its possible to use an output
diode pointed at an RC circuit with the audio voltage and Vdc fed back
to the FB input port of the IC, and thus get what is supposed to make
the most linear detector imaginable because there is MUCH more NFB
being applied than when using a cathode follower like I do. The IC
input impedance at IF frequency needs to be high though. I have
circuits for doing all that but the CF I use is just fine.

HP had an interesting AM demodulator in their AM/FM modulation meter.
The AM detector used transistors, not ICs. *A rough description as I
remember it is as follows. *The transistors were configured in what
was basically a darlington configuration. *Feedback was taken from the
collector through a capacitor to a pair of parallel diodes back to a
summing junction at the base. *The two diodes were connected in
opposite directions, with a pair of parallel connected complex cognate
networks in series with one diode at the end connected to the
transistor base. *The modulation output was taken across the network
that has the form of a low pass filter IIRC.

It'd be nice to have a schematic John. Then 1,000 words are told with
1 picture.

Hi Patrick,

Sorry that I forgot about your schematic request, however sometimes other
things
in life take precedence. The following files contain a description and a
simplified schematic of the HP AM Demodulator that I was talking about.

http://www.fmamradios.com/HP_AM_Demo...979-Cover1.jpg
www.fmamradios.com/HP_AM_Demod/HPJ-979-P12.jpg
www.fmamradios.com/HP_AM_Demod/HPJ-979-P13.jpg
www.fmamradios.com/HP_AM_Demod/HPJ-979-P14.jpg

The AM distortion is claimed to be less than 0.1%, although this article
doesn't
say what modulation depth this is good to. IIRC the service manual contains
those details. I believe that the ³audio² bandwidth of this detector is
something on the order of 250 kHz. I have a copy of the relevant pages from
the
service manual somewhere around here, with a detailed schematic and parts
list,
however I am not going to try and dig it out, the simplified schematic should
suffice to convey the basic idea. The specs as to modulation depth vs.
distortion are probably also in the relevant HP catalog, I believe some of
these
old HP catalogs are available on line, although I don't have any links to
hand.

Oops sorry I screwed the links up, they should be as follows.

http://www.fmamradios.com/HP_AM_Demo...979-Cover1.jpg
http://www.fmamradios.com/HP_AM_Demod/HPJ-979-P12.jpg
http://www.fmamradios.com/HP_AM_Demod/HPJ-979-P13.jpg
http://www.fmamradios.com/HP_AM_Demod/HPJ-979-P14.jpg

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/