[AMRadio] infinite impedance detector vs diode

John coleman wa5bxo at gmail.com
Thu Sep 4 18:10:44 EDT 2008

Very interesting Jim:
It seems to me from a designer / builder standpoint that the relatively high
Z input, of the "infinite impedance detector", would or could increase the Q
of the last IF XFMR thereby affording a sharper band pass if that was
desirable?  Of course we're still talking envelope detection here.
Is it possible to analyze a synchronized - product demodulator such as the
Sony IC or the old MC1496 circuits?  The MC1496 is just the mod/demod chip
and requires an external synchronized BFO.  So for your circuit test, you
could use the same carrier that you generate the AM signal with, as input to
the MC1496.  The BFO carrier insertion must be phased locked to the AM
signal input to be detected. 
See http://www.onsemi.com/pub_link/Collateral/MC1496-D.PDF

I built one of these detectors once.  It was a somewhat discrete design
using a 455KC OSC with a varactor diode in the circuit.  The OSC was fed to
buffer and then a RC phase separator where a 90 degree signal (Q) was fed to
a MC1496 and the other phase (I) was fed to the second MC1496.  The output
of the (Q)MC1496 was filtered and fed to the varactor diode to synchronize
the OSC with the IF signal which was fed to both of the MC1496 inputs
paralleled .  The (I)MC1496 output was where the audio was taken off.  

	If the station being tuned in had any phase modulation of 60HZ on
the harrier, as a lot of the BC610s had due to cabinet vibration, then the
60HZ audio would be heard very well.  I can remember one station that could
turn his modulator off and I could still detect his voice when he yelled
into the rig. HIHI  

The new SONY chips, I think, have it all built in except for tuned circuits.

John, WA5BXO

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