1. I've been digging around in the receiver of my Yaesu FT-817ND
transceiver and simulating with LTspice the AGC and AM detectors to
investigate possible distortions due to non-linear loading of the IF
feed they share with the SSB/CW product detector when I chanced upon
an unexpected behaviour: the envelope of a CW IF signal at the input
of the product detector is amplitude-modulated by the beat frequency
output. The modulation is asymmetrical and distorted and is perhaps
only 5-10% deep but its fundamental component is clearly the beat
frequency. I presume that the output is therefore distorted to the
extent that the output amplitude at any time is somewhat proportional
to the amplitude of the input envelope. Is that a reasonable
supposition?

2. I suspect the cause is a dynamic load impedance of the product
detector being fed by a high impedance IF source - a 100pF coupling
capacitor (approx 3600Xc at 455kHz) preceded by an IF buffer amp with
at least 1000R source impedance. The product detector is NJM2594,
stated in the datasheet to have a 600R input resistance and schematics
for test setups showing 50R sources. Is that a reasonable explanation?
Are there any other probable causes?

3. If the hypothesis in 2 is correct, a probable solution would be to
lower the source impedance and that of the coupling capacitor to
something on the order of 50 Ohms. Is that a reasonable conclusion?
Are there any other possible solutions?

I've posted an article to the Files section of the FT817 Yahoo!Group:
http://groups.yahoo.com/group/FT817/...ulation_of_IF/

73, Tom VE3MEO