On Nov 15, 8:13 pm, Tom wrote:
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/...rtion%20-%20Re...

73, Tom VE3MEO

Interesting that your posting should come just at this time. Perhaps
a week ago I was looking at a Spice simulation of a mixer to
baseband: really just a product detector. I noticed the same effect,
and concluded that it is indeed due to the change in effective load
impedance.

However, I do not believe that makes for distortion, directly. If the
source is linear, the loading that depends on the relative phase
between the LO and the IF signals does not cause distortion, just
modulation. On the other hand, if the source impedance looking back
into the IF is not constant with varying load, there will be
distortion.

If my analysis is correct, then there is not a need to prevent the
modulation of the IF output amplitude, at least not for the reason of
preventing distortion. A way to check this would be to put in two
signals and check the output for intermodulation products, or to just
check the sinusoidal output (appropriately filtered to remove high
frequency components) for harmonic distortion.

I'd be interested to know what more you learn about this. I may do
some playing with it myself, since the answer may matter quite a lot
to something I've just started working on.

Cheers,
Tom
@ K7ITM