In article ,
(Steve Kavanagh) writes:

There seems to be some difference of opinion on how to properly
terminate Tayoe detectors (and other similar circuits) at audio
frequencies. See the following published examples:

(1)G0BBL et al (QRP2001): load resistors are 220 ohms
(2)Hans Summers: load resistors are 1k
(3)9A2HL (on Hans Summers' website): Approx. 0 ohms, detector goes
directly to op-amp pins
(4)AC5OG (recent QEX series, Part 1): Approx. 0 ohms, detector goes
directly to op-amp pins
(5)AC5OG (recent QEX series, Part 4): 60 megohms (doubly balanced
circuit).

In addition references 1-3 use a series 47 ohm resistor at the input
of the detector, which seems unnecssary to me - the 50 ohm antenna
should perform the function of the filter resistor, shouldn't it ?

I did some Spice modelling and found that my detector was well matched
to the 50 ohm RF source for audio load resistances of a few hundred
ohms (470 ohms worked well), and the conversion loss was a bit more
than 3 dB when matched. With somewhat lower resistances the match was
not as good but the conversion loss was reduced - 220 ohms may be
close to optimum for both reasonable input match to 50 ohms and
conversion loss. With higher (and much lower) resistances both
conversion loss and input match degraded. The integrator capacitors
were 0.1uF in this model.

Note that I am defining conversion loss as the relationship between
the total power delivered to the 4 load resistances and the forward
power at the input. With high load resistances the voltage at each
load is indeed closer to the input voltage than for lower load
resistances, but most of the power incident at the mixer is reflected
back to the antenna. Therefore the conversion loss (defined in terms
of power, which is what matters for noise figure) goes up.

I would be interested in the comments of those of you with some
experience with these detectors. Also, can I assume (for the purpose
of computing noise figure of the audio preamps) that the source
impedance seen by the audio amps is simply equal to the load
resistance that gives the best match at the mixer input ?

I'd suggest you take a look at Dan Tayloe's own article in
RF Design for 1 March 2003. You can get a printable copy at

http://www.rfdesign.com

If you search for "Tayloe." Unfortunately, the figures aren't
available for downloading (I have my own paper copy) but the
details on the bits and pieces are all there. Good article.

Len Anderson
retired (from regular hours) electronic engineer person