AF6AY wrote:
On Sep 1, 10:01 am, Micha³ S³omkowski wrote:
I'm designing a HF transceiver and I'm going to use SA612/NE612 as a
product detector. I'm not sure what is the optimal signal level, which
should be delivered to the inputs from the IF amplifier to achieve the
best performance?

Not intended to be a high-level input device, the Gilbert Cell
structure of the
SA612 or SA612A should make a good, low-power SSB or CW detector.
Mixer ratings put the IP3 maximum at -15 dbm which comes out to about
40 mV p-p at the input. Mixer conversion gain is somewhere in between
14
and 19 db.

Both input and output impedances are 1.5 KOhms in parallel with about
2 pFd. Both inputs are DC biased internally so capacitive coupling
should
be used (very important so as not to disturb internal biasing).
Output
impedance is 1.5 KOhms from the internal collector loads, should also
be
capacitively-coupled to the audio output. Expect an audio output
level to
be about 1 V peak-to-peak maximum with an IF input of ~ 40 mV peak-to-
peak.
That is running from a +5 VDC supply rail. It is almost the same with
a +6
VDC supply rail.

Carrier re-insertion/mixing (for "product detector" use) would be
about
200 to 300 mV peak-to-peak. Try not to exceed 300 mV p-p as that will
cause distortion from "BFO" harmonic generation. Dropping below 200
mV p-p will cause the conversion transconductance to drop until it
almost
disappears.

Interesting application, breaking some new territory there! :-)
Basically an
HF to low-UHF design, it was intended solely as a low-supply-drain
(2.4 to
3.0 mA) front-end mixer, it should work equally well to "down-mix"
back to
audio range. Mixing is mixing, dependent only on the frequency
response
of the internal transistor junctions.

73,
Len Anderson AF6AY
Actually the SA602/612 (IIRC the '602 is the better part, the 612 is a
fallout of 602's that didn't meet spec) can work well. The Elecraft K2
uses this chip in several places, including the product detector and the
K2 has one of the best receiver spec's out there. They did make use of
pads in the signal and hfo paths to get the input levels correct, and no
doubt matched input and output impedances correctly as well. An
inspection of the K2 schematic's would be a good lesson in setting up
this part.