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?

On Tue, 01 Sep 2009 10:01:19 -0700, Michał Słomkowski 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?

There are graphs for input IP3, compression and gain in
the Philips data-sheet. The exact value of IP3 will
depend on the supply voltage, frequency and the configuration
of your circuit. The data-sheet figure for 45MHz is -13dBm.
You will want the IF input to be well below this level.
Somewhere in the -25 to -40dBm region should be ok.

The xx612 is a low power, high gain device. It is not a
particularly good choice as a product detector in a superhet.
A simple two diode balanced mixer or a diode ring mixer (DBM)
would give much better IMD performance and costs even less than
the 612.

73, Ed. EI9GQ.



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Thank you very much for your reply. I'm going to use NE612 because
it's quite popular and easy to use. Double balanced mixers are better,
but they need vary good impedance matching and high LO signal level. I
don't have much experience in building RF circuits, but I think the
IMD performance is not as important in product detector, as in the
first mixer.

My concept of the receiver stage is that: bandpass filters - variable
attenuators - first mixer AD831 - quarz filter XF9/B (a polish
replacement) with 9MHz IF - IF block based on variable gain
amplifiers LMH6505 and preamplifier LMH6624 - product detector.

I would use AD831 also as a product detector but this part is very
hard to find in Poland so I only managed to get one piece.

PS. I have read your webpage that helped me much in building my own
transceiver!

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

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.

On Tue, 01 Sep 2009 10:01:19 -0700, Michał Słomkowski 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?

I did this a long time ago with an NE602, in a receiver with audio-
derived AGC (see http://www.wescottdesign.com/articles/MSK/
page_79.html). With IF input levels the mixer would suffer diminishing
output. This would cause the AGC to get "stuck" in a positive feedback
mode and push the thing to ever-higher gain and ever-lower audio levels.
I fixed it with a pair of diodes, one in each direction, at the IF input
to the mixer (they're not in the schematic). They'd distort the signal
at those levels, but at that point it was much higher than desired anyway
-- it just needed to keep it low enough to let the AGC grab hold.

I _think_ they were germanium diodes -- IIRC silicon diodes let too much
signal through. You could use germanium, Schottky, or plain old silicon
diodes plus a 6dB pad, all to equal effect.

I'd tell you how good it sounded, but it was a data-link receiver and I
don't recall ever listening to it!

--
www.wescottdesign.com

I decided the IF amplifier will deliver the signal to SA612 at the
level -40dBm. Thanks everyone for advices.