I tried to use SA612 as a frequency mixer. Since it's a Gilbert cell, it has a high input impedance. Thus I added a pair of transformers as "Experimental Methods in RF Design" book suggests:

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The mixer seems to work:

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Here I'm using 15 Mhz 12 dBm LO signal, RF is -16 dBm and I change it in 1-3 Mhz range to make sure that the mixer works as it supposed to. Two smaller signals on the left and on the right are IF, e.g. mixing products. 15 Mhz signal in the middle is a LO leakage. It's something you would expect to see in a mixer. LO can be lowered to 8 dBm, which causes 3 dB loss in mixing products.

(For the record, I also tried to use a simpler circuit without transformers. It kind of worked but the level of mixing products where MUCH lower.)

The result surprised me. I've been using diode ring mixers before and observed a much higher level of IF. For instance here I'm using a balanced diode ring mixer made of two FT50-43 ferrite cores and four 1N5818 Schottky diodes:

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RF is the same -16 dBm, but LO is only -18 dBm. This mixer can be safely driven with 10-13 dBm which gives a much higher IF level.

This brings me to the question. Is an IF level which I got with SA612 an expected result or maybe I'm doing something wrong? Maybe you could suggest another test circuit?

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    $\begingroup$ I'm sorry, I'm a bit confused: the first spectrum analyzer plot is the output of the SA612 when you feed in 12 MHz & 2 MHz, but what is the second? $\endgroup$ – Marcus Müller Sep 5 '20 at 19:06
  • $\begingroup$ Second is an example of diode ring mixer output with similar input. $\endgroup$ – Aleksander Alekseev - R2AUK Sep 5 '20 at 19:57
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    $\begingroup$ What range of RF and LO did you try? In dBm? load impedance? $\endgroup$ – Tony Stewart Sunnyskyguy EE75 Sep 5 '20 at 22:54
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    $\begingroup$ Oh well , I cant fix your test, but I can give you a better Gilbert Multiplier. tinyurl.com/y59eseqg. Maybe that can help. $\endgroup$ – Tony Stewart Sunnyskyguy EE75 Sep 6 '20 at 15:01
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    $\begingroup$ According to the datasheet, the maximum IF output power before the onset of gain compression is between -15dBm and -20dBm, while the maximum output appears to be about -10dBm. Since the mixer output resistance is specified to be 1.5-k$\Omega$, it would be interesting to know what (differential) output you measure with a 1.5-k$\Omega$ resistor across the output pins. $\endgroup$ – Brian K1LI Sep 7 '20 at 14:08

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