I am using the AD8343 (frequency mixer DC - 2.5 GHz) for a 2 stages mixer. The first stage is a 640MHz + 400MHz -> 240MHz and is working well. The second stage is a 240 MHz + 242 MHz -> 2 MHz so it brings high-frequency signal to "normal" low signal, ready to be acquired with an ADC. The matching/tuning of the second stage is way more challenging than the first one because of this low signal. While I reached the +7 dB gain described in the datasheet at the output of the first stage (with a -10 dBm RF and -10 dBm LO), the output of the second stage never reached more than -20 dBm, that is to say 10 dB losses with regards to the RF signal.
I carefully followed the instructions of the datasheet for the differential input: a parallel capacitor gives a -20 dB peak reflection coefficient (S11 with the network analyser behind a balun). The output load is then adjusted with a potentiometer. Again, this procedure gave good results with the first stage.
For low frequency (let say < 5 MHz), the matching of the output should -- I assume -- not necessary be 50 ohms but more a high impedance. Despite the datasheet show interesting examples for down-conversion, the lowest output frequency is still 70 MHz and I am missing advices for tuning at lower frequencies.
Thanks for your inputs !