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I have a question about "best practice" in regards to how the 2nd mixer IF port is coupled to (in my case) a 20m double tuned filter on transmit.

I'm currently building out the transmit chain for a 20m single band SSB transceiver and I've got as far as the second mixer. It's a standard and simple single conversion superhet design. The mixer is a double-balanced ADE-1, with the 9MHz IF arriving on the RF port (as the output of the final IF amp) and the VFO feeding the LO port (si5351 ~ 5MHz). The 14MHz signal component from the IF port is currently around -30dBM, with the other mixer product (~4MHz) having the same power.

I'm going to follow this with a double tuned 14MHz BPF to eliminate any unnecessary mixing products. The BPF has a loss of around 8dB. From there, it's on to the amplifier chain.

My question is in relation to the 2nd mixer IF port on transmit. Best practice would say that you should terminate this port in 50 ohms at all frequencies. In the receive path for the same mixer, I terminated the IF port in a 9MHz (the IF freq) diplexer to ensure this happened. But, I'm not sure what to do for the transmit path and I don't see a definitive answer in my reference material. I'm tempted to just use a 6bB attenuator pad on the basis that I'm heading into a large amplifier. So, even though my signal budget isn't great, I can make up the loss in the following stage. But, I'm just not sure. Any advice would be greatly appreciated!

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    $\begingroup$ ADE-1 or ADE-1+, by the way? ADE-1 is "non-catalog", ADE-1+ is usually what you'd want. minicircuits.com/WebStore/dashboard.html?model=ADE-1 (other than being different in guarantees, the parts should be identical) $\endgroup$
    – Marcus Müller
    Sep 29, 2021 at 12:51
  • $\begingroup$ Well now @Marcus Muller, you've opened a can of worms! On closer inspection, it seems my ADE-1 is actually an ADE-1-24. I purchased these on aliexpress some time back (yes, I know I've placed a euro in the cheapskate jar - sob). That said, it seems they should work OK and they certainly work well on receive. However, since these are custom mixers, they don't have a datasheet. Can we say the mixer is a generic DBM diode mixer or is it more complicated than that? $\endgroup$
    – Buck8pe
    Sep 29, 2021 at 14:28
  • $\begingroup$ This is a bit of speculation, but given the naming scheme, I'd assume the architecture to be identical to ADE-1(+) $\endgroup$
    – Marcus Müller
    Sep 29, 2021 at 15:51

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It depends on your specific signal integrity needs. Ideally, I would check with some instruments to see the different behavior.

I'd bet it will "work" to just feed the output into your filter, however, you asked about best practice, and that would involve properly terminating your mixer with 50 ohms.

Your BPF will of course be 50 ohms or thereabouts at your frequency of interest, but at other frequencies it will be much different. So, putting a 3dB or 6dB pad would be the proper way to do it. The pad will make sure that the mixer sees 50 ohms at all frequencies and should avoid unexpected behavior and performance issues.

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