I came across a Mini-Circuit ZAD-1 double balanced mixer (DBM) at the TRW swap meet this weekend in good condition for 7 bucks so I grabbed it. This DBM with BNC connectors turned out to be a standard Level 7 mixer.
I connected my RTL-SDR dongle to the IF port with several adapters and fed the output of the Si5351 directly into the LO port. The antenna is connect to the RF port.
Using NT7S Arduino Library and a Mega 2560 board I set the clock0 to 125 Mhz to feed the LO. Below is the basic setup.
This set-up worked great for the Medium Wave (MW) AM broadcast band too. On 40 Meters I listen to several SSB stations just fine. Using the NT7S library for 125 Mhz may be a bit out of range since I do seem to hear a warble to the clock when I listen to the 125 Mhz signal directly on another receiver. I need to experiment with the original Adafruit library and see if there is a difference. All and all a cool experiment. If I can get the warble out of the clock and maybe add some band-pass filters this could be a very usable SDR receiver for HF.