Double Balanced Mixers (DBM) the secret to the secret sauce. NO, this is not about how they work but how to build one. If you search You Tube you will see three videos by N6QW on building the DBM.
Double Balanced Mixers find their way into our rigs used as Balanced Modulators, Product Detectors and of course Transmit and Receive Mixers. They are bidirectional and so work well doing double duty.
While their are but a few components in the above circuit some of the more bits play a huge role. Looking to the left core you see a pot and a resistor which provides a means of carrier balance and carrier unbalance. With the 39 Ohm resistor lifted from Ground you can balance out any carrier leaking through with the pot. When you ground the 39 Ohm, you unbalance the carrier and you have a means to provide a tune up signal.
It is OK to do this as W7ZOI included this in EMRFD with credit to W6JFR (me, old call). This will work with the SBL-1 on Pins 5 and 6.
The other curiosity is the Pi Filter on the right side. This is to limit the in and out to the audio range as you would have in a product detector and a balanced modulator. In a presentation to one of the UK Radio Clubs -- a knowledgeable person who designs Xtal Filters was adamant to limit the audio bandwidth input to his crystal Filter Design. He saw my schematic and said --Good.
A UK ham m0ntv (Nick) includes the Roy Lewallen diplexer in the design to prevent signal breakthrough. This does not have that feature.
I strongly support not using this circuit for 1st time builders as great care must be exercised in winding the cores and selecting matched diodes. Some of my contemporaries are like the sailors of old, any port in a storm or after 6 beers she looks pretty good and disagree. The other problem is how to connect everything without having wires running all over the place. The SBL-1, TUF-1 and ADE-1 are good commercial packages and once you get a circuit working then build away with the homebrew.
Here is how to do it and have short direct connections and get the wires in the right spot. I use Red/Green #26 twisted wire as the inner bifilar winding and that is the Red and Green reference .
I have just shared a secret of the secret sauce.
73's
Pete N6QW