HIgh level mixer LO/input isolation in "real world"

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clifford.c. wright:
Good Day! I've been trying out a TUF-1H+ mixer in my new home brew rig.
It is driven by a "Norton" rf stage preceded by a tunable BP filter over the HF range.
However I've run into some problems with local oscillator (45-75 MHz) feeding back through the
mixer rf input and the amplifier. The amp is stable and the Lo power is +17 dBm.
I could possibly use more shielding but it is definitely far too high, especially as the specs show
about 55 dB or better rejection.
Of course I am already considering using a 32 Mhz LP filter between the rf and mixer.
However I would really like to know how others have realised similar designs in practice.

FYI I have also determined that the W7ZOI hybrid cascode If amplifier is pretty good once the small mod
to the long tailed pair is used. Also if j309's are used the chain of biasing diodes has to be reduced to
one or two in series. Mine has an extra 9 MHz filter after the If amp and before the product detector.

So anyone with experience of a good layout for a high level mixer???
73's Cliff wright ZL1BDA ex G3NIA

Zack Lau:
That is the disadvantage of the "Norton" amplifier--relatively poor reverse isolation.

It can be improved upon by replacing the simple transformer with a directional coupler, but a lot of work needs to be done to get a version that is actually a desirable replacement for the original.

Zack W1VT

Peter Gant:
Years ago in Ham Radio magazine there was an article on high performance front ends by Ulrich Rhode. If you can find a copy (I'm still looking for you) then read through that because it gives you all sorts of layout tips. One thing I do remember is the amount of shielding he used including shields between the pins of the mixer.

If I find the article over the weekend I'll tell you which edition it's in.


Dan Mills:
A diplexer terminating the RF (and IF) ports will help a lot (And will do very good things for your mixer intercept point as well).

Given your high side IF setup, it should be possible to design a simple diplexer to send the IF leakage into a terminating load while passing the HF spectrum, and on the IF side to provide a reasonably broadband mixer termination (Which your 45Mhz first IF filter will not present without help).

Regards, Dan.

Peter Chadwick:

That was back in about 1973 or 4. The Intercept points quoted were very interesting, because the crystal filter specified just could not manage it! I wrote to HR about it and DJ2LR wrote back to say that he was just quoting somebody else's figures - which he didn't say in the article.

The old Plessey Semiconductors SL6440 had about -40dBm or a bit less LO signal at the input port, with a +30dBm input IP3 for a 0dBm input osc level, -1dB conversion gain and an 11dB DSB noise figure. Plus gain if you used a higher load impedance.

Long gone from production, I'm afraid.


Peter G3RZP


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