Mixer Diode Bias


Andy G4JNT
 

I'm looking to use an HP33801A mixer diode in a harmonic mixer arrangement to make a tracking generator for my HP8569B SpecAn.    The mixer diode came from an old, dismantled Ailtech SpecAn of the same vintage as the HP.   (Next month's RadCom DNotes carries some info about that).   The Tracking generator will be virtually the inverse of the SpecAn. input circuitry,  with the IF port of the mixer fed with a signal at 321.4MHz; the IF used in the HP6569B

The limited data I have on that mixer diode  - little more than a one-page summary - states that bias is typically 0.4V for harmonic mixer operation - which seems an odd way of specifying it.    I'd have thought a constant current would be the correct arrangement.   A DVM diode test (~ 200uA) shows a Vf of 0.42V. so 0.4V at the same temperature, would give virtually zero current, but increased sensitivity to RF drive.

SO, does anyone know if correct bias arrangement is a fixed voltage from a low Z source, which I guess would have to be properly temperature compensated,  or a constant current?  I hope it's the latter as that's just a resistor, but intuition says not.

Only ever met one case where forward junctions are biassed from a low-resistance temperature compensated voltage source, and that's class-B bipolar power amplifiers.   Does optimum use of this mixer diode rely on the same bias arrangement ?

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