[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik Kaashoek)

[ghf at hoffmann-hochfrequenz.de]

Am 2022-07-09 22:06, schrieb Erik Kaashoek via time-nuts:

> Ultra low noise opamps have been ordered to hopefully reduce the 
> internal
> noise of the PNA but the reference OCXO may already be the limiting 
> factor.
> The REF voltage output of the OCXO turned out to be rather clean. Much
> cleaner than a 7805 voltage regulator

The existence of my own ultra-low noise amplifiers was originally 
triggered
by this problem but has turned into a sport of it's own. Don't yield to 
the
temptation of driving this too far. A single AD797, LT1028, or ADA4898-2
all deliver an input noise density of abt. 1nV/rtHz which is the thermal
noise of a 60 Ohm resistor. ADA4898 has goof price/performnce.

The diodes in the mixer can easily feature RS = 20 Ohms, and the 2 
conducting
diodes then show 40 Ohms, which is not much less than the 60 Ohm equiv 
of the opamps.
RS is ohmic resistance of silicon and contacts, not the differential
slope resistance of the diode which is only half-thermal IIRC.

High level mixers often have additional resistors in series to the 
diodes.
It's no wonder then that high level mixers are usually not the winners 
in
dynamic range. Maybe an array of low-level mixers that are Wilkinsoned
together on RF and LO, with the IF ports in series would give good 
results.

1. Stephan R. Kurtz, Watkins-Johnson:  Mixers as Phase Detectors
2. Bert C. Henderson, W-J: Mixers: Part 2  Theory and Technology
Copyright © 1981 Watkins-Johnson Company
Vol. 8 No. 3 May/June 1981
Revised and reprinted © 2001 WJ Communications, Inc.

cheers, Gerhard



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