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

[Bob kb8tq]

Hi

Pretty much the best mixer to use for this in a basement / DIY basis
is a Mini Circuits RPD-1 or one of it’s siblings. It has a 500 ohm
output on the mix port instead of 50 ohms. Yes, you open circuit
terminate it ( so 5K  ) but as noted, it’s the Zout of the mixer that likely
sets what the op amp sees. With it’s higher output impedance, you 
are even less driven to nutty low noise op amps and 4 ohm feedback
resistors. The good old OP-27 / OP-37 sitting in the dusty back of your
parts drawer from back in 1993 will do just fine. 

Yes, this all gets back to being nutty as you get close to carrier. If you
are after -150 dbc / Hz at 1 Hz offset, you will need go a bit crazy. If
you head this way, there are a lot of posts back in the archives leading
you down various paths to get it done. 

While others have indeed fried expensive setups while loosing a supply
leg, I’ve never run into that problem. It most certainly can happen. I’ve
never taken any special precautions and have yet to “get bit” by the issue. 
As a rough guess, I’d say I’ve powered up various implementations these 
setups a couple thousand times over the years. 

Bob

> On Jul 10, 2022, at 6:32 AM, Gerhard Hoffmann via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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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