[time-nuts] Phase noise measurement (was - no subject)

EWKehren at aol.com EWKehren at aol.com
Sat Aug 21 11:07:25 UTC 2010

I have the Hp phase noise system with the 35601A but use most the time the  
Wenzel Audio Amp referred to in this email. Perfect! I drive with it a 
3561A and  a 7L5!  Works for me.  The only problem is getting any more 2SK369.  
Any recommendations?
Thanks   Bert Kehren
In a message dated 8/20/2010 6:54:05 P.M. Eastern Daylight Time,  
jmiles at pop.net writes:

>  Would anyone else like to suggest a known good low phase noise
> buffer  amplifier?  Maybe something from a Fred Walls paper?

You can  always build HF isolation amps by rigging MMICs and attenuators
together,  but this will not reliably get you below -160 dBc/Hz.  Bruce G.
has  given some good advice in this regard, with some circuit designs  at
http://www.ko4bb.com/~bruce/IsolationAmplifiers.html and  elsewhere.  I'm a
fan of this version (also from  Bruce):

This one has the advantage  of simplicity.  No weird parts, nothing that is
likely to be out of  production or hard to find, and dirt cheap.  I've
measured the  broadband floor at near -170 dBc/Hz at 10 MHz, and its noise
contribution  at 100 Hz is below what the 3048A can see.  These figures are
adequate  to measure any 10811-class OCXOs.

A practical PN measurement system for  10811-class oscillators can be made 
building two of those amplifiers and  using them to drive pretty much any
random double-balanced mixer found on  eBay with +10 dBm LO specs or more.
Both ports should be driven strongly to  reject AM artifacts and avoid
degrading the excellent noise floor offered  by the amps.  I'd hit the LO
port with +10 to +12 dBm and the RF port  with at least 0 dBm.

Then, see the Wenzel app note here  (
http://www.wenzel.com/documents/measuringphasenoise.htm ) to lock the  two
oscillators in quadrature and amplify the resulting baseband  output.  Any 
several sound-card FFT programs can be used to  generate an output graph,
although if you want absolute calibration in  dBc/Hz you need to be prepared
to sweep the actual test setup from mixer  output to FFT input to watch for
various sources of flatness  error.

A combination of an AD7760-EVAL board and a Digilent Nexys2 can  be used to
construct an excellent baseband digitizer for the DC-1 MHz  spectrum, but
most of the time a good-quality 192-kHz sound card is fine  for this sort of
work.  Most good crystal oscillators reach their  broadband floor by 10 kHz,
so there's no real need to go out to 1 MHz or  more.

-- john,  KE5FX

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