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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Aug 21 11:33:30 UTC 2010

The Wenzel Audio amp is a little noisier than it need be and it has a 
poor PSRR, so that a very low noise power supply with low ripple is 
Its not too hard to improve the PSRR and the input noise of such a 
current feedback amplifier.

There are JFETS (IF9030) with similar noise floors and significantly 
lower flicker noise.
However the minimum order from Interfet is about $250.
For noise measurements on several JFETS (including the IF9030 and the 
2SK369) see:
"/Ultra-Low-Noise High Input Impedance Amplifier for Low-Frequency 
Measurement Applications/"
Felix A Levinson, IEEE Transactions on Circuits and Systems Vol 55 No 7, 
August 2008 pp1815-1821.


EWKehren at aol.com wrote:
> Hi
> 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):
> http://www.ke5fx.com/norton.htm
> 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
> by
> 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
> of
> 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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