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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Aug 22 14:51:09 UTC 2010

dk4xp at arcor.de wrote:
>>> NXP   BF862, available from  digi-key.
>> Don't these devices have relatively high flicker noise?
> 1/f corner is well below 100 Hz. Look at the noise voltage plots of
> that audio guy I cited.
> My results for the BF862 were the same shape, absolutely somewhat worse
> in amplitude because I wanted a differential input and less FETs in parallel.
> Most of my BF862 had abt. 12 mA IDss, btw.
>> The input capacitance is relatively noncritical in this application
>> (phase noise measurement) since it is shunted by the much larger output
>> capacitance of the low pass filter at the mixer IF port.
> The 300 pF Cin of a single  IF3602 could seriously detune the input low pass
> and the 200 pF feedback capacitance in a stage with substantial voltage
> gain would destroy the bandwidth unless cascoding is provided.
The IF3602 is suboptimal with respect to flicker noise the IF9030 is 
Its capacitances are similar to those of the 2SK369 however it has 
somewhat lower flicker noise (see attachment).
At low frequencies the following opamp (in the Wenzel and similar 
circuits ) ensures that the voltage swing at the FET drain isnt large 
and the effect on circuit bandwidth is somewhat smaller than a naive 
analysis would indicate.
Cascoding done correctly, has the advantage of improving the input stage 
PSRR as long as the noninverting opamp input is bypassed to the drain 
supply of the cascoding FET.
Using a series string of LEDs connected to the drain supply and driven 
by a high output impedance to bias the opamp noninverting inputcurrent 
sink ensures that the PSRR is maintained down to very low frequencies.
For a sound card preamp used in a phase noise measurement setup the low 
pass filter capacitance is usually 10nF or more so a 300pF input 
capacitance has little effect on the filter cutoff.
> I think, I'll test some Analog Devices ADA9848-2 in parallel. It's hard to beat
> that combination of noise, 1/f, bandwidth, offset stability and price.
> Such a preamp can be used as an add-on to a scope or FFT-Analyzer, too,
> to characterize power supplies, references or oscillator bias circuits.
> It's fun to enter 60 dB probe gain into a scope channel menu
> and still see usable traces  with uV/div scale factors.
> ( with a low pass, of course)
> There are noise nuts, too!  ;-)
> Gerhard
If the desired bandwidth only extends down to 100mHz or so then a low 
ESR supercap can be effective in ensuring that the input stage doesnt 
have excessive dc drift.
>>> One heroic effort for  audio is here:
>>> http://www.diy-audio-engineering.org/index.php?board=2.0   HPS5.1
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