[time-nuts] Spec An for phase noise measurements

John Miles jmiles at pop.net
Mon Jan 21 18:39:31 EST 2008

> 	But would I be too simple minded to suggest that maybe some form
> of A/D PC/workstation input device with high dynamic range and decent
> sample rate (certainly available in high end audio stuff to 192 KHz)
> would be the logical vehicle for close in measurement in a quadrature
> locked PLL type phase noise setup ?

Correct, that's why I was complaining the other day about the lack of any
good high-dynamic-range DAQ dongles in the *medium*-speed market segment.

You can buy an ADC outfit for audio work ($) or a complete GNU Radio USRP
with >20 MHz acquisition bandwidth ($$$$), but there is nothing economically
available in the 1-MHz neighborhood.  There are some superb 24-bit
sigma-delta chips in the 0.5 MSPS-2.5 MSPS range, but nobody has anything I
can plug into my USB port and talk to with a C compiler.

> Otherwise why would you care about
> performance below 9 KHz ?

With quadrature conversion, the carrier is downconverted to 0 Hz, and the
analyzer sees the offset frequencies of interest directly.  If the analyzer
can't look down below 9 kHz due to limited RBW or a noisy LO, the
measurement won't be valid down there.

Phase noise at >= 10-12 kHz is important for adjacent-channel rejection in
narrowband communications work.  Below that, it's important for clean
recovery of the on-channel signal.  And when comparing crystal oscillators,
you'd like to be able to look down to 1 Hz, to bridge the gap between
phase-noise and Allan deviation measurements.

Conversely, you rarely need to look at offsets beyond 1 MHz.  Anything the
DUT is going to do, noisewise, it probably will do before that point.
(Spurs are a different story of course.)

> 	Why exactly does one need a wideband SA that goes down to 100 Hz
> (common spec) or 30 Hz (nicer and newer...) for this ?   Maybe I
> am missing
> something here.... (probably am, I often do...)

I can only look down to 100 Hz with my 8566B, and I'm fine with that for the
most part.  Someone testing extremely stable sources, again, may want a
closer look at the immediate vicinity of the carrier.

The 3047A/3048A outfits are also quadrature downconverters, but they have
two parallel outputs, one for an HF spectrum analyzer, and the other for an
FFT analyzer.  The software merges plots from both analyzers into one
wideband plot (offsets from 1 Hz-10 MHz).

> 	And should one want an actual SA for these measurements instead
> of a soundcardish thing and FFT software, I know that LF/HF boxes with
> better specs than any of the affordable general coverage SA families on
> Ebay show up regularly for not too much money.

Yes, noting that the FFT analyzers generally are limited to DC-100 kHz.
They also work better with switchable lowpass/highpass filters that
eliminate signal components outside the decade currently being measured.
(That's one of the things you get with a 3047A or 3048A outfit that you
don't get with the 11729B/C by itself.)

Chances are you already have an RF/microwave SA on your bench already if
you're doing this kind of work, so it makes sense to use it, as long as its
minimum frequency limit isn't a problem with the measurements you want to

-- john, KE5FX

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