[sdiy] Plotting the phase of a filter with FFT or a Phase Comparator?

[Neil Johnson]

cheater00 . wrote:
> Hi guys,
> I have been looking for a good way to measure the phase shift of a
> filter. One way is to use an FFT spectrograph, but it isn't always the
> best idea:
> - they're a bit cumbersome to use. tiny screens and even tinier
> resolutions don't help.

Which specific "FFT spectrograph"s (I guess you mean a spectrum 
analyser?) with tiny screens and even tinier resolutions are you 
referring to?

More importantly, what resolution are you looking for?

> - it's difficult to find an FFT which does audio frequencies, and if
> you do they're usually a bit expensive

For a rough estimate you could use a PC sound card.

> - there's little resolution in the low end (not great if you want to
> e.g. design a filter network for a subwoofer, or see how your vcf
> behaves in the low end of the spectrum)

Oh I don't know ... my HP 35660A can resolve down to microHz if you give 
it long enough.

> An alternative I've found is to use a phase comparator. First off, I
> stumbled upon something called a "phase writer", which is one of those
> wax roll printers. What it does is it takes two sine waves as input -
> one is the reference and one is the tested wave, and plot the phase
> shift between them (either in the range -tau/2..tau/2 or 0..tau).

Sounds like a mechanical version of a network analyser.

> The less cumbersome version is the Phase Comparator (not really sure
> if there's much of a connection with the circuit used in PLL's). It
> takes two sine waves, again, but this time it outputs a DC voltage or
> current which is proportional to the phase shift. Two that seem to be
> useful for low-frequency work are the Dranetz 305 (apparently very
> precise, and somewhat inexpensive), as well as the Tracor 905a (google
> thinks it's a tractor if you don't include the "a" so beware). Manuals
> or brochures are available for both.

HP did a phase-magnitude meter that seems quite popular with the audio 
crowd.  HP 3575A Gain-Phase meter, although it only goes down to around 
40Hz.  Below that you're really looking at mechanical or FFT.

> One thing that's bothering me about phase comparators is that they are
> made to compare the phase of static frequencies, and they have a
> settling time, which means you have to turn off the Z axis or the
> storage in the scope while the comparator is settling. Ideally you'd
> want to have a sweep, and no gating at all.

*ANY* frequency or phase mesaurement takes some time, and the lower the 
frequency the longer the time it takes to get, more so if you want more 
significant digits.  That's life.  Or start with an HP 141T and the 
30Hz-300kHz plugin, and then you really appreciate what "slow" means....

> Is there any other equipment useful for this purpose that you could
> recommend? I know there's the impulse method, but again that's
> deconvolution/fft. I'm not sure there's a box which does that
> specifically, it's always integrated into fft spectrographs.

Audio (specifically 20Hz to 20kHz) can be done on the cheap with a PC 
sound card and software.  Below 20Hz or above 20kHz and you start 
heading towards proper test equipment.

The classic HP spectrum analyser for audio work is the 3580A, covering 
1Hz to 50kHz.

> BTW, can someone recommend good spectrographs which go down to 1Hz?

Check out the FFT-based HP Dynamic Signal Analysers.  To properly 
measure phase you need a dual channel unit.  The 3562A still commands 
good prices, but if you hunt around you can find something like the 
35660A or 35665A for reasonable prices.

Neil
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