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

[Neil Johnson]

cheater00 wrote:
>> Which specific "FFT spectrograph"s (I guess you mean a spectrum analyser?)
>
> Yeah - thanks! Living in Germany is really killing me.

I hope not literally!!

>> with tiny screens and even tinier resolutions are you referring to?
>
> Well, the HP 35660A :-) for one thing. Those things just seem so
> cumbersome, especially if I compare to a full-screen spectrogram  on a
> TV!

Or monitor.  Agreed, although for what it is I find the screen good enough.

>> More importantly, what resolution are you looking for?
>
> I think it's less about what resolution I'm looking for, and more
> about what resolution can be had..

Well, you can have whatever resolution you have the time and money for.

>> For a rough estimate you could use a PC sound card.
>
> Sure, but that's not really going to give me any certainties. Agreed
> though, a PC sound card is an OK first approximation.

You can use a stereo card for a basic network analyser: one channel
samples the input, the other channel samples the output.
But only useful over the audio range, and at typical audio amplitudes
(there's no range switching like you get on a proper hardware
analyser).

>> Oh I don't know ... my HP 35660A can resolve down to microHz if you give it
>> long enough.
>
> Yeah, I have looked at the 35660A and 3561A. Which one would you
> recommend and why? Is there any "trick" to finding spectrum analyzers
> that go this low in frequency?

I first used a 3561A.  It's a portable analyser, but only single
channel.  The 35660A is dual channel, but much larger and not very
portable.  It is very much a bench machine.  Both have an upper range
of 100kHz.  The 35660A splits that into 50kHz in dual mode, but you do
get very precise phase information.  So it's fine for audio use, ideal
for audio filters :)

There's no trick as such, for very low frequencies you need to go for
FFT instruments as there's no practical way for analogue to go sub-Hz
- the settling time as the analyser filter is swept would give a sweep
time measured in hours.

> Would you suggest the HP141T as a good spectrum analyzer?

Only if you like ancient machines or want to look like Charles Atlas!
It's fine for RF use, but for audio the lowest range is 30Hz, and even
then the sweep times can get very long for narrow bandwidths.

> Can you compare the 3580A to the 3561A? The 3561A does 125uHz-100kHz,
> but not sure about much more.

3580A - analogue, slow, covers 1Hz to 50kHz
3561A - FFT, much faster, does computation, covers 125uHz to 100kHz.
3562A - dual channel version of 3561A
35660A - modern version of 3562A, but sample rate reduced to 50kHz in dual mode
35665A - as above, but sample rate still 100kHz in dual mode, plus a
few other extra features

Also, price varies.

> Again.. would you be able to tell the differences that 3562A brings
> in? And the 35665A?

Both are good machines.  The 3562A is the older one.  It depends what
you can find for the price you're prepared to pay.
As a rough guide, Stewarts of Reading have:
 a 3562A for £1,500, and a 35665A for £1,850.

Neil
--
http://www.njohnson.co.uk