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Old 25th November 2012, 10:48 AM   #1
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Default Interfacing to a spectrum analyser

Hi,

There must be DIYers out there who have used a spectrum analsyer for audio measurements, so for you guys i have a question.
How does one overcome the interface troubles of a node in an amplifier, could be of a vastly different impedance, driving the extremely vulnerable analsyer 50R input? Max 30dBm.

I am thinking either an audio transformer, or less appealing to me, a low distortion op-amp with ac coupling.

Thanks!
Craig

Last edited by Craig405; 25th November 2012 at 11:41 AM. Reason: Spelling retardedness
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Old 25th November 2012, 11:00 AM   #2
dmills is offline dmills  United Kingdom
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Resistor L pad, easy, linear and does not need power.

Actually, for audio work a PC with a good soundcard often makes a better SA then a real one which often lacks perforance at such a low frequency range, plus the soundcard approach can do things like plot group delay and phase shift which a scalar analyser cannot.

Regards, Dan.
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Old 25th November 2012, 11:29 AM   #3
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Hi dan, thanks very much.

A quick google points to this being a resistive divider basically, is that right?
If i wanted to measure a high impedance node for distortion, i guess this is still tricky with a 50R input.

I have never actually made a distortion measurement before so maybe people always select low impedance points.

Cheers
Craig
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Old 25th November 2012, 12:15 PM   #4
dmills is offline dmills  United Kingdom
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What analyser do you have?
Most of the ones that go down low enough for audio work have a high Z input option.

You will find that an RF spectrum analyser is the wrong tool for audio distortion, dynamic range is usually not that great, and LO phase noise is often awful due to the very wide tuning range required.

Do note that you only need about -20dBm at that 50 ohm input, so the input resistor can be quite large sometimes, also note that some spectrum analysers will suffer damage if you put DC on the input.

Regards, Dan.
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Old 25th November 2012, 08:11 PM   #5
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Hi,

I am using a R&S analyser, 20Hz to something like 30GHz. I dont have the model number to hand at the moment.
Point taken on AC coupling and amplitude limitation, if i blow it I bought it, and that would bankrupt me!

The input will tolerate a power of -30dBm with zero DC component and is only an RF 50R terminated input unfortunately, no high Z input.

I recall reading dynamic range was something ridiculous like 160dB, if I am correct then this is plenty for audio, although I dont recall the phase noise of the LO.

Is there a good way to measure from highZ nodes?
This may be a silly question, as it may turn out that all circuits I measure have a lowish impedance output due to feedback.

Cheers.
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Old 25th November 2012, 10:22 PM   #6
dangus is offline dangus  Canada
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Maybe use a differential amplifier or buffer?

If using a sound card, I like the idea of using outboard ADC and DAC connected to the sound card through Toslink and powered from separate isolated supplies. That may not have enough resolution or bandwidth without spending big bucks.
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Old 25th November 2012, 10:44 PM   #7
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Quote:
Originally Posted by Craig405 View Post
Hi,

Is there a good way to measure from highZ nodes?
This may be a silly question, as it may turn out that all circuits I measure have a lowish impedance output due to feedback.

Cheers.
My colleagues who frequently measure all sorts of things with spectrum analysers usually use a differential active probe for high-impedance nodes. Of course as soon as you use a differential active probe, you have to start worrying about its distortion. As a check, you could measure distortion, screw an attenuator on the active probe, measure again, see if you find the same answer.

You can measure low distortion levels on a spectrum analyser by using the smallest resolution bandwidth available (often 1 Hz) and increasing the attenuation until the distortion of the analyser (and probe, if any) is below the distortion you want to measure. Still, I don't believe you will get anywhere near -160 dB. By the way, on some types you explicitly need to switch on the DC coupled-option to get a reasonable sensitivity at low frequencies.
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Old 26th November 2012, 02:04 AM   #8
DUG is offline DUG  Canada
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Craig405

I used to use a R&S specA...20Hz to 26.5GHz...($186,000 CDN) back in early 90's.

It had a tracking generator that went to 5GHz.

If yours has one:

Use a resistive divider and (terminate the gen in 50ohms) sweep the response to make sure it is dividing properly at the frequencies you are interested in.

Yes they are expensive to repair.
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Old 26th November 2012, 09:42 AM   #9
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As others have noted, a RF spectrum analyzer likely does not have sufficient resolution for any serious audio distortion measurements. Run the test--connect the generator you intend to use directly to the analyzer (with appropriate attenuation etc.) and see where the harmonics are.

The simplest form of medium-Z input would be simply a series resistor (plus perhaps a series capacitor for AC coupling). Take a 4950 Ohm resistor, and you have 5 kOhm input Z with 40 dB attenuation. When working with line- or even speaker-level signals, this should do the trick.

Samuel
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Old 26th November 2012, 09:06 PM   #10
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Great responses, thanks.

@ Dangus, I think this will be the way to go for open loop measurements in the input stages of amps. I'll have to knock up a low distortion op-amp buffer to do it.

@ MarcelvDG, it looks like the frequency resolution is in the data sheet as 0.01Hz with a display resolution of 0.1Hz, I cant seem to get the display to do better than 1Hz/div with a 10Hz span about 20kHz though. I dont have a low distortion tone generator at the moment.

@ DUG, no tracking generator installed, doh.

@Samuel Groner, I will probably give this a go, I have a Hifi Power amp that has a data sheet distortion spec I would be interested to test out, should be able to see if i can get measurements of 0.000x% ballpark.

Cheers
Craig
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