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sameerdhiman 7th June 2011 07:51 AM

Oscilloscope as Spectrum Analyzer/Frequency Response Analyzer

Does someone know about any DIY adaptor to turn Analog Oscilloscope X-Y mode into Spectrum Analyzer and Frequency Response Analyzer ?

It would be great help for poor hobbyist like me who can purchase Spectrum Analyzer in dreams only.

Svein_B 7th June 2011 09:19 AM

You do not have to buy a spectrum analyzer if you have a PC with soundcard near your workbench.
Several free or inexpensive PC software do the job nicely.
A lot of user ARTA: ARTA Home

You will have to make a small interface to adapt the power and signal level to the input of your soundcard.


sameerdhiman 7th June 2011 09:58 AM

Thanks Svein,

I have the setup (PC+ARTA) but I am not satisfied with the speed of screen refresh. It is most problematic when I have to tune notch filter's pot, adjusting a pot takes ages to be updated on screen.

If I can use my Analog Oscilloscope as Frequency Response Analyzer It would be TRUE REAL TIME Analyzer.

gpapag 7th June 2011 10:03 AM

As Svein has pointed out, you can use your sound card.
You can then record the signal using a wave editor free software and save it in “wav.” Format. This “wav.” File can be later analysed (FR/Spectrum/Distortion) by importing it either to ARTA or RMAA or some other SW package.

Your main question though is for the hardware part, that is, what physical connections (wiring, and/or adaptor) you will use to bring the signal to the input of your soundcard and to an appropriate amplitude level.
This seems to be a well thought interface:

Soundcard Interface

If you are using specifically your oscilloscope for the “front part” of signal probing, the “Y Out” of the oscilloscope is a convenient pick up point for the input of your soundcard. See here


[Note: If your oscilloscope has a “Y Out” you can use it to feed your soundcard.
The probe will be connected to the Y Input of the oscilloscope, so effectively you are using the scope’s preamplifier/adjustable attenuator as a good quality adjustable preamplifier.]
and here:

If your oscilloscope does not have a “Y Out” then, you may have a look here (it is an "ongoing" :D project, as I have to clear up some issues :mad:)


gpapag 7th June 2011 10:21 AM

There is somethink that I overlooked in your first post.
Are you specifically interested in capturing and analysing the X-Y mode signal of your oscilloscope?

If this is the case, then the "Y Out" will not do the job.

You have to tap both the CH1 and CH2 outputs from inside your oscilloscope and feed them to your sound card's Lin and Rin respectively.

Then manipulate them with software.


sameerdhiman 7th June 2011 11:15 AM

Thanks George,

Well the concept is having standalone adapter

Sweep Generator --> Adapter --> DUT --> Demodulator --> Y input
|--> Ramp Voltage --> X input

Sweep Generator can be stand alone or PC's sound card. Ramp Voltage will be provided by an adapter.

macboy 7th June 2011 12:58 PM

You don't need to use the XY mode to do this. You can do it with normal triggered sweep mode. The trick is to set the horizontal of the oscilloscope to a timebase that will give a full sweep in the same amount of time (or slightly longer) as the sweep generator takes to do a full sweep. The trigger output of the sweep generator can be sent to the trigger input of the scope, or just trigger off of the signal. One caveat is that you need to use a somewhat long sweep time (certainaly no less than 1 or 2 seconds) so it is beneficial to use a digital storage scope. Otherwise you depend on your persistence of vision to see the full sweep. You don't even need a demodulator to get a nice looking display. Another caveat is that the vertical is linear not logarithmic (dB).

sameerdhiman 7th June 2011 02:19 PM

Thanks macboy :)

Is there any IC available which can convert linear to log voltage ?

If I use PC as a sweep generator then from where I shall get the trigger signal ?

SY 7th June 2011 02:36 PM

Sound Cards
I use AudioTester and set the average number to 1 for real time measurements. With a 16k sample length, and using the old POS computer in my lab, the refresh rate is at least once per second.

hitachi_nut 8th June 2011 04:06 AM

Common IC used by hobbyist is the AD8307 log amplifier Got excellent dynamic range and comes in through-hole package.

I've used it for filter sweeping together with an HP 8601A sweep generator and a scope (with X-Y) capability.


Originally Posted by sameerdhiman (
Thanks macboy :)

Is there any IC available which can convert linear to log voltage ?

If I use PC as a sweep generator then from where I shall get the trigger signal ?

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