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Old 13th January 2010, 12:09 AM   #1
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Talking Function Generator

Can a function generator such as the Victor VC-2002 be substituted for a audio generator?
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Old 13th January 2010, 02:28 AM   #2
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Sure. From the specs it sounds like many other similar low cost generators. Even some expensive ones. They usually have a slight peak on the top and bottom of the sine wave, since they generate a triangle and then round it off to form the sine wave. IOW, don't expect really low distortion, but it will work fine for frequency response and looking at square wave response on the scope. Many can sweep, so you can do visual response curves for filters and such on the scope.

CH
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Old 13th February 2010, 12:55 AM   #3
cliffyk is offline cliffyk  United States
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One issue for audio work is that function generators tend to have pretty crappy distortion specs for their sine wave outputs--also, (and this is 100% IMHO) the Victor instruments tend to be crap. I have a Victor VC-3165 counter that I wish I'd not bought--fortunately it only cost $50 and it to does work well to monitor the frequency of my pirate FM station.

The VC-2002 sine wave THD is specified as being "-40 dBc" (40 dB below the set frequency's level), this is 1.0% THD which is not even close to being a clean enough source for serious audio work.

Look for older low distortion RC oscillators like the Kikusui ORC-11 (0.002% THD) and it's clones. The Leader LAG-120B and LAG-126 (0.005% THD) also often appear on eBay.

I have a Kikusui ORC-11 and it's output from 10Hz to 50kHz is < 0.0019% THD...
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Old 14th February 2010, 02:17 PM   #4
Telstar is offline Telstar  Italy
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Quote:
Originally Posted by cliffyk View Post
I have a Kikusui ORC-11 and it's output from 10Hz to 50kHz is < 0.0019% THD...
What kind of waves does it to?
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Old 15th February 2010, 09:41 AM   #5
cliffyk is offline cliffyk  United States
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Quote:
Originally Posted by Telstar View Post
What kind of waves does it to?
Sine and square, it's an audio signal generator not a function generator--as I said function generator's tend to have relatively poor distortion specs...

Click the image to open in full size.

PC sound card based instrument's can be useful for audio work, however their output is only as good as the sound card itself--and while most sound card's have low audio distortion they will also produce ultrasonic artifacts of the sampling frequency, and have lower level outputs in the range of 1.0V RMS or so.

A "not too bad" and inexpensive instrument is the Velleman PCGU1000 function generator, it's THD is less than 0.08% across the audio band. Here's a review I did just after I got one...
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Old 15th February 2010, 09:55 AM   #6
Telstar is offline Telstar  Italy
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Thanks. The price of the Kikusui is a bit out of my range.
I exlcuded soundcards, that even they may be pretty good, but even good software such as Spectra plus is limited to 22050hz. What I need the most is a FAST HF square wave (20khz to 100khz min.)

Have you measured the rise time of the Kikusui and of the Velleman?
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Old 15th February 2010, 07:35 PM   #7
Frex is offline Frex  France
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Telstar,
SpectraPlus is not limited to 22050kHz, it's your soundcard !
You can use sound card like " ESI Juli@" which allow 192kHz sampling rate. With it you have 96kHz bandwidth (90k more really).

Frex.
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Old 16th February 2010, 12:45 AM   #8
cliffyk is offline cliffyk  United States
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Here are screen shots of the PCGU1000 and ORC-11 20%-80% and 10%-90% risetimes.

The PCGU1000 is not bad at 14.66ns (10%-90%), however the ORC-11's performance is not impressive as fast rise time was compromised for low-distortion...

PCGU1000:
Click the image to open in full size.

ORC-11:
Click the image to open in full size.

As to sound card instruments, and please I do not mean to offend proponents of same, there is no way you will get either a decent looking, or fast rise time square wave from even the best of the breed.

Here is the rise time for a 48kHz sample rate, created with a reasonably good Creative USB "card":
Click the image to open in full size.

16.06us, or over 1000 times slower than the PCGU1000. Granted this is a 48kHz card, however even with a 192kHz card you'd be lucky to get rise times of 3-4us.

Here's what the full wave looks like:
Click the image to open in full size.

By contrast here is the full wave from the PCGU1000:
Click the image to open in full size.
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Old 16th February 2010, 12:50 AM   #9
star882 is offline star882  United States
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There are programmable signal generators that are basically just ring buffers (loaded with whatever waveform you can fit inside and clocked at whatever frequency the PLL is capable of running at) connected to DACs. The HP/Agilent units at my university are that type and they produce a very clean signal. But they're probably way too expensive for your application. It should be easy to build something a little less elaborate out of a microcontroller and high speed DAC.
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Old 16th February 2010, 10:39 AM   #10
Telstar is offline Telstar  Italy
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Quote:
Originally Posted by star882 View Post
There are programmable signal generators that are basically just ring buffers (loaded with whatever waveform you can fit inside and clocked at whatever frequency the PLL is capable of running at) connected to DACs. The HP/Agilent units at my university are that type and they produce a very clean signal. But they're probably way too expensive for your application. It should be easy to build something a little less elaborate out of a microcontroller and high speed DAC.
Not expensive at all, they cost (used of course) less than the 600$ of the ORC-11.
I'm checking some datasheet, raise time seems to be usually around 20ns and distortion <0.01% iirc.
Looks like the Vellerman is really a best buy.
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