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Old 24th March 2012, 06:47 PM   #151
PChi is offline PChi  United Kingdom
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Location: Staines, UK
Hi L,
Thanks for the comments. The sample and hold output is clean because the low charge injection analogue switch, the Analog Devices ADG1221, is good though the 10 pin Mini Small Outline Package with a 0.5 mm pin pitch is a challenge to hand solder.
The multiplier output signal level varies from about:-
Frequency = 10 Hz. Multiplier output amplitide at IC10 pin 14 = 50 mV peak to peak
1 kHz, 60 mV.
10 kHz, 6 V peak to peak
100 kHz, 2 V peak to peak
I think that the multiplier output level could be reduced at high frequencies by adjusting the extra loop gain compensation components that I have added.
I have ordered an E-MU 0204 sound card which you mentioned earlier which looks good. I will try the ARTA software when the sound card arrives.
Regards Paul
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Old 4th April 2012, 07:46 PM   #152
pol098 is offline pol098  United Kingdom
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Saw this on eBay, don't know if it's old news to everyone but:
Ultra low distortion (<0.00005%) 1kHz sine generator assembled and tested PCB

From Latvia, US$51 shipped

Second and third harmonic levels
unloaded or loaded 600ohm < 0.5 ppm ( < 0.00005%)
Output frequency 1kHz +/- 1%
Unloaded output voltage level 300mV - 2.6V (RMS)
Output impedance 600 ohm
External power supply DC voltage/current 35V / 25mA
Dimensions (W x D x H) / Weight 60mm x 70mm (50mm - board) x 27mm / 20g

Ultra low distortion (<0.00005%) 1kHz sine generator assembled and tested PCB | eBay

(No connection with me, was looking for an inexpensive tunable low-distortion oscillator, thinking of Farnell LFM4).
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Old 5th April 2012, 02:41 AM   #153
grenert is offline grenert  United States
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So, you going to test it out for us?
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Old 5th April 2012, 08:24 AM   #154
PChi is offline PChi  United Kingdom
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Hello,
Good post. The specifications are impressive. My guess is that it is fixed frequency which influences the design because the loop gain can be trimmed at 1 kHz which means that the gain control circuit can be set to have little effect hence cause little distortion.
It's going to be difficult to verify the really low second and third harmonic distortion levels.
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Old 6th April 2012, 01:44 AM   #155
pol098 is offline pol098  United Kingdom
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Quote:
Originally Posted by PChi View Post
... My guess is that it is fixed frequency which influences the design because the loop gain can be trimmed at 1 kHz which means that the gain control circuit can be set to have little effect hence cause little distortion.
It's going to be difficult to verify the really low second and third harmonic distortion levels.
Thanks. Sorry if my post wasn't clear, it is definitely fixed frequency. I suppose the best that can readily be done is to check that the harmonics are buried in noise with the equipment available; no better is needed.

Responding to another post, whether I buy one or not, no, I'm not going to test it for you - I'm not in a position to do anything useful, sorry.

Many years ago I built a Linsley Hood low-distortion oscillator which worked very well (I'm a fan of thermistors), but can't find it now. People tended to build oscillators and see what amplitude they got - I worked out how to calculate the amplitude from the thermistor specification, something I've never see anywhere. I submitted it to Wireless World for publication; they published something with my graph and formulas, but described it as a new type of oscillator, and the information sank without trace except for one letter criticising me for claiming the oscillator was a new idea.

I used the oscillator to help with designing and building a very cheap distortion analyser with an unusual technique, working at zero frequency - much easier to get a bandwidth of a few Hertz than at higher frequency where you require crystal filters. I think it worked quite well; I took out a provisional patent for some of the techniques but never quite completed the project, deciding there was no market, or at any rate it wouldn't help me much in making a living. I have the prototype, but all paperwork, including circuit, has gone; I don't even know if I would remember how to use it. The idea came from a suggestion in M G Scroggie's Radio Laboratory Handbook. It did give readings, harmonic by harmonic, from the low-distortion oscillator - it was sensitive enough. Nowadays you'd probably do better with a good computer sound card and software to generate waves and analyse them.

I haven't touched any of this stuff for a very long time, and am completely out of it. For some reason I thought it would be nice to have a low-distortion variable-frequency oscillator when I couldn't find mine, but probably won't find one which is low-distortion at a price I can justify (for probable shelfware). I was toying with the idea of getting a mediocre Wien-bridge one with passive components and switches, and replace the active circuitry, but realistically I'd never get it done.

Given this cheap Latvian 1kHz PCB, I played with the idea that continuously variable frequency is not necessary for many purposes; it might make sense to buy, say, three, and modify two for other frequencies.

But really I'm here under false pretences; I'm not active in the field.
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Old 6th April 2012, 10:07 AM   #156
PChi is offline PChi  United Kingdom
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[QUOTE=pol098;2974530]Thanks. Sorry if my post wasn't clear, it is definitely fixed frequency. I suppose the best that can readily be done is to check that the harmonics are buried in noise with the equipment available; no better is needed.

Thanks for the clarification abut it being fixed frequency.

I too had a circuit published in Electronics and Wireless World. It was for an Oscillator using Analogue Multipliers to generate Sine and Cosine Squared then summing to give a low ripple gain control voltage. I acknowledged the reference I copied the idea from. A few months later someone who no doubt completely innocently thought the idea was new had a letter published describing the concept. So much for an informed Editor.

I agree that a sound card is good for the variable frequency measurements. For really low distortion and noise a few fixed frequencies is probably adequate.
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Old 24th April 2012, 01:32 AM   #157
Oddeo is offline Oddeo  United States
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I recently built a version of the APF oscillator that was linked in the first post of this thread, and have gotten fairly good initial results for this simple circuit.

The level controller was redesigned. I used a couple of BJTs and a couple of JFETS to implement a simple two-stage track and hold. The BJTs form a differential pair that look at two phases of the quadrature signal and generate complementary control signals for the JFETS. The unused stage of the TL084 is used to buffer the first T&H section; and the LED driver stage subsumes the buffer for the final T&H stage.

Without tweaking, the oscillator is delivering the results shown below. These seem consistent with the results that coluke reported in post #7, though he approached the level control in a different way.

Click the image to open in full size.

Cheers,

Curt
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Old 25th April 2012, 08:33 AM   #158
PChi is offline PChi  United Kingdom
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[QUOTE=Oddeo;2998351]I recently built a version of the APF oscillator that was linked in the first post of this thread, and have gotten fairly good initial results for this simple circuit.

Those are impressive results.
I ordered a Creative E-MU 0204 USB Audio Interface but gave up after having to return a couple of wrongly delivered items. I need to find an alternative.
What sound card or external audio interface did you use to take the measurements?
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Old 25th April 2012, 04:19 PM   #159
Oddeo is offline Oddeo  United States
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Quote:
Originally Posted by PChi View Post
What sound card or external audio interface did you use to take the measurements?
The measurement above was taken with an ESI Juli@ card. I also have an e-mu 0404PCI card. The two give comparable results, though their noise floors and spuriae are of course not identical.

I decided not to go the USB route as I was not sure that my lab PC could meet the transfer demands (it is rather old). I preferred to face the challenge of conducted and radiated noise associated with placing an analog interface inside the PC case :-)

Cheers,

Curt
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Old 25th April 2012, 04:35 PM   #160
PChi is offline PChi  United Kingdom
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Quote:
Originally Posted by Oddeo View Post
The measurement above was taken with an ESI Juli@ card. I also have an e-mu 0404PCI card. The two give comparable results, though their noise floors and spuriae are of course not identical.

I decided not to go the USB route as I was not sure that my lab PC could meet the transfer demands (it is rather old). I preferred to face the challenge of conducted and radiated noise associated with placing an analog interface inside the PC case :-)

Cheers,

Curt
Thanks for the answers.
My first thought was to use an external interface connected via USB with possibly fewer noise issues. But after my failed attempt to buy one possibly I should follow your example and use an internal PCI card with no chance of any USB throughput issues.

Regards
Paul
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