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Old 7th September 2012, 03:19 PM   #331
1audio is offline 1audio  United States
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An active filter is limited by its amplifier and in most implementations by the common mode non-linearity of the input of the opamp. A passive low pass filter is really the only way to improves on one of the really low distortion sources. And at these levels non-linearity of passive components may be the limiting factor.

The TI board has two crystal oscillators with about the same foot print as the Crystek oscillators. They are on the attached schematic. You do need to improve the power supply to get the best results. I have attached an ultra low noise supply I created for this task that works quite well. The sim should open in LTspice with no fiddling.

The shunt regulators for Victors oscillators are set to 15V ea. Once the batteries drop below 15V the system will shift. Most likely the output voltage will drop and eventually the batteries will be drained. Adding batteries will only sell batteries, no change in performance. Adding a cascode may not help and may require more voltage to get to the same place.
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Old 7th September 2012, 04:29 PM   #332
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Thanks for the replies! Much appreciated.
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Old 8th September 2012, 11:25 AM   #333
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Can an active peak filter be used (tuned to a desired frequency) to reduce THD? I realize the active filter will introduce some distortion, but if the Q is high enough won't the net result be lower distortion? Is this idea already built into these low distortion oscillators? (I guess "peak filter" is an old fashioned term. I guess what I mean is a narrow bandpass filter.)
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In general, the distortion of the filter swamps the distortion of the very-low distortion oscillator.
Oscillators are basically 2nd order filters (tuned to infinite Q). So the fundamental distortion limit from the amplifiers and passives of an additional filter stage is the same as for the basic oscillator alone. However, an additional low-pass filter (a bandpass seems unnecessary) can reduce the distortion contribution from the multiplier and from level detector ripple.

SRS DS360 (DS360 - Low Distortion Function Generator) uses such a filter, although the source in this case is a DAC, and overall distortion performance is not state of the art.

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It is possible to use LC passive filters to do what you suggest, but that's really only practical for fixed frequency oscillators.
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A passive low pass filter is really the only way to improve on one of the really low distortion sources.
I doubt that, at audio frequencies, a passive LCR filter would provide lower distortion than a decent active filter, as inductors are notoriously nonlinear. Using composite opamps, or high-performance discrete implementations, the distortion performance of the filter is easily pushed to a level where the passives and layout effects dominate. Not saying this is something for the casual designer, though.

Passive filters are used in the MHz region, but this is another matter...

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An active filter is limited by its amplifier and in most implementations by the common mode non-linearity of the input of the opamp.
Of course one'd use a suitable filter topology, e.g. multiple feedback.

Samuel
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Old 8th September 2012, 03:59 PM   #334
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Normally I would agree that inductors would limit what can be done. However the lowest distortion system I have, the Radiometer CLT-1, uses sophisticated LC filters with large Ferrite cores to make a -170 dB source. I have no idea how to duplicate those inductors and would not suggest that its an easy task. At -130 to -140 dB harmonics from the oscillators as they are I don't see a reason to try to improve on them. These are numbers that even the Halcro amps couldn't achieve.
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Old 9th September 2012, 06:44 AM   #335
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Yes, I'm aware of the CLT1 (current version CLT10). The spec sheet says -150 dB guaranteed, -160 dB typical--where does your -170 dB come from? The secrets of it is probably revealed in their patents; I have yet to give them a detailed read.

Doesn't however convince me that an active filter wouldn't be the better (or at least easier) approach for a general-purpose bench oscillator.

Samuel
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Old 9th September 2012, 06:55 AM   #336
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can an HP339A distortion analyzer be tweaked for lower thd measurment? Oh, say by anouther 20dB? And, if so - how? Thx - RNM
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Old 9th September 2012, 07:20 AM   #337
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Originally Posted by Samuel Groner View Post
Yes, I'm aware of the CLT1 (current version CLT10). The spec sheet says -150 dB guaranteed, -160 dB typical--where does your -170 dB come from? The secrets of it is probably revealed in their patents; I have yet to give them a detailed read.Samuel
Just reading the meters on the front. Only certain combinations of settings get there. And not necessarily under load. But it will deliver more than 1 watt into the DUT.
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Old 9th September 2012, 08:51 AM   #338
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Originally Posted by RNMarsh View Post
can an HP339A distortion analyzer be tweaked for lower thd measurment? Oh, say by anouther 20dB? And, if so - how? Thx - RNM
If I remember well the venerable 339A employs a Bridged-T active notch with auto tuning/nulling circuitry based on LDR/LED optocouplers, which are likely to be the responsible for the analyser residuals - imho if you're going to measure THD in the -120 dBc range (or less) it's better you build your own tool.

My favourite one is a manual-tuned double notch based on Bainter topology - a sample run of a 239A clone thru the notch is attached: @ +40 dB post notch gain noise floor is quite low, and you can easily (and stably) get more than 100 dB fundamental suppression - I'll post the schematics when I'll find it...

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Old 9th September 2012, 08:58 AM   #339
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Originally Posted by Samuel Groner View Post
Yes, I'm aware of the CLT1 (current version CLT10). The spec sheet says -150 dB guaranteed, -160 dB typical--where does your -170 dB come from? The secrets of it is probably revealed in their patents; I have yet to give them a detailed read.
Samuel, just for the sake of curiosity: on what concepts this beast is based on?

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Old 9th September 2012, 12:52 PM   #340
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On what concepts this beast is based on?
I haven't studied the details enough to give a competent answer. Right now I can't find the patents with all the details, I think I have them at work. As far as I can tell from the information available at the manufacturers website (Danbridge), they use a tuned LC network which has very high output Z at 30 kHz (the 3rd harmonic of the used 10 kHz fundamental). This inherently prohibits any 3rd harmonic to flow into the load.

Note that this is not a general-purpose distortion analyzer; it can just analyze two-ports (in particular passives), and it's just the 3rd harmonic residual which is extremely low (and not e.g. the 2nd). One could imagine adapting this for a normal low distortion oscillator, and making the tuned network switchable between 2nd and 3rd harmonic reduction. But this sounds like a tremendously complicated undertaking...

Samuel

Last edited by Samuel Groner; 9th September 2012 at 12:55 PM.
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