Low-distortion Audio-range Oscillator - Page 70 - diyAudio
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Old 12th October 2012, 11:10 PM   #691
1audio is offline 1audio  United States
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Sorry, was at work. Its a B&K 1607 frequency and distortion measuring bridge. Manual dates from 1962.
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Old 12th October 2012, 11:21 PM   #692
RNMarsh is offline RNMarsh  United States
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The B&K 1607 is a fine unit and shouldnt be hard to copy/duplicate... uses 1% cap values.... paper.... new PP type will have higher Q and maybe thus greater notch depth. The only issue with old models that dont get regular use is the shafts of main control will be oxidized and hard to turn... needs cleaning and lubrication or the vernier control wont have enough torque to turn the main dial/shaft.
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Old 12th October 2012, 11:23 PM   #693
1audio is offline 1audio  United States
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Richard:
Do you have a schematic? (Not wanting to open something untouched for a generation. . .)
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Old 12th October 2012, 11:48 PM   #694
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
Sorry, was at work. Its a B&K 1607 frequency and distortion measuring bridge. Manual dates from 1962.
Still can't find anything from Google. Not a wonder you're having trouble finding a service manual.

The only thing that came up was a reference to an old post on this forum by you.
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Old 13th October 2012, 03:39 PM   #695
KSTR is offline KSTR  Germany
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That's pretty close in a sense to what is happening. On the KH4400 it locks with about 5-10 mV. I think a filter won't get to as low a distortion as an injection locked oscillator. And you have the digital system coming along for the ride with its junk to deal with. I am transformer isolation on the KH4400 to reduce this issue.

This is only useful if you can access the clock of the digital system to lock to.
Demian,
Does this work by having the ACG skewed so as to just not start from noise/spuriae alone even when the multiplier/FET is full on, then inject a small sinewave at the tuning freq? If so, I might try this experimentally on Victor's Osc that I just ordered. Would be perfect to use with the AP's or a soundcard's D/A sine on a bin center for heavy time domain averaging without needing to trigger on the sent or incoming signal which causes drop of HF from jitter. Is the injection method effectively jitter-free (assumed no RFI-spikes and hum upsetting the injection)?
- Klaus
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Old 13th October 2012, 06:44 PM   #696
richiem is offline richiem  United States
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@KSTR -- or increasing the negative feedback to where oscillation just quits -- then the small injected signal gets strongly amplified and purified.
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Old 13th October 2012, 09:35 PM   #697
davada is offline davada  Canada
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One form of level control I played a bit with is offsetting the frequency, of an injected sine stimulus, from the resonance frequency of an SV filter. With a a high Q filter the gain drops rapidly as the stimulus frequency is changed about Fr. It's a poor approach to ALC because
the shift in stimulus frequency produces strong side bands on either side of the fundamental.
In turn the side bands produce harmonics extending several octaves above the Fr. The intensity of the side bands is proportional to the delta f of the stimulus. FM to AM.

Any form of automatic frequency control will have this effect to some extent.
Jitter and phase noise will have the same effect. All this is in addition to distortion contributions of the stimulus.

I think an automatic frequency control would fight a bit with the ALC.
I think these same effects would be present in auto tuned notch filter as well and
may contribute to a limit of realizable notch depth.

What about arranging the oscillator as a phase locked loop?
The oscillator could be fine tuned by a reference frequency and the reference frequency doesn't have to sinusoidal.
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Last edited by davada; 13th October 2012 at 09:59 PM.
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Old 13th October 2012, 10:11 PM   #698
1audio is offline 1audio  United States
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First, injection locking lends itself better to a SVO since you have two low pass filters to exploit to reduce any contributions from the locking signal. There is a lot on serious analysis on injection locking and associated jitter/phase noise in the lit. Usually the close in phase noise is from the external source (XO in my case) and the noise further from the carrier comes from the locked oscillator. So a good low distortion oscillator locked by a good crystal based oscillator should be even better.

Second, even with a PLL you need a frequency control element that is voltage controlled. The Boonton does this with an external CPU controlled PLL and tuning via voltage on an analog multiplier in the cosine section I think. It works quite well actually with the output frequency very precisely locked.

Locking Victor's oscillator can be done with a small signal into the bottom of the AGC (for example). The larger the pull needed the larger a signal is needed with the distortion going up as well. Pulling under 1% should be pretty benign however.
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Old 13th October 2012, 10:22 PM   #699
davada is offline davada  Canada
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"What about arranging the oscillator as a phase locked loop?
The oscillator could be fine tuned by a reference frequency and the reference frequency doesn't have to be sinusoidal. "

This could be done easily by modifying the inverting summing amplifier of an SVO by injecting a small amount of signal into the non inverting input. A dc controlled element can be connected to the inverting node and ground (a second multiplier). The dc off the output of a phase comparator controls the element. The draw back is any ripple on the control signal will produce FM and of course there will be elevated distortion from inserting all this.
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Old 13th October 2012, 10:28 PM   #700
1audio is offline 1audio  United States
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Default B&K 1607 Internal view

Here are some pictures of what is inside the B&K. Tracing this won't be so easy especially with the really high grade construction.

The box is the housing for the tuning pot. The panels are plastic, probably to minimize external capacitance. All of the tuning caps are polystyrene, the input cap is paper (8 uF).

I'll do a little probing to see if I can figure out the harmonic compensation trick.
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