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Old 11th February 2013, 12:51 AM   #91
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
I havent found the limit to be the jFET yet. I started with reaching the limit with an A-Prec model One; then I am now at the low end of the the ShibaSoku. Never changed the jFET... did try but went back to original now. I can get the 339a oscillator to -125-ish. Not sure due to noise there.... need the ADC to look at it now and seperate the noise from the harmonics.

I think the Passive filter I have already (B&K 1607) will be the best approach, also. The box is plenty big to house active circuit AND it is continuously variable over a wide range. Anyone else have such a pasive filter or going to get one asap? :-)

Thx- Richard
I don't know. If I type Twin T into the Ebay search all I get is car parts coming up.
B&K 1607 is no help either.
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Old 11th February 2013, 02:29 AM   #92
RNMarsh is offline RNMarsh  United States
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Try Twin-T notch filter?
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Old 11th February 2013, 03:34 AM   #93
davada is offline davada  Canada
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Try Twin-T notch filter?
I did and every variant of that.
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Old 11th February 2013, 03:55 AM   #94
richiem is offline richiem  United States
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@Richard -- Victor has used an old but good technique to verify levels of harmonics, which is to use a second generator to make a marker with frequency very close to the harmonic of interest, attenuated by a known amount relative to the fundamental under test, say -120dB, and mixed with the fundamental.

Then despite any gain error -- filter, ADC, software, etc -- the level of the harmonic relative to the marker signal will give accurate results, so long as they are close in frequency (if being in the center of an FFT bin is critical, this should be checked/adjusted). This should eliminate a lot of uncertainty about levels. Any old generator will do, even a function generator, since it's harmonics will have no affect on the fundamental being examined.

Just throwing this out because I know you care about the real levels being seen in plots.
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Old 11th February 2013, 04:47 AM   #95
RNMarsh is offline RNMarsh  United States
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@Richard -- Victor has used an old but good technique to verify levels of harmonics, which is to use a second generator to make a marker with frequency very close to the harmonic of interest, attenuated by a known amount relative to the fundamental under test, say -120dB, and mixed with the fundamental.

Then despite any gain error -- filter, ADC, software, etc -- the level of the harmonic relative to the marker signal will give accurate results, so long as they are close in frequency (if being in the center of an FFT bin is critical, this should be checked/adjusted). This should eliminate a lot of uncertainty about levels. Any old generator will do, even a function generator, since it's harmonics will have no affect on the fundamental being examined.

Just throwing this out because I know you care about the real levels being seen in plots.
Thank you for the reminder. I saw his scheme and have a mental note about it. Very nice idea as a cal for reality check. Just keep pushing down the distortion source and measurement levels.... I think 10 more DB's and I'll be happy with a close to -140 Real number that can be duplicated and used easily (well sort of easily). But thats coming harder and harder and a little slower now. But looks attainable. Thank you -
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Old 12th February 2013, 12:20 AM   #96
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OT so forgive me. I've been following the various oscillator threads passively (except when I fixed my 339A) but seeing the "Morrey" name rang a bell - I wondered if it could be my elementary-high school friend Walt and looked up the ref'd article on his Heathkit mods and indeed it is him - a fun side-effect of following the thread. I last saw him at our 45th HS reunion ... probably will again in 2014, have corresponded infrequently.

OK ... back to your regular discussion.

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@jez -- I was attracted to the Morrey mods when they first appeared, then the problem reports started to show up. I was a bit put off by the complexity, though, and so never built one. The mod of the IG-18 to the HP 239 circuit is on hold for the moment, waiting for further small mods to the discrete circuit in IG-18 #1.

Try using a notch filter between your Morrey and the PC spectrum analyzer -- even a passive one can give you a lot of info if you adjust the displayed levels of the harmonics for the attenuation factors of the filter. A passive Twin-T will attenuate the 2nd H by roughly 9dB and the 3rd by roughly 5dB.
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Old 12th February 2013, 03:01 AM   #97
richiem is offline richiem  United States
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Nice to find old friends by almost accident....

Just to add a note, the 239 mods to the IG-18 turned into a 339 mod and works quite well. Info on my webpage at www.moorepage.net
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Old 13th February 2013, 02:19 PM   #98
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Yes, truly it is fun to make these connections. BTW for any with an interest there is a clean, calibrated 339A about to close on eBay at $300 with no bids. I have no connection with it ... have just been watching. John
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Old 13th February 2013, 04:51 PM   #99
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Yes, truly it is fun to make these connections. BTW for any with an interest there is a clean, calibrated 339A about to close on eBay at $300 with no bids. I have no connection with it ... have just been watching. John
Thanks, this may be deal I have been waiting for!
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Old 13th February 2013, 07:13 PM   #100
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@Dennis - HTH
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