Check.
In exploring places/weaknesses of existing low distortion oscillator designs and what the issues found -how to affect them for the better - A lot of very fine adjustments and tuning for gains of a few db at a time results -- Resolution counts a lot, not accuracy.
Thx-RNMarsh
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Well, that would be smart, wouldn't it?
Of course, me not being that way, I didn't think of it.
Hi dirkwright,
Sorry I'm late getting back to you on this, and I think you've gotten mostly the same answer I'd give from Samuel. Virtually all of the DUTs I would measure with the DM, mostly power amplifiers, have a low-pass characteristic that is very close to being minimum phase. In fact, mostly first-order to a good approximation at audio frequencies. For that reason, when I did the DM I wanted to model the "reference path" from the oscillator input source to look the same. This also made it easier for the phase and amplitude settings to remain close to right if I changed the test frequency. Think of the whole thing as a bridge. In any case, I picked the range to include amplifiers with first-order rolloffs in the range of perhaps 100 kHz to >1MHz (I don't remember exactly), so the needed phase would be whatever it needed to be.
However, there is a consideration that I did not take into account, which could influence the discussion and possibly tilt it in favor of an all-pass (but still with whatever disadvantages there are with respect to the all-pass not correctly modeling the minimum phase amplitude characteristic of the DUT). That is the situation where the power amplifier includes an input filter. Many amplifiers have such an input filter as it is good practice against EMI and other things. Moreover, some amplifiers incorporate an input LPF at a lower frequency than the intrinsic closed-loop bandwidth of the amplifier so as to reduce the amplifier's susceptibility to so-called TIM (slewing induced distortion). Here we would now have a DUT with more like a second-order minimum phase LPF characteristic. Indeed, there have been a few power amplifiers with a simple multi-pole R-C input filter to yield an approximation to a linear phase characteristic. So, such instances might end up being better modeled by an all-pass function.
Cheers,
Bob
It's quite possible then that the DM could benefit from having both AP and LP phase networks available. I do have some information on 2nd order all pass networks if needed.
I'll have a look. I seem to recall seeing some passive AP networks as well. In the interest of not adding more op amp distortion and noise.
I'll have a look. I seem to recall seeing some passive AP networks as well. In the interest of not adding more op amp distortion and noise.
Good point. If so, I'd be tempted to use the LPF as coarse adjust and the all-pass as the fine adjust. The all-pass would then only need s few degrees of range.
Cheers,
Bob
Electronic Filter Design Handbook.
Check out CH7 NETWORKS FOR THE TIME DOMAIN of ELECTRONIC FILTER DESIGN HANDBOOK.
I think you can download it from here
Hotfile.com Electronic Filter Design Handbook, Fourth Edition
Check out CH7 NETWORKS FOR THE TIME DOMAIN of ELECTRONIC FILTER DESIGN HANDBOOK.
I think you can download it from here
Hotfile.com Electronic Filter Design Handbook, Fourth Edition
Thank you very much for your reply Bob. I have your book about amplifiers and of course the LA article. There are things about the DM that I do not understand, but I am not about to challenge you on your design decisions. I'm just going to accept the fact that I don't understand and try to figure something out myself (the school of hard knocks). I'm not about to violate your's and LA's copyright either by providing too many details of your design.
I've designed a basic circuit board based on my ideas and ordered it last night. I do not know if it will work until I actually build it and try it. Thanks for the inspiration and for all of your hard work.
Hi dirkwright,
Thanks for your very kind words about my work. I wish you the very best of luck with your project. Its always good to scratch your head about why somebody did something a certain way, and suggest alternatives. Nobody has all the answers, no matter how many books they've written or how much experience they have.
Cheers,
Bob
In the early eighties I did build the Bob Cordell THD Analyzer from the articles in Audio Magazine. It worked very well and I used it for a few years. Now I did recently discover it in my storage room and decided to upgrade it (if possible) to today's standards as far as components are concerned. I have read the threads about this analyzer on this website, but I have yet to see a comprehensive upgrade plan (with proven results such as reduced noise etc.) with a list of components (opamps, resistors, capacitors etc) to replace and without changing the circuitry of the analyzer.
I would like to hear from anyone has successfully undertaken such a complete upgrade with significant improvements.
I would like to hear from anyone has successfully undertaken such a complete upgrade with significant improvements.
It was probably illegal and got shut down.
Dick if you interested I can send you a copy.
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