Testing for self-resonance: ringing, transients and accuracy - UK Vintage Radio Repair and Restoration Discussion Forum

Al (astral highway) · 15th Jan 2008, 11:32 am

I recently acquired a lab-grade pulse generator S/H (Textronix Type 114) and want to use it to measure the self-resonant frequency of a series resonant circuit. It's basic stuff, only twice line frequency, but I have a couple of serious concerns. Bear with me if what I'm asking seems obvious or I'm not the exactly the right terminology.

Please not that I don't need an explanation of the thoery of series resonance: it's the mechanics and accuracy of the test and protecting the equipment that most concerns me.

I'm really working in the dark here.

1) Does lab-grade equipment automatically have protection against fast transients or do I need to put a small ultra-fast diode in parallel with the output to short reverse current to earth? I mean this at any pulse voltage range.

2) Although I am really concentrating on twice line frequency, I want to make sure I don't distort the waveform from my source. What is the best way to couple it to the two components under test?

3) Are there any oscilloscope considerations I should bear in mind for easiest analysis and most accurate interpretation of what's happening? I have an old V-509 'scope that I'm gradually getting to know.

Many thanks in advance. Sorry if this is kidergarten stuff.

Skywave · 15th Jan 2008, 9:39 pm

Hi Astral -

Well, it's not "kindergarten stuff", for a start

I understand (more-or-less) what you're asking and what you intend to do, although using a pulse gen. to do this is, well, unconventional to say the least! As such, you may not get many really "on the spot" replies here.

However, let me throw in a few things that may help you.

First, the pulse gen. you have is a quality item (Tektronix). If it's not stating the blindingly obvious, do you have the manual? This should answer your concerns about spike protection (Tek. are usually very thorough with their tech. documentation). Generally speaking, quality analogue T & M kit usually has some form of protection provided - e.g. sig. gens. often have reverse power prot. built in; OTOH, spectrum analysers usually have a warning notice on the front panel stating "Input level above 20 dBm will damage this Item!" - or something like that. Mind you, having said that , there is a particular sig. gen. made by Marconi I know of that when the attenuator setting is changed, puts out a substantial spike - which does MOSFET VHF RF amps. no good whatsoever!

Question: when you say you are "concentrating on twice line frequency", does this mean that the p.r.f. of the signal from the pulse gen. is at 100 Hz?

As far as coupling the gen. to the cct. under test, etc., all I can say is that with a sine-wave gen., my technique is to use a high-ish value resistor in series with the gen. O/P - that way the low Z at resonance doesn't "short" the generator - and a similar R between the cct. and the 'scope probe. The overall idea being to try to 'isolate' the cct. under test from the measuring gear. Obviously, there will be a lot of attenuation in this arrangement - so the O/P from the gen. needs to be fairly high.

I can't give specific values of components, since so much depends on the relative and discrete values of the L and the C in the cct. under test and the freq. of measurement. A certain amount of "cut and try" is required.

HTH.

Al / Skywave.

Al (astral highway) · 15th Jan 2008, 11:19 pm

Hi Al,

Thanks for your tips.

Quote:

when you say you are "concentrating on twice line frequency", does this mean...?

Erm, no. Sorry that wasn't clear. I just meant twice mains frequency!

Skywave · 16th Jan 2008, 5:22 pm

Which is what I suspected: 100Hz.

Al / Skywave

Skywave · 16th Jan 2008, 8:32 pm

Astral -

I found this on the 'Net:

http://cappels.org/dproj/minmassrfloop/loopantring.html

It isn't exactly what you want to do, but it's pretty close & should give you a lead. Ignore the first few bits - scroll on down to the part titled "The Q of a Tiny Loop".

Al / Skywave.

15th Jan 2008, 11:32 am	#1
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	Testing for self-resonance: ringing, transients and accuracy I recently acquired a lab-grade pulse generator S/H (Textronix Type 114) and want to use it to measure the self-resonant frequency of a series resonant circuit. It's basic stuff, only twice line frequency, but I have a couple of serious concerns. Bear with me if what I'm asking seems obvious or I'm not the exactly the right terminology. Please not that I don't need an explanation of the thoery of series resonance: it's the mechanics and accuracy of the test and protecting the equipment that most concerns me. I'm really working in the dark here. 1) Does lab-grade equipment automatically have protection against fast transients or do I need to put a small ultra-fast diode in parallel with the output to short reverse current to earth? I mean this at any pulse voltage range. 2) Although I am really concentrating on twice line frequency, I want to make sure I don't distort the waveform from my source. What is the best way to couple it to the two components under test? 3) Are there any oscilloscope considerations I should bear in mind for easiest analysis and most accurate interpretation of what's happening? I have an old V-509 'scope that I'm gradually getting to know. Many thanks in advance. Sorry if this is kidergarten stuff. __________________ Al

15th Jan 2008, 9:39 pm	#2
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Testing for self-resonance: ringing, transients and accuracy Hi Astral - Well, it's not "kindergarten stuff", for a start I understand (more-or-less) what you're asking and what you intend to do, although using a pulse gen. to do this is, well, unconventional to say the least! As such, you may not get many really "on the spot" replies here. However, let me throw in a few things that may help you. First, the pulse gen. you have is a quality item (Tektronix). If it's not stating the blindingly obvious, do you have the manual? This should answer your concerns about spike protection (Tek. are usually very thorough with their tech. documentation). Generally speaking, quality analogue T & M kit usually has some form of protection provided - e.g. sig. gens. often have reverse power prot. built in; OTOH, spectrum analysers usually have a warning notice on the front panel stating "Input level above 20 dBm will damage this Item!" - or something like that. Mind you, having said that , there is a particular sig. gen. made by Marconi I know of that when the attenuator setting is changed, puts out a substantial spike - which does MOSFET VHF RF amps. no good whatsoever! Question: when you say you are "concentrating on twice line frequency", does this mean that the p.r.f. of the signal from the pulse gen. is at 100 Hz? As far as coupling the gen. to the cct. under test, etc., all I can say is that with a sine-wave gen., my technique is to use a high-ish value resistor in series with the gen. O/P - that way the low Z at resonance doesn't "short" the generator - and a similar R between the cct. and the 'scope probe. The overall idea being to try to 'isolate' the cct. under test from the measuring gear. Obviously, there will be a lot of attenuation in this arrangement - so the O/P from the gen. needs to be fairly high. I can't give specific values of components, since so much depends on the relative and discrete values of the L and the C in the cct. under test and the freq. of measurement. A certain amount of "cut and try" is required. HTH. Al / Skywave.

16th Jan 2008, 5:22 pm	#4
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Testing for self-resonance: ringing, transients and accuracy Which is what I suspected: 100Hz. Al / Skywave

16th Jan 2008, 8:32 pm	#5
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Testing for self-resonance: ringing, transients and accuracy Astral - I found this on the 'Net: http://cappels.org/dproj/minmassrfloop/loopantring.html It isn't exactly what you want to do, but it's pretty close & should give you a lead. Ignore the first few bits - scroll on down to the part titled "The Q of a Tiny Loop". Al / Skywave.