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Oscillator question

Started by Spehro Pefhany May 6, 2015
Hi, 

As an alternative to a bridge type setup I'm thinking of measuring a
relatively low Q inductance (a lot of series resistance) by using a
tank circuit/oscillator. 

What kind of oscillator will be the least sensitive to the Q
(oscillate closest to  (2*pi* sqrt(LC))^-1?) ? 

And respond reasonably fast to changes in LC.. (seems like that's a
conflicting requirement..) 

I have control over the frequency, probably hundreds of kHz or low MHz
(finding the SRF of the coil is a to-do), only need ~kHz response. 

Tks all, 		


On Wed, 06 May 2015 16:41:32 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>Hi, > >As an alternative to a bridge type setup I'm thinking of measuring a >relatively low Q inductance (a lot of series resistance) by using a >tank circuit/oscillator. > >What kind of oscillator will be the least sensitive to the Q >(oscillate closest to (2*pi* sqrt(LC))^-1?) ? > >And respond reasonably fast to changes in LC.. (seems like that's a >conflicting requirement..) > >I have control over the frequency, probably hundreds of kHz or low MHz >(finding the SRF of the coil is a to-do), only need ~kHz response. > >Tks all, >
A series RLC is purely ohmic at the LC resonant frequency. Maybe you can do something with that. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On 5/6/2015 4:47 PM, John Larkin wrote:
> On Wed, 06 May 2015 16:41:32 -0400, Spehro Pefhany > <speffSNIP@interlogDOTyou.knowwhat> wrote: > >> Hi, >> >> As an alternative to a bridge type setup I'm thinking of measuring a >> relatively low Q inductance (a lot of series resistance) by using a >> tank circuit/oscillator. >> >> What kind of oscillator will be the least sensitive to the Q >> (oscillate closest to (2*pi* sqrt(LC))^-1?) ? >> >> And respond reasonably fast to changes in LC.. (seems like that's a >> conflicting requirement..) >> >> I have control over the frequency, probably hundreds of kHz or low MHz >> (finding the SRF of the coil is a to-do), only need ~kHz response. >> >> Tks all, >> > > A series RLC is purely ohmic at the LC resonant frequency. Maybe you > can do something with that.
My Measurements model 59 'Megacycle Meter' is the best thing I have for that. Otherwise, an AC bridge would get my vote. The active device in a self-limiting oscillator presents wildly varying impedances to the tank circuit. ALC helps a lot, but is more work. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On 2015-05-06 1:41 PM, Spehro Pefhany wrote:
> Hi, > > As an alternative to a bridge type setup I'm thinking of measuring a > relatively low Q inductance (a lot of series resistance) by using a > tank circuit/oscillator. > > What kind of oscillator will be the least sensitive to the Q > (oscillate closest to (2*pi* sqrt(LC))^-1?) ? > > And respond reasonably fast to changes in LC.. (seems like that's a > conflicting requirement..) > > I have control over the frequency, probably hundreds of kHz or low MHz > (finding the SRF of the coil is a to-do), only need ~kHz response. >
A regular logic inverter with the inductor from output to input an a cap from input to ground pretty much always oscillates. Of course, a low Q would result in lousy phase noise but that's probably something you don't care about in this case. If there is a chance that some of the tested coils are high Q you might want to add a resistor in front of the input, to avoid potential bzzzzt situations. Also, provide 1M or so to VCC to tie the input when no coil is inserted. Otherwise it might spew EMI. -- Regards, Joerg http://www.analogconsultants.com/
On 2015-05-06 4:52 PM, Phil Hobbs wrote:
> On 5/6/2015 4:47 PM, John Larkin wrote: >> On Wed, 06 May 2015 16:41:32 -0400, Spehro Pefhany >> <speffSNIP@interlogDOTyou.knowwhat> wrote: >> >>> Hi, >>> >>> As an alternative to a bridge type setup I'm thinking of measuring a >>> relatively low Q inductance (a lot of series resistance) by using a >>> tank circuit/oscillator. >>> >>> What kind of oscillator will be the least sensitive to the Q >>> (oscillate closest to (2*pi* sqrt(LC))^-1?) ? >>> >>> And respond reasonably fast to changes in LC.. (seems like that's a >>> conflicting requirement..) >>> >>> I have control over the frequency, probably hundreds of kHz or low MHz >>> (finding the SRF of the coil is a to-do), only need ~kHz response. >>> >>> Tks all, >>> >> >> A series RLC is purely ohmic at the LC resonant frequency. Maybe you >> can do something with that. > > My Measurements model 59 'Megacycle Meter' is the best thing I have for > that.
Yay! Got one as well. Problem is, they don't make'em no mo'h.
> ... Otherwise, an AC bridge would get my vote. >
I have to confess that I have become decadent and sometimes use a network analyzer or for stuff above 200MHz a Signalhound analyzer/gen combo. I know, I know, it always feels like to fire up the big truck to go buy a couple of rolls.
> The active device in a self-limiting oscillator presents wildly varying > impedances to the tank circuit. ALC helps a lot, but is more work. >
That's where the 74HCU04 or CD4007 comes in handy. -- Regards, Joerg http://www.analogconsultants.com/
On 5/6/2015 8:00 PM, Joerg wrote:
> On 2015-05-06 4:52 PM, Phil Hobbs wrote: >> On 5/6/2015 4:47 PM, John Larkin wrote: >>> On Wed, 06 May 2015 16:41:32 -0400, Spehro Pefhany >>> <speffSNIP@interlogDOTyou.knowwhat> wrote: >>> >>>> Hi, >>>> >>>> As an alternative to a bridge type setup I'm thinking of measuring a >>>> relatively low Q inductance (a lot of series resistance) by using a >>>> tank circuit/oscillator. >>>> >>>> What kind of oscillator will be the least sensitive to the Q >>>> (oscillate closest to (2*pi* sqrt(LC))^-1?) ? >>>> >>>> And respond reasonably fast to changes in LC.. (seems like that's a >>>> conflicting requirement..) >>>> >>>> I have control over the frequency, probably hundreds of kHz or low MHz >>>> (finding the SRF of the coil is a to-do), only need ~kHz response. >>>> >>>> Tks all, >>>> >>> >>> A series RLC is purely ohmic at the LC resonant frequency. Maybe you >>> can do something with that. >> >> My Measurements model 59 'Megacycle Meter' is the best thing I have for >> that. > > > Yay! Got one as well. Problem is, they don't make'em no mo'h.
I know--I heard about them from you some years back. I now have a couple of my own, courtesy of eBay. (Well, one back end and two heads.)
> > >> ... Otherwise, an AC bridge would get my vote. >> > > I have to confess that I have become decadent and sometimes use a > network analyzer or for stuff above 200MHz a Signalhound analyzer/gen > combo. I know, I know, it always feels like to fire up the big truck to > go buy a couple of rolls.
Well, if you have a network analyzer, it's a natural tendency. What with all the optical stuff, my lab is failing the cat test fairly spectacularly at the moment, or I'd have one myself. I've been meaning to update the lab photos on my web site, but it hasn't been neat enough for some years now. ;) Cheers Phil Hobbs
>> The active device in a self-limiting oscillator presents wildly varying >> impedances to the tank circuit. ALC helps a lot, but is more work. >> > > That's where the 74HCU04 or CD4007 comes in handy. >
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Wed, 06 May 2015 19:52:05 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 5/6/2015 4:47 PM, John Larkin wrote: >> On Wed, 06 May 2015 16:41:32 -0400, Spehro Pefhany >> <speffSNIP@interlogDOTyou.knowwhat> wrote: >> >>> Hi, >>> >>> As an alternative to a bridge type setup I'm thinking of measuring a >>> relatively low Q inductance (a lot of series resistance) by using a >>> tank circuit/oscillator. >>> >>> What kind of oscillator will be the least sensitive to the Q >>> (oscillate closest to (2*pi* sqrt(LC))^-1?) ? >>> >>> And respond reasonably fast to changes in LC.. (seems like that's a >>> conflicting requirement..) >>> >>> I have control over the frequency, probably hundreds of kHz or low MHz >>> (finding the SRF of the coil is a to-do), only need ~kHz response. >>> >>> Tks all, >>> >> >> A series RLC is purely ohmic at the LC resonant frequency. Maybe you >> can do something with that. > >My Measurements model 59 'Megacycle Meter' is the best thing I have for >that. Otherwise, an AC bridge would get my vote. > >The active device in a self-limiting oscillator presents wildly varying >impedances to the tank circuit. ALC helps a lot, but is more work. > >Cheers > >Phil Hobbs
It's not an official "oscillator" but a small ARM could (depending on the resonant freq) DDS a sine wave, and a dumb comparator or phase detector could measure the current. When the current is in phase with the voltage, you're at the LC resonance. Could be cute and cheap. We're about to use an LPC1758 to software-DDS generate a sine wave at up to, maybe, 5 KHz, out its 10-bit DAC. Numbers like 50 KHz shouldn't be too hard. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Wednesday, May 6, 2015 at 4:37:42 PM UTC-4, Spehro Pefhany wrote:
> Hi, > > As an alternative to a bridge type setup I'm thinking of measuring a > relatively low Q inductance (a lot of series resistance) by using a > tank circuit/oscillator. > > What kind of oscillator will be the least sensitive to the Q > (oscillate closest to (2*pi* sqrt(LC))^-1?) ? > > And respond reasonably fast to changes in LC.. (seems like that's a > conflicting requirement..) > > I have control over the frequency, probably hundreds of kHz or low MHz > (finding the SRF of the coil is a to-do), only need ~kHz response. > > Tks all,
At low Q there is some resonance frequency Q relation that falls on a circle. Brian Pippards "Physics of Vibration Vol 1." ? It's either here or at work. George H.
George Herold <gherold@teachspin.com> wrote:

> On Wednesday, May 6, 2015 at 4:37:42 PM UTC-4, Spehro Pefhany wrote: >> Hi, >> >> As an alternative to a bridge type setup I'm thinking of measuring a >> relatively low Q inductance (a lot of series resistance) by using a >> tank circuit/oscillator. >> >> What kind of oscillator will be the least sensitive to the Q >> (oscillate closest to (2*pi* sqrt(LC))^-1?) ? >> >> And respond reasonably fast to changes in LC.. (seems like that's a >> conflicting requirement..) >> >> I have control over the frequency, probably hundreds of kHz or low MHz >> (finding the SRF of the coil is a to-do), only need ~kHz response. >> >> Tks all, > > At low Q there is some resonance frequency Q relation > that falls on a circle. Brian Pippards "Physics of Vibration Vol 1." ? > It's either here or at work. > > George H.
See Equation (3), Page 2, Radiotron Designer's Handbook, Chapter 9 http://headfonz.rutgers.edu/RDH4/CHAPTR09.PDF See the comment: "This relationship shows that in practice there is little difference between these two frequencies. Q must, for example, be less than four to make f,, differ by 1% from f,. (equations omitted.) Q normally exceeds fifty, for which value the two frequencies differ by about one part in twenty thousand." Main url: http://headfonz.rutgers.edu/RDH4/
On Wed, 06 May 2015 19:52:05 -0400, the renowned Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

> > >My Measurements model 59 'Megacycle Meter' is the best thing I have for >that. Otherwise, an AC bridge would get my vote.
I'm using an AC bridge with an SRS DSP lock-in but want to do better in terms of resolution on small changes in inductance. Something like this with R 50x higher and a 1kHz update rate would be nice: http://img.directindustry.fr/images_di/photo-g/appareil-mesure-rlc-haute-vitesse-8981-5274409.jpg
>The active device in a self-limiting oscillator presents wildly varying >impedances to the tank circuit. ALC helps a lot, but is more work. > >Cheers
I guess the servoing the phase so it is resistive is what I want if I'm using an oscillator.