[SI-LIST] Re: Using Current Probes to Measure Cable Resonance
- From: Doug Smith <doug@xxxxxxxxxx>
- To: olaney@xxxxxxxx
- Date: Fri, 11 Jan 2008 22:16:02 -0800
Hi Orin,
Thanks for the reply. I have a B&K grid dip meter that I still
occasionally use. Your discussion brought back many memories.
I have been doing some research on measuring the resonant frequency of
physical structures lately and will publish a subset of the results over
the next few months on my website. The first article from this work is
the January 2008 Technical Tidbit on my site.
Doug
olaney@xxxxxxxx wrote:
>An echo box is a different kind of gadget -- a passive, resonant cavity
>tuned to the center frequency of a radar pulse, also called a phantom
>target. What you described is a grid dip oscillator, or GDO, called that
>because it was based on a small vacuum tube back when these were first
>used. The meter monitors current through the control grid, a milliamp or
>thereabouts when the circuit is oscillating vigourously. If the GDO is
>tuned to the freq. of a nearby resonant structure, the resonator will
>soak energy from the GDO, causing it to resonate less vigorously, hence
>the dip in grid current. Eico made one, and so did Heathkit, among
>others. There was a follow on version based on a tunnel diode ("tunnel
>dipper"). These days it's just called a dip oscillator, and probably
>uses a JFET. I don't know if these are still offered commercially,
>though I'm sure there are ham radio DIY articles for it. The "external
>antenna" is not an antenna per se, but the externally mounted coil for
>the LC resonant circuit of the dipper. Energy is coupled magnetically to
>whatever you are checking. Any EM radiation in the antenna sense is
>coincidental. I have an old Eico 710 in my personal museum; it only goes
>to 250 MHz. These days I'd probably try a small magnetic probe on a
>network analyzer and monitor S11. The probe has to be lightly coupled to
>get a good, sharp resonance. A small loop works best. If you try the
>classic split toroid it will be overcoupled, and you get messy results.
>Doug's technique is a good approach for multi-conductor cables and for
>low Q resonances. The two techniques should be considered complementary
>-- they have similarities, but Doug is measuring coupling (S21) rather
>than resonance as such. It's a better technique for the intended
>purpose, IMHO.
>
>Orin
>
>
>
--
-------------------------------------------------------
___ _ Doug Smith
\ / ) P.O. Box 1457
========= Los Gatos, CA 95031-1457
_ / \ / \ _ TEL/FAX: 408-356-4186/358-3799
/ /\ \ ] / /\ \ Mobile: 408-858-4528
| q-----( ) | o | Email: doug@xxxxxxxxxx
\ _ / ] \ _ / Website: http://www.dsmith.org
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