[SI-LIST] Re: Using Current Probes to Measure Cable Resonance
- From: olaney@xxxxxxxx
- To: ron@xxxxxxxxxxx
- Date: Fri, 11 Jan 2008 09:19:11 -0800
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
On Fri, 11 Jan 2008 06:49:27 -0800 "ron@xxxxxxxxxxx" <ron@xxxxxxxxxxx>
writes:
> Hi Doug
> A simple way is to use an echo-box:- an oscillator with a loop
> antenna
> tied into the tuning circuit with a
> meter measuring the current in the oscillator with a fixed voltage
> source. As the circuit is tuned, the
> resonances peak. The loop antenna is connected by a dual banana
> plug so
> different bands loops can
> be inserted.
>
> Coupling is by proximity of the loop so anything that resonates will
>
> show up on the current meter and no
> physical connection is necessary.
>
> I havent seen them in many years but they should still be around.
>
> Ron Miller
>
> Doug Smith wrote:
>
> >
> >Happy New Year everyone!
> >
> >I been running lots of experiments and taking data over the
> holidays. The
> >first result of all this activity is my Technical Tidbit for
> January, 2008.
> >The link to the article is the picture of a spectrum analyzer and
> two
> >currentprobes at the bottom of the page at http://emcesd.com[1] .
> >
> >This month's Technical Tidbit discusses a method of using a
> spectrum
> >analyzerwith a tracking generator and a pair of current probes to
> determine
> >the resonant frequencies of wires and cables. This method can be
> used to
> >findEMC and other noise problems in equipment.
> >Abstract: A technique for using current probes to measure cable
> resonances
> >isdescribed. A swept frequency voltage is induced on a cable by one
> current
> >probe and the resulting current measured using a second one. This
> technique
> >can be especially useful in troubleshooting EMC emissions problems
> as well
> >asother noise related problems in equipment designs.
> >
> >This quick and easy method can be applied to a new design for which
> circuit
> >boards do not even exist yet to avoid EMC and noise problems. The
> method has
> >many potential uses inside and outside of system enclosures.
> >
> >Doug
> >
> >-- ------------------------------------------------------- ___ _
> 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[2] \ _ / ] \ _ / Website:
> http://www.dsmith.org[3]
> >-------------------------------------------------------
> >
> >--- Links ---
> > 1 http://emcesd.com
> > 2 mailto:doug@xxxxxxxxxx
> > 3 http://www.dsmith.org
> >------------------------------------------------------------------
> >To unsubscribe from si-list:
> >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
> field
> >
> >or to administer your membership from a web page, go to:
> >//www.freelists.org/webpage/si-list
> >
> >For help:
> >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> >
> >List technical documents are available at:
> > http://www.si-list.net
> >
> >List archives are viewable at:
> > //www.freelists.org/archives/si-list
> >or at our remote archives:
> > http://groups.yahoo.com/group/si-list/messages
> >Old (prior to June 6, 2001) list archives are viewable at:
> > http://www.qsl.net/wb6tpu
> >
> >
> >
> >
> >
>
>
> --
> Ronald B. Miller, Microwave/SI Engineer RAIL = NOISE + REFLECTIONS
> Gigahertz Data Div of MI Corp. \\ // \\ //
> 7721 Sunset Avenue \\// \\//
> Newark, CA 94560 ->JITTER<- EYE
> ->JITTER<-
> tel 510-793-4744, //\\ //\\
> fax 510-742-6686 // \\ // \\
> www.ghzdata.com RAIL = NOISE + REFLECTIONS
>
>
>
>
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
> field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
