LC notch filter not working!

Hi,
I am trying to filter out a 13.56 MHz signal (and if possible I would want to filter some of its harmonics in succeeding circuits).
I have tried a LC parallel resonance circuit put in series with the load. In theory the impedance of the LC parallel circuit becomes infinite at resonance frequency, i.e. the circuit becomes open.
I used a fixed inductance LuH and a variable C, i.e. a trimmer to get the product L*C = 1 / (2*pi*13.56MHz)^2 right. C should be approx 14 pF, however, due to +/-20% tolerances in L, I use a trimmer.
However, I can turn the trimmer (in the range from 10 to 20pF) as much as I want and I don't see ANY effect at all on my scope.
Any hints ? What am I missing ?
I have also looked at active notch filters, but this seems to be rather difficult at these high frequencies (see http://focus.ti.com/lit/an/slyt235/slyt235.pdf ).
Thank you!!
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On a sunny day (26 Apr 2007 02:36:19 -0700) it happened snipped-for-privacy@googlemail.com wrote in

Well, 'infinite'.... it depends how much is your termination (the impedance you drive _from_ and drive _into_?
L ------------ ---------------- Zin C Zout --------------------------------- But Zin _and_ Zout need to be a lot lower then the high impedance of the parallel LC in resonance for anything significant to happen.
Often something like this is easier:
Zi --------------------------Zo | L | C | ///
Now Zi and Zo can be a few kOhm, and will be practially shorted at resonance.
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On Apr 26, 2:36 am, snipped-for-privacy@googlemail.com wrote:

What input impedance is your scope? In a very slightly more accurate theory, and a much more useful one, the impedance does NOT become infinite, but rather becomes Q times the reactance at resonance. The reactance in your case is about 850 ohms. The Q I have little idea about: it could be 10 (pretty easily), it could be 1000 (with quite a bit of difficulty). You may do much better if you put a lower load resistance on the output of the filter -- in the RF world, 50 ohms would be usual, but at least something much lower than a 1 megohm scope input (as I suspect you're using).
There are better circuits for implementing RF notches. With only passive parts (and no superconductors), you can't get a infinite Q peak, but you can get an infinitely deep notch. Google 'bridged T notch circuit.' The key concept is that if you give the RF two paths to follow that cause phase shifts that are exactly 180 degrees out of phase at the output at one frequency, and you can adjust the amplitudes while summing them back together, you can get them to cancel perfectly. The bridged T notch should work quite well for you at 13MHz.
Cheers, Tom
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Say Tom,
Any suggestions for building electronically adjustable notch filters where there's a ballpark of a watt flowing around (i.e., 30dBm in a 50 volt system -> 10V peak voltage)? Such high voltage seem to rule out using a varactor diode as the capacitor or using a DC bias current in an inductor to push it towards saturation. For UHF and above the obvious answer seems to be YIG filters, but how about for HF and VHF? A single octave of tunability would do wonders for me at times...
Thanks, ---Joel
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Joel Kolstad wrote:

One approach would be to use a coaxial stub filter with a bunch of PIN diode shunt switches arrayed along its length. It would tune in steps, of course, but depending on the notch width you want, that might work.
I often use a coax patch cord and a thumbtack for this sort of thing.
Cheers,
Phil Hobbs
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Hi Phil,

Yeah, that's similar to what I typically do now -- albeit with lumped L-C's by the time I'm down at HF. By the time you get, e.g., 5% tuning steps though, that's 15 sections to make an octave. Not bad, I'm just hoping someone knows some magic that will work even better. :-)
I have seen some commercial notch filters that were electro-mechanical in nature: something like a motor-driven roller-inductor. That and a handful of capacitors gets you a very wide tuning range, tons of power, etc. with the only drawback being that the time to change frequencies is going to be measured in seconds. It almost seems like the most elegant approach sometimes...
Thanks for your input, ---Joel
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Joel Kolstad wrote:

In the '60s it was done with motor driven roller inductors and motor driven variable capacitors.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
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wrote:

Seconds? You should check out the old Collins 490T antenna tuner. As I recall, the spec was something like 5 seconds max to tune any new load within the specified range, any new frequency within range, but you knew that if it didn't tune in a second and a half, something was most likely broken. Motors don't have to be slow. The inductor went from one end to the other in I suppose under half a second. Seems like it was 8 or 10 turns. Obviously PIN diodes would be faster though. Also, I wouldn't count varactors out of the race, for at least part of the job. You might not use diodes rated specifically for varactor service, since you'd be biasing them with tens of volts most likely. But there's a long ways between these embryonic ideas and a working design, and I leave that to you. ;-)
Cheers, Tom
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HI Tom,

Yeah, I suppose that if you start back-biasing your diodes at 100V, suddenly a volt starts to look like a small signal again.
Someone else mentioned that a straightforward means to drop the voltage swings is by dropping the "system" impedance. A 1:100 transformer takes 10V to a mere 100mV, although now the 50 ohm system is 5 ohms so Q has to be 10 times better to obtain the same notch depth. Still, probably worth pursuing.
Speaking of Collins radios, here's a rather sad site: http://cgi.ebay.com/Collins-490T-1-Radio_W0QQitemZ120112545170QQihZ002QQcategoryZ4673QQcmdZViewItem
---Joel
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Joel Kolstad wrote:

Nope, you've got the square root in the wrong place: a 100:1 transformer changes the impedance by 10,000:1, not 10:1. Try 5 milliohms. You'd need as many varactors in parallel for that as you'd need in series-parallel for the other approach.
Cheers,
Phil Hobbs
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Oops, thanks for catching that Phil! Sheesh...
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Tom Bruhns ( snipped-for-privacy@msn.com) writes:
Who's the idiot who suddenly decides to cross-post this to so many newsgroups: sci.electronics.design, alt.engineering.electrical, rec.radio.amateur.misc, sci.electronics.equipment
You bozos think that just because it might be relevant to a newsgroup, it's acceptable to cross-post.
The reality is that there are virtually no times when crossposting is necessary, and it's the mark of iditots too lazy to find the right newsgroup, or too clueless.
ANd when you see fit to add in newsgroups, you're even greater idiots.
Michael
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On a sunny day (27 Apr 2007 07:47:56 -0700) it happened Tom Bruhns

Many years ago I made a digital system, it consisted of 4 coils 10, 20, 40, and 80 uH, and 4 relais to put these in series as needed (this was a rather high power system). So the the tuning software switched the relais, giving 15 presets. That was accurate enough for antenne matching. And there were 12 of these in a rack....
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Other than a pre-tuned Collins commercial transmitter at an Army station in the early 1950s, the first time I recall seeing an automatic antenna tuner was in the T-195 transmitter built by Collins for a USMC contract (forget the AN/ number, its companion receiver was the R-392, the 28 V counterpart to the R-390 and R-391). On a quickie demo in 1955, the officer doing the demo disconnected one of the Jeep's whip antenna sections. The T-195 retuned its antenna is a few seconds, indicated by a little lamp on the front panel. Most amazing to me at the time, used to the huge built-to-last-forever HF monsters that were always most fussy on manual tuning. :-)
Much later I got a PDF of that T-195 TM and believe that this set might have been the first military radio to incorporate the Bruene voltage-current detector necessary for the automatic antenna tuning servos. Any delays in operation might have been just from the detector-sensor output time-constants in addition to motor speeds. The "Bruene Bridge" as it is sometimes called, is the basic form for nearly every other automatic antenna tuner built since then.

Yes, the all-important "gestation stage." Ask any mother. :-)
73, Len AF6AY
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wrote:

You could reverse feed an isolator into a selective load.... The beauty of this approach is low (even very low) insertion loss. The drawbacks or course are deep notches, and maybe only 20MHz BW at UHF. The latter being primarily a function of the isolator response.
You would still have to make the load (cavity, line section, etc...) electronically adjustable if you truly wanted to meet your above criteria, but it should work.
-mpm
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Hi Tom,

The input impedance of the scope is 1 MegOhms for the passive probes I use, but i can change the coupling to 50 Ohms as well.
Do you know where the Q in an LC notch filter comes from ? Is this the Q of the inductor defined as (2*pi*f * L) / R ?
What kind of inductor and capacitor is best suited for a notch filter at 13.56 MHz (RF frequencies) ? I use a ceramic trimmer right now, but I am not sure what kind of inductor is best suited for RF circuits. It seems that the value of an inductor (even the L) is very frequency-dependent.
When you say a low load is much better, do you mean for a parallel LC notch filter, or for a series LC notch filter ?
Thanks for the great help!
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snipped-for-privacy@googlemail.com wrote:

What are the circuit impedances?
Consider that your circuit more or less looks something like this,
Rtrap input signal >-----+----/\/\/\/\/----+-----> output | | / / \ \ Rin / / Rout \ \ / / \ \ | | | | ----- ----- --- --- - -
Hmmm, looks just like a plain old RC attenuation pad! Except the Rtrap is actually a parallel tuned circuit that is a high impedance at one frequency and lower impedances at other frequencies. So lets pick any handy set of values for Rtrap, as an example. Maybe your LC circuit is more, or maybe less... the effect is what you want to understand. Lets assume the value for Rtrap approaches 100 Ohms for non-resonant frequencies, and say 10,000 Ohms at the resonate frequency.
So, if Rin happens to be high, say 100,000 Ohms or more we can just ignore it. (Which is practical, as all it does is provide a constant load for your source, and we'll assume it is sturdy and can handle anything from 0 to 1000 megs!)
That means you have two circuits, one at the resonate frequency and one at all others, which both look like this,
>----+ | / \ 100 Ohms, or / \ 10,000 Ohms / | +------> out | / \ / Rout \ / | ----- --- -
It's just a plain old resistance divider. If Rout is 100 Ohms the output will be 1/2 the input at non-resonate frequencies (insertion loss), and at the resonate frequency it will be 1/100th of the input.
Obviously if the Rout value is 100,000 Ohms your divider is going to have virtually no effect at all! And if it is 10 Ohms the effect will be even greater than it was at 100 Ohms.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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Can that circuit ever produce any depth of notch? If Rtrap is a parallel tuned circuit then it has in parallel with it an effective resistance of Rin+Rout, and to get any reasonable selectivity Rin+Rout must be high compared to L/C.R, the dynamic impedance of the tuned circuit at resonance.
If that is so, are there actually any values for Rin and Rout that could produce a reasonable selectivity?
--
Tony Williams

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snipped-for-privacy@googlemail.com wrote:

Can you sweep it to determine where it is resonant? Sounds like it is either off frequency or has miserable Q or maybe both. 'Course as others have indicated the meaurements set up may be part of the problem.
Ed
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ehsjr wrote:

What is your load? If it is high impedance, then your parallel LC has to exhibit an even higher impedance at resonance, which is difficult for real inductors. To suggest a solution we should know more details on the source and the load.
Pere
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