# LC notch filter not working!

• posted

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 L=10uH 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

Thank you!!

• posted

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.

• posted

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

• posted

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

• posted

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

• posted

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...

---Joel

• posted

In the '60s it was done with motor driven roller inductors and motor driven variable capacitors.

• posted

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

• posted

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

• posted

an LC won't touch harmonics.

depends on Q

parasitic capacitance, parasitic inductance.....

if you have room you could maybe try a stub filter.

Bye. Jasen

• posted

From: snipped-for-privacy@googlemail.com on 26 Apr 2007 02:36:19 -0700

Only if you have the theoretical zero-loss components.

OK. At 13.56 MHz, the reactance of the inductor is ~ 852 Ohms. The impedance magnitude of a parallel-tuned circuit at resonance is very close to Q

• X where Q is the total quality factor of both inductance and capacitance and X is the resonance reactance of either L or C.

Using a toroid inductor with a Q=150 will get you a resonance magnitude of about 127.8 KOhms. Using a solenoidal form inductor the Q is closer to

50 and the resonance magnitude would be about 42.6 KOhms.

A series impedance between source and load, the load being a finite resistance of some sort, makes a simple voltage divider...the impedance of the parallel-resonant circuit being resistive at resonance. If the load is on the order of 100 KOhms or more, the voltage drop will be small; if it is on the order of 100 Ohms, the voltage drop at resonance is great. But, with a low load resistance, there will be a decided loss of voltage on either side of resonance due to finite impedance magnitude of the tuned circuit.

That depends on just where you are observing. Putting a scope probe on the load end will detune the L-C since the probe itself has a capacitance which is very close to what you've chosen. That can be calculated and proven but the impedance math gets more complicated. Note: The load end also has some capacitance at this frequency and that will detune the resonance as well.

Someone else suggested a shunting trap of a series L-C rather than a parallel L-C. That wouldn't be much better since the off-resonance impedance of a series trap will affect the source end's impedance and thus its gain. Such an application needs to take into account the entire circuit's impedances including circuit capacitance of both source and load, as with the parallel L-C that needs to include pass frequency as well as notch frequency..

In general, the "trap" circuits used in the past (early TV receivers of 50 years ago) were only partially-successful, primarily concerned with bandpass shaping without assuming anything close to high attenuation at a single frequency. They worked fine at the high source and load impedances for tubes but not at all optimum for solid-state active devices.

There are some bridge circuits that might work at a specific frequency for attenuation, but those would need to be analyzed for their response you want to pass. A better bet for attenuating both a specific frequency - and - harmonics is to use a lowpass L-C. If your desired bandpass frequency is only about a third of the "trap" desired, an Elliptic (aka Cauer) lowpass with one of its maximum attenuation frequencies at 13.56 MHz could do that and attenuate the higher harmonics. The Elliptic function filters have definite attenuation frequencies in their stopbands.

I'd like to suggest an easy way out, but there really isn't any...without going to a more elaborate circuit than first realized.

If you wish to pass a rather narrow band of frequencies but attenuate a specific frequency well away from those, an ordinary tuned circuit might be better. Depending on the frequency desired and Q of the L and C, the impedance magnitude drop-off away from resonance might be enough to do whatever it is you want to do.

73, Len AF6AY
• posted

On a sunny day (26 Apr 2007 17:17:20 -0700) it happened AF6AY wrote in :

Drive from a low impedance with a series resistor. One big advantage of series LC to ground is that you can connect the trimmer cap to ground, so it does not detune when you stick a normal screwdriver in it :-) Somebody may remember the 'tol-trimmer'.

Else I agree with yet an other poster that the T filter is likely the way to go.

• posted

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

• posted

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

• posted

On Apr 26, 5:17 pm, AF6AY wrote: ...

This particular part is not true. A parallel-resonant trap placed in series is not detuned by load capacitance at the output. It's still parallel-resonant at the same frequency. A simulation shows this easily. The same is true of a series resonant shunt trap.

Cheers, Tom

• posted

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.

---Joel

• posted

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

• posted

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

• posted

On a sunny day (27 Apr 2007 07:47:56 -0700) it happened Tom Bruhns wrote in :

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....
• posted

Why are you asking me? I have no idea what the load is - I'm not the OP.

Ed

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