LC notch filter not working!

From: Tom Bruhns on 27 Apr 2007 08:18:57 -0700

Yes, you are correct...caught me with a low level of caffeine on Thursday. :-)

Yes on the parallel L-C for the trap frequency. But, under low source impedance and high load impedance, with the approximate L and C given, there is a voltage increase at a frequency below the trap frequency. [there are four combinations of 3 components for L and C circuits, each with a peak versus dip impedance response, me has to keep reviewing those to avoid confusion] To explain, my (later) analysis model was as follows:

One-Ohm impedance current source. Parallel L-C in series with load, L1 = 10 uHy with Q of 150, C1 = 14 pFd. Load is 1 MOhm in parallel with C2, C2 varying 10, 20, 30 pFd. Capacitors were assumed essentially lossless since their typical Q at these frequencies can be 1000 or more. Minimum voltage response was at a nearly constant frequency regardless of C2 value. Maximum voltage response frequency varied considerably. Using 1.0 V RMS reference for 0 db, the response v. C2 value was:

C2 = 30 pFd, Vout peak +22 db at 7.8 MHz, Vout minimum -35 db. C2 = 20 pFd, Vout peak +26 db at 8.25 MHz, Vout minimum -32 db C2 = 10 pFd, Vout peak +21 db at 10.4 MHz, Vout minimum -26 db

I could have done the above with L1 Q of 50 but that would simply decrease the lower frequency peak voltage, show a lesser voltage minimum at the upper trap frequency, the rest about the same.

• At this point someone will get hot about "ya can't have voltage

• gain with no amplifier!" or equivalent. :-) Yes, one can since

• a voltage increase only means a current decrease at one

• frequency...the only power loss is in the Qs of the components.

Yes, but only for the series resonance frequency. There's a variation in the overall voltage response depending on the load resistance and its parallel load (and probe) capacity. For sure, a series-resonant circuit across the source is going to affect the gain of the driving source from its frequency variation of impedance.

This is one of those seemingly-inocuous circuit applications which can get very tricky to apply with any repeatability. Especially so when the source and load were unspecified. It's safe to say that EVERYTHING interacts over frequency and one cannot just assume anything. That includes scope probes which far too many apply thinking just of their

10 Meg input resistance and forgetting they all have capacity to ground in parallel. :-(

Thanks for reminding me to go back to earlier basics, Tom. A number of years ago I worked the math on impedance of the four basic 3- component combinations and wrote it up for a work application (that would have been a high production failure situation if used as-is) and thought memory "would always be there." Actually it was but my mind gets cluttered with other stuff on a disorganized basis. :-)

BTW, I used my own LINEA (DOS-only) analysis program and LTSpice (free Windows compatible full package from Linear Technology) to run this simple circuit model. Results agreed.

73, Len AF6AY

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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

site:

I've trimmed off a couple of the groups since someone seems to have gotten his knickers all twisted up over the posting in, wow, four pissibly relevant newsgroups. Wish he'd take his venom out on the idiots that cross-posted from the wierd alt.* groups a couple weeks ago.

Anyway, Joel, WHY would you think that the Q needs to be any different?? You'd scale the impedance, and as Phil noted you got the impedance ratio vs turns ratio backwards, but you'd want the same Q at that frequency. It might be practical to scale by a 3:1 turns ratio or possibly even 4:1 at these frequencies, but I'd be wary of going beyond that.

Cheers, Tom

...

Trimmed off a couple of the groups and much of the message, though all was noted. Thanks for the additional info; I trust the OP will finde it useful, if he's still around. (Pet peeve: posters who don't bother to get back to say "Hey, that helped," or "Huh?" or give some indication they are still lurking.)

Yes, to be sure the response depends on the load. In fact, even at the notch frequency, if you start with a high load and add capacitance, you significantly affect the depth of the notch with the added capacitance.

It can be quite useful to add another capacitor (or inductor) to a series or shunt trap, to get the response at a frequency you specifically want to pass to be high. You can do the same thing with transmission line stubs, which becomes practical at higher frequencies. For example, you can put a shorted stub across a line, where the stub length is 1/2 wave on the frequency you want to "kill." But then the response at nearby frequencies will also be attenuated. You can then view that first stub as a reactance at the frequency you want to pass, and add another stub of the same reactance magnitude but opposite polarity. You'll find, of course, that the two stubs total a wavelength, assuming both are shorted at the ends away from the point the join the through line. With low loss line, this can be a very effective way to get rid of a large signal in a fixed- frequency receiver system. The capacitor-or-inductor-added-to-the- trap is a lumped equivalent of this idea.

I suppose in an absolutely accurate analysis, the impedance versus frequency charaterisitic of a load that includes capacitance may be such that the frequency of the maximum attenuation of a finite-Q notch is shifted ever so slightly, but for sure it won't be shifted enough to notice; the proximity of the metal in the probe to the coil is likely to affect the resonant frequency more.

Cheers, Tom

Hi Tom,

I was probably unclear in that I meant Q of the inductor (and am assuming Q of the capacitor is high enough to be ignore); *not* Q of the system.

Say you're in a 50 ohm system. If, at resonance, your series shunt L-C exhibits a resistance of 0.5 ohms, that's about a 40dB notch. However, in a

5 ohm system, it's only a 10dB notch. You need to get the resistance down to 0.05 -- implying a Q ten times large than what you started with -- to maintain the same notch depth.

Do you buy this? :-)

Thanks,

---Joel

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Agreed. However, the coax cable stub idea might be a tad impractical considering that 13.56 MHz is lower in wavelength than the 20m band. Stubs could get as long as around 15 feet at that frequency. :-)

I once had to "rotate" the impedance of some SAW filters (8 of them) for the 60 to 70 MHz region and couldn't get any more space for the matching other than some slots in a machined-out chassis. I couldn't have done it without skinny lil 1/8" OD coax held in place by some RTV. [roughly 3/8 of a rotation on the Smith Chart] I hate to think about doing that at 13 MHz.

73, Len AF6AY

This should, of course, be 20dB. I know I was thinking 20dB, but clearly I typed 10dB. Oops.

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!

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,

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            
Ukpeagvik (Barrow, Alaska)                         floyd@apaflo.com

In message , Floyd L. Davidson wrote

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

The first thing you must do is determine the self-resonance of the coil. That can be done with a "grid dipper". If that frequency is below the frequency you want to filter, it won't work. The next ting to do is determine the resonsnce frequency this time installed in the circuit witout a trimmer. Again, if that is below the frequency to be filtered, it won't work. Only when that resonance is above, 13 MHz in this case, can a trimmer be applied to tune it.

The stray capacitance, possibly multiplied by the chip gain, may rule out operation at 13 MHz.

Angelo Campanella