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

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

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.

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?

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