From: Tom Bruhns said:

This particular part is not true.

Yes, you are correct...caught me with a low level of caffeine on

Thursday.

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.

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.

The same is true of a series resonant shunt trap.

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