From: firstname.lastname@example.org 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
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
resonance reactance of either L or C.
Using a toroid inductor with a Q0 will get you a resonance
about 127.8 KOhms. Using a solenoidal form inductor the Q is closer
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
of the parallel-resonant circuit being resistive at resonance. If the
on the order of 100 KOhms or more, the voltage drop will be small; if
on the order of 100 Ohms, the voltage drop at resonance is great.
with a low load resistance, there will be a decided loss of voltage on
side of resonance due to finite impedance magnitude of the tuned
That depends on just where you are observing. Putting a scope probe
the load end will detune the L-C since the probe itself has a
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
circuit's impedances including circuit capacitance of both source and
as with the parallel L-C that needs to include pass frequency as well
In general, the "trap" circuits used in the past (early TV receivers
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
There are some bridge circuits that might work at a specific frequency
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
only about a third of the "trap" desired, an Elliptic (aka Cauer)
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
specific frequency well away from those, an ordinary tuned circuit
be better. Depending on the frequency desired and Q of the L and C,
impedance magnitude drop-off away from resonance might be enough to
do whatever it is you want to do.
73, Len AF6AY
On a sunny day (26 Apr 2007 17:17:20 -0700) it happened AF6AY
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.
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.
Yes, you are correct...caught me with a low level of caffeine on
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.
are four combinations of 3 components for L and C circuits, each with
peak versus dip impedance response, me has to keep reviewing those to
avoid confusion] To explain, my (later) analysis model was as
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
C2, C2 varying 10, 20, 30 pFd. Capacitors were assumed essentially
lossless since their typical Q at these frequencies can be 1000 or
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
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
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
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.
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