Home |
Search |
Today's Posts |
#14
|
|||
|
|||
From: "Jeff" on Tues, Oct 11 2005 2:47 pm
Im confused regarding the choice of iron powder toroid for a 7Mhz high pass filter. I need to use a small size (T68) toroid. For the freq range, some reference texts say T68-2 is the one, others say T68-6" grade. I see differing "best Q range" specs for the 6 grade.....2-30Mhz......10-50Mhz...etc. Have similar need for 20Mhz LPF, but assume "6" is really the choice there. Any suggestions as to the relative differences and reasons to choose between grades 2 and 6 for the HPF in T68 size? Been there, done that. In the practical case, what you can expect is a very slight change in insertion loss in the passband, perhaps a less sharp transition of attenuation at cutoff, depending on the type of highpass. It can be modeled in any SPICE analysis program with accuracy if you make a special model that adds a series R element computed from reactance divided by Q. For a highpass filter, the usual filter type has the inductor in shunt. In that configuration the inductor has a reactance directly proportional to frequency and will have little effect on the insertion loss in the passband. The equivalent series R due to Q won't matter much at 1/3 to 1/4 the cutoff frequency. It might matter on attenuation in the stopband region but the analysis on that is more difficult and a practical build-and-measure is the quicker way to go. For a lowpass filter, the usual configuration has inductors in series and there the Q of the inductors will affect insertion loss more. Those would use the lower frequency range for powder mix. In a highpass filter it is important that the capacitors have a minimum series inductance so that the 5x to 10x cutoff frequency isn't disturbed. Since capacitors are usually in series, their Q will effect the insertion loss. Fortunately, most capacitors will have a good Q up around 500+ and won't be a factor. Use what you have and measure the results (you've got a spectrum analyzer handy so that's taken care of). I made a fairly good, practical Synthesis-Analysis program for L-C filters that includes automatic modeling of (separately) all capacitor Qs and inductor Qs. I can attach that to private mail if you want it. Freeware. Proven by practical test comparison. |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
OT- The Five Iron | CB | |||
PRO-95 good choice? | Scanner | |||
Iron powder vs. ferrite cores for low-power inductors | Homebrew | |||
Iron powder vs. ferrite cores for low-power inductors | Homebrew |