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Author Topic: Capacitor selection question  (Read 992 times)
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WA2SQQ
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« on: January 08, 2024, 11:38:07 AM »

Please refer to the attached photo.
I'm looking at this circuit where Q2 is providing a signal at ~ 40 khz through caps C10 & C11. I suspect that the combination of the two caps and  R1 are rolling off some of the lower frequency content.

Here's my question - the combination of C10 + C11 = 180 pf.
Why would this circuit use two cap instead of just using a 180pf cap? Am I missing something?


* Filter_Caps.jpg (117.83 KB, 790x884 - viewed 130 times.)
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KD1SH
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« Reply #1 on: January 08, 2024, 12:49:28 PM »

  I suspect that it's something to do with avoiding self-resonance issues. In solid-state printed circuit board design, particularly with surface-mount integrated circuit packages, you'll often find bypass capacitors in parallel—one larger value and one smaller—which at first glance seems odd, since you'd think that it would be easier and less expensive to use one cap of a value equal to the parallel pair. It's done that way to avoid self-resonance issues, particularly when the noise frequency is a known quantity, as in a switching power supply. Every capacitor (or inductor) is likely to have a self-resonant frequency, due to leakage resistance and inductance within itself, and using two different capacitors can be a way to avoid that.
  Your capacitors are in series, and they're not bypass caps but rather coupling caps, but I'm thinking that the designer was attempting to "straddle" a potential self-resonant point, whereas a single cap might have wound up sitting right at that point, or uncomfortably close to it.
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« Reply #2 on: January 08, 2024, 04:55:12 PM »

Look at your parts list; Do the two capacitors have the same temperature coefficients?

What is the circuits application and what equipment is it used in?

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WA2SQQ
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« Reply #3 on: January 08, 2024, 08:45:29 PM »

There is no parts list. This is part of an ultrasonic motion sensor.
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W4AMV
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« Reply #4 on: January 22, 2024, 06:42:52 PM »

Design test point at the junction of the 2 C's. The voltage gain and frequency response controlled by the 2 sets of (3904) devices. The HP corner of the cascaded NPN's ~ 70 kHz. The FET phase splitter, at ~ 10 kHz. They are different enough that I expect the design(er) desired a test point to check the individual gains and responses. Those NPN voltage gain units, they are beta sensitive, but simple.
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