R22/C16 values for LPF/HPF?

[Winston2021]

There's not much discussion I can find here about experimentation with those. Here's what I've found. Anything anyone wants to add from theory or experience is welcome.:

From:


Hammerhead
Pulse Induction Metal Detector
by Carl Moreland
Revision 1.5 - 21 July 2006


"The output signal of the preamp can be optionally filtered by R22-C16, which can be installed as a low-pass filter as shown in Fig. 2, or transposed to provide high-pass filtering. Both filter types have shown up in some commercial designs, and values for R22 and C16 will depend on the application. They are not used in this project so values are not provided; omit C16 and short R22."


Any examples of what commercial designs?


------------


Thread: TP4 and TP9 won't stabilize


https://www.geotech1.com/forums/show...4586#post94586


"...I installed R22 150 ohms and C16 .1UF the low pass filter..."


https://www.geotech1.com/forums/show...4631#post94631


"hi BobCat which in these used c22 .1 uf or .47 uf"


https://www.geotech1.com/forums/show...4671#post94671


"Hi rajesh c22 is .1uf
Bobcat"


Using the RC Low Pass filter calculator here:


https://www.trance-cat.com/electrica...calculator.php


That's a 10.61kHz cutoff LPF.


------------


Thread: Auto-tune modifications?


https://www.geotech1.com/forums/show...9378#post69378


"Second, if you want to add a HPF (sic LPF) after the preamp then Tinkerer is right, you just need to set it for minimal impact on the decay. As an example, if you have a 500uH coil and a 500-ohm damping R, then the decay tau is 1us (this ignores any effects of the clamping R). You want the HPF (sic LPF) to have a tau much faster so that it does not slow down the decay. Let's say 10x faster, or 0.1us. That's 1k & 100pF. - Carl"


https://www.geotech1.com/forums/show...3469#post73469

NOT about using R22/C16, but an active filter between stages:


"I think I have discovered the one modification that greatly improves the operation of the HH. An active filter, 2 pole Bessel with 3.5hZ cutoff, followed by an inverter (to get the signal back to the correct polarity for the autotune circuit), placed after the output of the the differential integrator (IC8a). The output of the inverter feeds the existing autotune circuit. Solves all my instability problems and give smooth operation. Sweeeet! Used a TL072 to implement the two stages (active filter and inverter). The values for the active filter used were determined by FILTER PRO (see attachment):


https://www.geotech1.com/forums/atta...1&d=1213623375 "


https://www.geotech1.com/forums/show...597#post185597


"Using the okawa-denshi.jp MFB tool have recreated his filter in jpeg format, so hopefully KingJL's efforts will be clearer to view:


https://www.geotech1.com/forums/atta...4&d=1394013040 "

[Munthir]

What are the performance improvements when LPF or HPF are used?

[YIVANKA]

As an example, if you have a 500uH coil and a 500-ohm damping R, then the decay tau is 1us (this ignores any effects of the clamping R)？