I've built a band pass filter to detect speed using a radar motion detector (IPM-165 http://nic.ath.cx/PDF/sensors/radar/CDM324-doppler/InnoSenT/InnoSenT_application_note_III_11_06.pdf).

Using PSpice I have simulated the circuit as well as building and testing on a breadboard. From the radar I receive values of approximately 5.4mV (Vamp) and 21.0143MHz.

There appears to be no AC voltage and therefore frequency after capacitor C1, is there any obvious reason for this that I am missing?

Any help would be greatly appreciated, I'm at a bit of a loss with it.


  • \$\begingroup\$ A 10u capacitor is too large, consider using 100n instead. The larger the capacitor, the higher the capacitors series resistance and inductance, so for larger capacitors the inductance dominates at higher frequencies. \$\endgroup\$ – C_Elegans Mar 8 '18 at 14:24
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    \$\begingroup\$ Also, if all you’re needing is a band pass filter, consider using an LC filter. Getting op amps that can do what you want at that frequency is going to be difficult \$\endgroup\$ – C_Elegans Mar 8 '18 at 14:29
  • \$\begingroup\$ Which op amp works at 21MHz? \$\endgroup\$ – JIm Dearden Mar 8 '18 at 14:30
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    \$\begingroup\$ @jandy12 Is your frequency of interest correct? A 21 MHz Doppler frequency indicates a velocity greater than 477 000 km/h, according to the app note you referenced. \$\endgroup\$ – AlmostDone Mar 8 '18 at 15:10
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    \$\begingroup\$ Looking at the specs for the MC33078 it quotes the gain-bandwith product as 16MHz - not good enough for a 21Mz signal. Also 1nF @ 21MHz is about 7.6 Ohms \$\endgroup\$ – JIm Dearden Mar 8 '18 at 17:04

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