Electrical – Choosing values for a low pass RC filter

Here is a circuit which only lets through pulses with a repetition frequency of less than 1KHz.

When triggered by the leading edge of each input pulse, Monostable IC1 generates a 1ms pulse which clocks D F/F IC3's output high. IC3 is reset when the input pulse goes low again, so the output follows the input. If the frequency is higher than 1KHz then IC1 is continuously triggered and doesn't produce any clock pulses, so IC3 stays in reset.

R2, C2 and IC2B provide a short delay to ensure that IC3 is out of reset when it receives the clock pulse from IC1.

BTW this circuit can be simplified down to just one IC by configuring the other half of the CD4528 as a basic flip-flop. Connect pins 15 and 14 to GND, pin 11 to Vdd, apply clock input (from pin 6) to pin 12, and connect the input frequency direct to pin 4 (+TR) and pin 13 (/RESET). Output is on pin 10.

The LM358 and LM741 do not have enough bandwidth to do what you want. At 200kHz the LM358 can only manage about 10dB of gain. It also has poor slew rate and a high phase shift at this frequency, resulting in bad filter performance and distorted output.

If you don't have access to better opamps then you will have to use 'passive' filtering. Conventional IF amplifiers use a tuned transformer between each stage, with a tapped primary and loosely coupled secondary to achieve high Q. The downside of high Q is narrow bandwidth, but by staggering the center frequency of each stage the overall bandwidth can be improved without compromising stop-band attenuation.

You could build your own transformers from scratch, but it might be easier to scavenge some from an old AM radio and modify them to suit (you might even be able to leave them on the PCB and use the entire IF circuit as is). Inside each transformer you should see a small capacitor in parallel with the primary winding, which creates the tuned circuit. Wiring another capacitor across this will lower the frequency. To get from 455kHz to 200kHz you need to add about 4 times more capacitance (might be 2~10nF, depending on the value of the original capacitor).

The circuit below shows the IF part of a typical transistor radio. I have highlighted the capacitors that are in each IF transformer. The transformers are often coded with a color to indicate what stage they are in:- yellow = 1st, white = 2nd, black = third.