While designing the receiver for PFM (Pulse Frequency Modulation) circuit, the output signal was distorted with high frequency carrier signal. The transmitter used frequency of 25KHz and that was riding on the input message signal. After troubleshooting for a while, I found out that I needed a better filter to filter out the high frequency carrier noise. So, I opted for Sallen-Key 2nd order filter, and I was able to get rid of this noise.
Circuit Operation
The receiver follows a four-stage process to drive the loudspeaker:
Optical Detection: The photodiode (D2) detects incoming light pulses and converts them into electrical current.
Signal Preamplification: The first op-amp stage (U3:A) acts as a high-gain transimpedance amplifier. The 100MΩ resistor (R9) provides the massive sensitivity required to pick up weak light signals.
Demodulation & Filtering: The second op-amp stage (U3:B) and its surrounding components (R10, C6) function as a low-pass filter. This stage is critical for removing the 25kHz carrier frequency pulses, leaving behind the smooth audio signal.
Audio Power Amplification: The final stage uses an LM386 (U4) to provide the necessary current to drive the loudspeaker (LS1). RV4 acts as the volume control to adjust the signal level reaching this power amp.
The output waveform of the received signal, the phototransistor output and the output signal after the filter is shown below.
As you can see the output waveform before the speaker is very weak.
Then I added the Sallen key 2nd order gain filter as shown below.
The
Sallen-Key 2nd Order Low Pass Filter is formed by
R10, R11, C6, C11, and U3:B. This is the "secret sauce" of the circuit to amplify and get rid of high frequency 25KHz carrier signal frequency. This filter was designed with cutoff frequency at 3kHz using the online
Sallen-Key 2nd Order Unity Gain LPF Design Calculator.
The following shows the input and output waveform of the received signal.
As you can see the magnitude of the signal just before the speaker has significantly improved.
However, the amplitude is still small to drive the speaker. The yellow signal is the microphone signal at the transmitter, which is about 200mV peak to peak, the pink pulsed signal is the photodiode output at the receiver, which is about 100mV peak to peak, while the green signal after filtering and after the LM386 audio amplifier is just 4mV peak to peak.
I tried to use equal component gain Sallen-Key LPF by converting te unity gain Sallen-key 2nd order filter but the circuit was not stable.
