EEVblog Electronics Community Forum
Electronics => RF, Microwave, Ham Radio => Topic started by: rfguy2020 on January 30, 2024, 12:40:23 pm
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Hello,
I'm designing Zero IF demodulator circuit. Output of my IQ demodulator has signal, which as significant DC offset. My received signal has AM modulation.
My challenge is that my signal has first strong AM modulation (leakage from transmitter) followed by weak AM modulated signal, which is my desired signal, with DC offset caused by transmitter. DC offset can vary, but it has level of ~1V. My desired signal has amplitude around 1mV. I would like to have voltage gain around 1000-10000 for my ADC.
I have tried AD8338 with AC coupling. The problem with AC coupling is that my desired signal (square wave) has distortion due to high pass filter. I could live with that but bigger problem is the transition from transmit phase to receive phase. AD8338 is in saturation during transmit phase (modulation is on/off signal ~0V/~1V). If I'm using gain of 1000, it takes too much time to recover back to 0V output.
Ideally, I would like to compensate DC offset, so I would not have distortion in my wanted signal. I have evaluated servo loop circuits (opamp integrator in amplifier output and integrator output summed to amplifier input). I have an analog switch, which is turned on, when I need to compensate dc offset. It looks promising in LTspice, but it looks a bit complicated.
Do you have recommendations for good differential DC offset compensation circuits?
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My challenge is that my signal has first strong AM modulation (leakage from transmitter) followed by weak AM modulated signal, which is my desired signal
Sounds like your problem isn't DC offset from the demodulator itself, but rather RF power leaking from the transmitter. I don't know what your modulation scheme is, but it's unlikely that the receiver side can correct for this unless the unwanted leakage and desired signal can be modulated differently from each other.
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My challenge is that my signal has first strong AM modulation (leakage from transmitter) followed by weak AM modulated signal, which is my desired signal
Sounds like your problem isn't DC offset from the demodulator itself, but rather RF power leaking from the transmitter. I don't know what your modulation scheme is, but it's unlikely that the receiver side can correct for this unless the unwanted leakage and desired signal can be modulated differently from each other.
Carrier leakage cancellation circuit is another option, but it is going to be more complex (expensive) than removing dc from baseband signal.