I am sending a data with On-Off keying over the air using an LED and receiving it with photodiode.
A figure below is a stream transmitted (blue) and a stream received (yellow). A symbol rate of this test is 2,000 Symbols/sec. It means, each duration of HIGH and LOW is a multiple of 0.5 ms, 500 us.
As I use the light as communication medium, interference from fluorescent light exists. And it is a sinusoid with a frequency of 120 Hz. And it appears in the figure as the upper and lower envelopes.
Therefore, I need to filter out that 120 Hz sinusoid from the received signal.
I have two choices:
- RC HPF with 120 << cutoff << 1 kHz
- (Passive or active) Twin T notch filter at 120 Hz
I used filter design and analysis tool to determine proper resistances and capacitances of each circuit.
- RC HPF with R = 51, C = 10u
- Twin T notch filter with R1 = R2 = 4.7k, R3 = 470, C1 = C2 = C3 = 1u
I think both are sufficient to have a gain nearly -10 dB at 120 Hz so that the interference can be reduced significantly. In addition, it shows nearly 0 dB gain when frequency is higher than 1 kHz.
But I wonder that how phase distorts the received signal significantly or not. With my knowledge, different phase over frequencies are related with group delay as below:
But, I'm not sure about phase and group dealy since I have little knowlege about it.
- Which implementation would you recommend to reject f = 120 Hz and pass f > 1 kHz?
- Will phase do something serious to the received signal in filtering out f = 120 Hz?