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PlutoSDR Sink (upconverts and) transmits an OOK modulated signal and RTL-SDR Source captures (and downconverts) signal between the BW of [(center_freq - 1/2*sample_freq) , (center_freq + 1/2*sample_freq)]. LPF 1 is to suppress extra peaks in the received signal, (refer to Lab 6 Q4.2.a, the majority used a BPF then, LPF is also a valid choice, despite LPF might allow more noise to be included.) LPF 2 is to reject the two peaks centered at +- 2fc and retain the demodulated signal centered at 0 Hz. , downconverts to 0 centered.. Recall how to calculate 1st null band of a random binary signal from Lab 5, the data rate is given by fysm (Symbol rate). Based on your calculation, choose a proper cutoff for both signals. Notice that with LPF 1 in use and cutoff set properly, you are likely to always detect the message with comparably good PSR. It is added for removing noise and also enables you to verify the transmission is done correctly. Otherwise, if we bypassed it, when interference is enabled, you are unable to receive the message transmitted properly. You are unable to tell if this is caused by the wrong setting, hardware issue, or interference. Eventually, LPF 1 will be replaced by your circuit design. |
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