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This section contains scenarios and debug hints. If you are facing the same problem, try the solutions here first.

Expected Results:

  1. Bode Plot shows enough suppression at noise signal frequency.
  2. Waveform shows clear HIGH and LOW signals, a good example is shown below. (Green: input noise signal, blue: output filtered signal)
  3. Successfully demodulated messages with a reasonable PSR at 1000 packet counts. 


Characterise your design (LTSpice and Breadboard Circuit)

To characterise your design, you could make use of the following:

  1. Analyse the frequency response (Bode Plot):
    1. LTSpice: Small Signal AC Analysis (sweep)
    2. Waveforms: Network
  2. Analyse the frequency components and power level:
    1. LTSpice: FFT
    2. Waveforms:  Spectrum
  3. Observe the signal waveform:
    1. LTSpice: Simulate with recorded .sig file and probe the in/output
    2. Waveforms: Scope
  4. Eye Diagram:
    1. Waveforms: Scope

Common Debug Hints (when no PSR value observed)

Possible cause 1: Signal Recording Error

Solution: Double check the recording setting and redo the following:

Setting → Device Manager → Option 2 (2x16k)

Config → Samples: 32768, Rate: 4MHz, Base: 7.5ms

Zero Offset after recording

Possible cause 2: Transmitted Signal is weak 

Solution: Check the PSR count when clean signal is being transmitted. If it is relatively low (i.e., ~0.3 or lower), adjust your antenna length, position or consider give the receiver side a boost. To do so, go to the RTL-SDR Source block, change the Gain method to "Manual", RF gain to 20dB. Run the transmission again, it should boost up to a relatively high PSR (~0.9) with clean signal.

You are suggested to record signals at a private place rather than in the lab, so that other teams who are transmitting at the same time will not affect your recording. 

Possible cause 3: Cut-off too early


 

Possible cause 4: Cut-off too late


 

Possible cause 5: Suppression not enough


 



 



 



 



 

 

Scenario 1: When Bode Plot cutoff at designed frequency but no message can be demodulated.

Debug routine, possible causes and solutions:

  1. Check waveform, if signal is still noisy → Suppression level of noise signal is not enough. Consider shift cutoff frequency lower, change filter type, increase order of filter, etc.
  2. Check waveform, signal is not noisy. Pass clean signal without interference through the filter circuit, demodulate the recorded signal again, successfully demodulated with PSR. → Wanted signal is being suppressed/distorted. Consider shift the cutoff frequency higher and chec
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