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This section contains scenarios and debug hints. If you are facing the same problem, try the solutions here first. |
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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. |
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Possible cause 3: Cut-off too early |
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Solution: Check the bode plot with AD2, make sure your wanted signal is not being suppressed. When you are using trimmers, vibration and impact during transport may cause its resistance to vary. You might want to tune the circuit regularly and eventually replace trimmers with fixed resistors. Sometimes, your circuit too noisy that it is hard to determine the passband gain from the bode plot, you can also pass the clean signal without interference, to see if you can recover any PSR. |
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Possible cause 4: Cut-off too late |
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Solution: Check the bode plot with AD2, make sure the interference signal is being suppressed enough. You are the one who should decide what is a good suppression. Higher suppression usually means more components and thus higher cost, you should weigh between various aspect while designing a product. Sometimes, your circuit too noisy that it is hard to determine the passband gain from the bode plot, you can also make use of the FFT response function and exam the amplitude of signals at different frequencies. |
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Possible cause 5: Suppression not enough Passband signal distorted. |
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Possible cause 6: Passband signal distorted. |
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Solution: Check the bode plot with AD2, you might notice crazy passband gain or ringing presents. High passband gain might be introduced when you do not tune R1 and R2 together to match the unity gain, ringing my due to the nature of the design of filter. They are unavoidable sometimes, but you can choose to shift the gain hump to the right place so that it will not affect your passband signal. |
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Possible cause 76: Unstable Unstable DC signal overriding |
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Solution: The circuit is drawing power from your laptop USB adaptor, the DC power could be unstable, especially if you are not connecting your laptop to the power source. You might notice the signal output is rather "wavy" in LTSpice simulation and actual circuit experiment. You can always add a passive high pass filter at the output of the circuit to regulate the DC instability. This high pass filter will not count towards your final design stage.
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Possible cause 7: |
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Scenario 1: When Bode Plot cutoff at designed frequency but no message can be demodulated.
Debug routine, possible causes and solutions:
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