Ham Radio Blog
Listening to an AM station on 954kHz.
If you can demodulate a CW signal, you can also demodulate AM signals by just changing your side tone frequency, usually 700Hz or so, to 0Hz.
An AM carrier component is translated to a near DC signal, which can easily be eliminated by applying, say, a HPF.
https://people.csail.mit.edu/hubert/pyaudio/
https://www.jetbrains.com/help/pycharm/install-and-set-up-pycharm.html
This is Gqrx and my SDR program side by side.
Listening to the same CW signal with slightly different LO frequencies.
You will hear a higher pitch sound from the left audio channel, and a lower pitch from the right.
This is listening to two CW signals at 7011.058kHz and at 7024.991kHz.
A QRS CQ comes from the left channel, and a QRQ conversation from the right.
Waveform 1 shows an incoming 7076kHz CW signal converted to a 2kHz IQ signal (only the real part is shown in the figure) with a receiver tuned to 7074kHz.
The LO is adjusted to generate a 2kHz signal, and Waveform 2 shows the multiplier output after decimation. (The real part in red, the imaginary part in green, and the absolute value in white.) The signal does not become a pure DC signal, because there is always a minor discrepancy everywhere.
The multiplier output is downsampled with the decimation ratio of 960 (= 192,000 / 200) to allow only the signals very close to DC.
After decimation, the (almost) DC signal is 40 times upsampled to obtain 8k samples per second and is multiplied with the output of a 400Hz side tone oscillator to finally generate the audible signal with the sample rate of 8kHz.
In this way, if there is another CW signal with the frequency difference of +f Hz, you will also hear one more audible signal at 400+f Hz.
This figure is for the case when the LO is detuned downwards, and you will hear a slightly higher pitch sidetone.
And this figure for real CW signals. Are you now happy with QRMs and QSBs?
This is demodulating a CW signal with a BFO.
A 7076kHz CW signal comes into the receiver as a 2kH IQ signal, and the BFO is set to 2.7kHz to get a 700Hz audible tone.
Monitoring my TX signals sending CQ.
You can see “CQ TEST” as a waveform.
Am I getting close to implement a multichannel CW decoder?
You see a crosshair cursor in each VFO window. A yellow vertical line is to select which signal to be demodulated, and a cyan horizontal line to adjust the threshold value for emitting the audio signal.
Notice that you will hear the same 700 Hz tone whether you detune the receiver or not.
This is my control panel, but what about the sound?
Here is an audio signal presented to your speaker. The message is the usual “5NN BK” as you see.
Notice that you hear a pure sine wave with no background noise.
The array self.s contains the FFTed IQ signal, and the self.audio_data is a 700Hz sine wave.
So, you don’t like it because you love QRMs, QRNs and everything that is related to ionospheric propagation?
