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6

Let's first start by developing some intuition of what a complex signal looks like. We can use GNU Radio to generate a signal that's just an unmodulated carrier, and then put that into a UI that will display the real and imaginary components over time: The result for 80 Hz is this: Notice how the real part is 90 degrees behind the imaginary part. If you ...


5

I like your PLL approach, because it doesn't try to "recover the original signal from noise", but actually goes ahead and detects what you're actually interested in, the presence of a specific frequency, and uses that to generate a "perfect" tone. Much cleverer than spending hundreds on the best thinkable crystal filter on the market! (I'm always baffled ...


4

Ringing in narrow filters may already have an answer here: https://dsp.stackexchange.com/questions/2170/why-do-i-see-ringing-in-the-output-of-a-digital-filter-with-a-narrow-transition "looking into the future", or more realistically, a filter delay, seems to be a requirement to reliably estimate the shape and time location of the falling (and rising) edges ...


3

The answer can be fond in The ARRL Handbook 2019, Vol 3, although it's spreaded across different chapters. In short, SNR is typically calculated for the noise floor of 2500 Hz SSB signal. Particularly this is how WSJT-X calculates negative SNRs for FT8 and other modes. Now the trick is that by deviding the bandwidth in half you decrease the noise floor by 3 ...


2

...it looks like a "quadrature mixer" is really just two separate mixers, with one fed by an LO to which a [constant] phase delay has been applied relative to the other's LO. Yes ... and. Quadrature versions of the modulating signal may comprise the "baseband" inputs to the quadrature mixers: Here's where "the phase shift is 90 degrees for all frequencies" ...


1

IMD will respond more to attenuation as the receiver will be operating in a more linear region. This is your best bet for detecting IMD: If you add 3 dB of attenuation (or reduce gain by 3 dB) and you see some signals or the noise floor go down by more than 3 dB, you've significantly reduced IMD. There is another way to determine if attenuation should be ...


1

On some (most?) receivers, if the RF gain is turned up too high, intermodulation distortion increases due to increasing non-linearities (maybe even clipping, etc.) I'd argue that's all receivers, even, since that's exactly what "turned up too high" would imply. You get a non-linear effect, and that always leads to intermodulation. If that doesn't ...


1

An ideal frequency mixer simply multiplies its inputs. One input is the RF signal we wish to shift in frequency, and the other is the local oscillator (LO). For the moment, let's consider just an ideal frequency mixer where the LO is a pure sinusoid. Since mixers are used to shift frequencies, we can use the Fourier transform to better understand their ...


1

If your signal was at 500 MHz, and you successfully captured it with a 6 Msps sample rate, the signal is no longer "at" 500 MHz. Indeed it was transmitted there, but either: your receiver has an analog mixer, and mixed this signal down to baseband such that it could be sampled at 6 Msps, or the digital signal was originally sampled at a much higher rate, ...


1

You need to convert the .ogg file into something usable by the flowgraph. How to open satnogs iq.dat files covers a similar issue for IQ files. For example, an APT recording with noaa_apt_decoder.grc resamples outside the normal demodulation chain before saving to the .ogg file: You need to reverse this resampling and possibly add a throttle depending on ...


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