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16

The bits per sample will affect the dynamic range of your receiver. There's a lot of math that I'm sure you can find, but here's the intuitive explanation: A digital signal can represent only discrete quantities, where an analog signal can represent infinitely many quantities between any two discrete quantities the digital signal might represent. The ...


15

Nothing is preventing an SDR from transmitting the D-STAR protocol, other than the need to implement it as such. It is open, and currently I know that Digital Signal Decoder (DSD) can actually decode some data and textual frames. What prohibits transmission of D-STAR Digital Voice is the codec used to encode the voice - it's AMBE2000, which is patented and ...


13

Sure, it would work. In fact, if you've ever used something implementing some digital mode that interfaces with an SSB tranciever, this is exactly what is happening. Many TNCs and most PSK-31 software is an example of this. The reason I/Q is more frequently used is that it's simpler. If you want an SDR with 50 MHz bandwidth, you can do that with a single ...


13

Your “B” is the stereo difference signal of broadcast FM stereo. It is placed at twice the pilot frequency so that it can be recovered by having the receiver lock onto the pilot signal and frequency-double it to obtain the subcarrier signal marking the position of the difference signal. The receiver uses this subcarrier to shift it in frequency down to the ...


11

Is the raw data coming from the USB dongle literally samples of the ~1090 MHz wave? Or is the carrier frequency first demodulated (in that case, what actually am I receiving?) It's downconverted, not demodulated, using a local oscillator, mixer, and filters. That is, the signal you obtain is the same as if the transmitter had its carrier frequency set to 0 ...


10

One controls the hardware, and the other controls the software. The hardware selects some section of the entire RF spectrum (by a local oscillator and mixer), and down-converts it into a frequency an analog-to-digital converter can handle, filters it (to discard out-of-band signals), samples it, and delivers that data to the computer. This data determines ...


9

Signal strength and dynamic range Generally speaking, dynamic range is the ratio between the strongest and weakest signal that can be received. In a digital signal, the dynamic range is determined by the number of bits per sample: the strongest signal is one which uses the full range of the sample values, and the weakest signal is one which uses only two ...


9

Software-defined radio is radio where the signals are sampled and converted to digital data early in the process. This means that like any digital data, they can be copied without loss of quality. Computers can execute the same calculation many times. Thus, a SDR program can take one input and process it differently, as if it were many different radios. ...


8

You could check out ShipPlotter which appears to be a windows-based AIS receiver. It mentions in the webpage that it accepts audio through your sound card. In the case of RTL-SDR, you'll want to use something like "Virtual Audio Cable" or "VB-Audio Cable" to route the audio from sdrsharp to ShipPlotter.


8

Are the oscillators in SDR hardware (that are used to mix the input to sinusoidal waves) purely digital? The answer is the same as to your other questions: The term "SDR" doesn't describe a single device architecture, and hence, all imaginable solutions to the problem of generating a tone for a mixer exist¹. There's SDR devices that have a numerical ...


8

I expect to see the sum & difference frequencies. You're multiplying two complex sinusoids, not a $\sin$ and a $\cos$, but $$e^{j2\pi f_1t}\cdot e^{j\left(2\pi f_2t-\frac\pi2\right)}= e^{j2\pi(f_1+f_2)-j\frac\pi2}$$ So, only the sum frequency, as it should. I am seeing a big peak at sum frequency followed by a series of multiple peaks on the FFT. I'...


7

There are actually three different distinctions one could mean by referring to “narrowband” FM. Wideband FM in this context generally refers to the type of FM used for broadcast stations — those picked up by consumer FM receivers — as opposed to that used by two-way communications, including amateur transceivers. You are right that there is no fundamental ...


7

I'm working on a system (theoretical at the moment) whereby I wish to receive data from multiple sources all transmitting at the same time. I've read that a helical antenna is capable of this. I'm afraid you're misinformed. No single antenna, no matter what the design, can be better than another at receiving multiple signals at once. A receiving antenna ...


7

You're on to something very right! In signal processing, we define a basic waveform by its frequency, number of samples within the period and its amplitude. I'd go a step further: In digital signal processing, the actual frequency doesn't "exist" any more. It's just "a periodic signal with a period of $T$ samples". So that's exactly why for example ...


7

Yes. To actually do it, you need: SDR receiver hardware with (slightly more than) 20 MHz bandwidth to capture the entire FM broadcast band. A computer, fairly powerful (definitely not a Raspberry Pi or such). SDR software which can be configured to actually do this. Many receiver programs are focused on receiving a single signal; my own ShinySDR can do ...


7

This is very likely to be be due to overload of the receiver. A quick, rough way to tell the difference: Tune your SDR receiver so the waterfall center frequency is not the same as the transmitter's frequency. Transmit. Check whether the spurs you see are symmetric about the transmitter's frequency or the receiver's frequency. This tells you which side ...


6

What you are really looking for in a SDR depends on your needs. Here are some factors to consider: ADC/DAC resolution: this is measured in bits. It pretty much represents how finely the analogue to digital converter or digital to analogue converter can represent changes in the waveform. Usually higher is better (12-16 bits), but you can get away with lower ...


6

Swapping I and Q reverses all the frequencies. For example, a signal 5 kHz above the mixer's LO will appear at -5 kHz, instead of 5kHz. CW and AM are symmetrical in the frequency domain, so it doesn't matter for the purposes of demodulation, though your software is likely to display the wrong frequency. SSB is not: reversing I and Q will make USB look like ...


6

Implementing a CW receiver in an SDR is pretty much like implementing a SSB receiver. You will tune the RF bits to some band of interest. Next, you will multiply the I/Q signal so that the CW signal you want to receive is at 750 Hz, if that's your desired pitch. Next, you must filter. There are two reasons. The obvious reason: you don't want to hear ...


6

There are a lot of things wrong here. Neither plot looks correct. There is no way that Qt plot is realistic for anything but a signal generator. Where's the noise? Where are the three missing constellation points in the Qt plot? APRS isn't QAM (it's AFSK over FM), so I'm not sure why you are expecting QAM. You don't have any filters, clock recovery, or ...


6

Almost every SDR receiver is capable of receiving the FM broadcast band. The ones which can't are typically SDR transceivers designed for specific HF bands. The harder part of your requirements is the waterfall of “the band … or a significant portion of that range”. The FM broadcast band is 20 MHz wide, and to display all of it straightforwardly requires ...


6

Very interesting (and clever)! Yes, the first counter is being used as a delta-sigma modulator to turn the incoming analog signal into a high-speed string of bits. Then the counters generating the I and Q signals are defining the lower-speed sample rate. Two other counters (labeled "mixer") are essentially integrating (i.e., low-pass filtering) the high-...


6

An RTL Dongle is Receive-Only. You can't transmit with it.


6

What the FFT sink shows as frequency axis actually has no basis in "real world signal" – it just takes the sample rate you set (here, you set 1 MHz), and scales the full nyquist bandwidth to that. If you used a different number in the sample rate field of the FFT sink, the spectrum would look absolutely the same, just the frequency axis would have different ...


6

If I read the instrumentation correctly, this is but 20 kHz; that's pretty wide for what I'd expect for Ham operation on HF. It's not "very wide" per se. (I'd define "wide" based on the ratio of bandwidth to center frequency, and that's but 0.2% here.) This looks a bit like an OFDM signal, especially with the hints of side lobes. 20 kHz OFDM in HF sounds a ...


6

The simplest squelch, applicable to almost all modes, and the one provided by most amateur radios' “squelch knob” as far as I've experienced, is power squelch: measure the signal power (in SDR, $I^2 + Q^2$), average over some short time interval, and compare against a threshold. This is done before the demodulator as the demodulator removes amplitude/power ...


6

Given that I don't have a frequency standard or other known transmitter available, what is the easiest way to calibrate the NooElec NESDR SMArt which is using a TCXO rated at 0.5 PPM.? I.e. to determine the PPM calibration value? Find any continuous broadcast signal that has a carrier and you know the nominal frequency of, and adjust the PPM value until its ...


6

You've nailed this! So, I'm sometimes involved in these kinds of investigations. It all gets easier when you have control over the transmitter – when some spurs disappear as soon as you turn it off, but others remain, you've ruled out the transmitter as the source of these. Then, digital receivers typically, as you notice, have different sources of spurs. ...


6

Fun setup! I have many of the same components and need to try them out myself, but here's a general outline. (Plenty of this may be review for you, but I'm including it for others who may not be as familiar with your equipment.) The antenna A miniwhip (see also this article and this PDF) is an active antenna taken to the extreme: it uses a tiny conductor ...


6

The billon code refers to a generation of transmitters that could transmit up to ~3.5 billion different codes. Although there were a large number of codes possible, the transmitter always transmitted the same code each time the button was pressed, unlike more modern versions where the code changes each time the button is pushed (a rolling code). The DIP ...


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