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18

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 ...


16

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

What limits the use of "solid state variable capacitors" like these for dynamically filtering (low power/received) RF signals? The main issue is bandwidth. Although there are some varicaps that claim a 10:1 variation (or more) in capacitance, not all of this is useable. One end of the range may be low Q, or have a very steep curve. Once you have a ...


12

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 ...


12

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 ...


12

You might be able to detect a regenerative receiver. This is because this receiver topology uses positive feedback to increase the Q of the tuned circuit. If the feedback is too much it becomes an oscillator which transmits quite conspicuously. Detecting just any huperhet receiver is a different matter. The coupling between the LO and the antenna is not ...


11

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'...


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 ...


10

I think it's more intuitive if you unlearn some things first. Oscillation is not: $$ \cos(\omega t) $$ where $\omega$ is the angular frequency in radians per second, and $t$ is time. Rather, oscillation is: $$ e^{i \omega t} $$ By Euler's formula this can be expanded to: $$ \cos(\omega t) + i \sin(\omega t) $$ If you plot this function on the complex plane, ...


9

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 ...


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

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 ...


8

The Hilbert block in GNU Radio creates an analytic signal from a real-valued one by applying the Hilbert transform to create the Q (imaginary, quadrature) component. The Complex to Mag block then discards the phase information in that analytic signal, keeping only the amplitude. Generally speaking, this is AM demodulation. This further agrees with your Add ...


8

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 ...


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 ...


7

Yes it can be done, and there are some huge advantages: Better use of bandwidth Existing chipsets/support/implementation Low power In fact there's little reason why one couldn't essentially replace DSTAR and competing systems with a standard based on GSM and GPRS technologies. Even for those parts that are patented, the patents running out means we'd be ...


7

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 ...


7

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


7

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. As ...


7

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 ...


7

GNU Radio, like any DSP system, works primarily in terms of sample counts, not time. Therefore, you have to add on time information — a sample rate — to get correct frequency-domain information. GNU Radio does not automatically figure out what the matching sample rates between parts of your flow graph are, so you have to set them up correctly yourself. The ...


7

As you might suspect, the data rate impacts the required RF bandwidth. Besides the data rate, the modulation method, encoding, packet overhead, duplexing, and other factors ultimately determine the required RF bandwidth. As an example, the 4G network has a data speed of ~20 Mbps and consumes a bandwidth from to 5 to 20 MHz. In the United States, the 70 cm ...


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 ...


7

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 ...


7

I can't dig up my source for this, but I'm pretty sure these spurs are just the result of rpitx using a fractional-N PLL (that isn't in any way optimized for phase noise) to generate a square wave. You get pulses of different (discrete) widths, which means you get spurs at predictable intervals. There isn't really anything you can do to make them go away, ...


6

Nothing. In fact, the UDR56k is an SDR and transmits D-STAR. If you want D-STAR digital voice, you do need to purchase an add-on card which contains an ASIC from DVSI implementing the propritary AMBE codec, or pay a substantial (but undisclosed publicly) licensing fee. One can safely assume that any licensing agreement would forbid any free proliferation of ...


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 ...


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