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17

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 ...


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 ...


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

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'...


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

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

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. ...


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

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

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


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

2m isn't a good band if you want to do anything digital. Digital modes just aren't very popular on 2m. I think the only digital activity you are likely to find is: D-STAR APRS D-STAR uses GMSK, but the data for voice transmitted on that channel relies on a proprietary codec called AMBE. Any software that can encode or decode it is almost surely illegal, ...


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

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

Based on the information you have given in the post and comments, the simplest way to construct a "omni" for 433 MHz, is by using an SO239-chassis-connector, and solder/screw the antenna elements directly onto the connector. Make sure the ground-plane elements are bended down, to ensure a close match to 50Ohm impendance. The single-strand/1mm wire will do ...


6

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 ...


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

In general, the HF bands (1.8 MHz through 30 MHz) do not have sufficient bandwidth to support live transmission of a video signal. In the US, the FCC does not authorize an emission mode for live video on HF. The lowest available band in the US that supports live video is 70 cm (~440 MHz). There are, however, options for transmitting pictures on HF. One of ...


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

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 ...


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