Hot answers tagged

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

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 ...


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

I will need a UHF/VHF diplexer on either end to suitably merge/split the signals from each antenna Yes, this is correct. A tangent: If you wanted to save some money by using mass-market parts, you could use 75 Ω power dividers (coax splitters) instead of diplexers. This has 3 dB loss because the signals are not directed exclusively to the intended ...


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

Here is an assortment of common ideas and recommendations for RTL-SDR devices: Mount the device directly on your antenna, or otherwise minimize the amount of feed line used. (This does not directly remove noise but rather increases signal, but that's just as good if not better. It may also remove noise by moving the device further from your computer ...


7

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 ...


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

One approach would be to capture the GSM Cell ID. This uniquely identifies a particular GSM site. You can then lookup this ID in a database such as one offered by Cobain to determine the tower location. If your application serves a very limited geography, you could build your own database to suit your needs. In order to calculate distance, you will need to ...


7

Since you said you're using an RTLSDR, it's very likely that you aren't receiving a station on 22,695 kHz, but rather one on 6,105 kHz. NHK World Radio does broadcast on that frequency (according to the schedule I'm looking at, they broadcast between 0200 and 0400 UTC from a transmitter in France). The RTLSDR "direct sampling" mode is a hack, and ...


6

The interfering harmonics are spaced at around 15.5 kHz. Do you have an old (CRT) TV set nearby? That may be the source, since TV line frequency is 15.625 Hz. If not, look for other interfering electronics, such as compact fluorescent lights or other switched-mode power supplies (your own PC?).


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

This is how the mathematics of complex signals work. The proof begins with Euler's formula: $$ e^{i\varphi} = \cos \varphi + i \sin \varphi \tag 1 $$ For signal processing, instead of $\varphi$, we are usually thinking about some sinusoidal oscillation at angular frequency $\omega$ that varies with time $t$, which we can write as: $$ e^{i\omega t} \tag 2 $$ ...


5

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 ...


5

The R820T and the R820T2 differ. The R820T can set a narrow IF filter (300 kHz) - but not if the gain is set at maximum. The R820T2 can not set a narrow filter - minimum bandwidth is about 800 kHz. This is easy to see when one runs the dongles with a spectrum display software having nothing connected to the antenna input. I use Linrad, set the IF filter to ...


5

The answers given so far provide good food for thought. I would like to add a slightly different perspective. Today's home is loaded with sources of RFI (radio frequency interference). Routers, computers, wall warts, LED lamps, solar inverters, video cameras, etc. all are potential sources of RFI that interfere with the weak signals of short wave signals ...


5

Well, the need for one ADC that's twice as fast is definitely a very important factor! The same "it gets harder with rising frequency" argument applies to the anti-aliasing filter: Building a low-pass filter of pass band edge frequency $f_{cutoff}$ and stopband start frequency $f_{stop}$ depends in complexity on how narrow the transition between pass- and ...


5

Just to answer the second part of the question, you do not want an amplifier that has too wide a bandwidth, mainly to lower the complexity (and hence also the cost) of the amplifier. All users of radio spectrum (including radio amateurs) must not cause any interference to other users of the radio spectrum, and that means that out-of-band transmissions must ...


5

You may be overthinking this. To make a long story short, just connect it and tune the capacitor to resonance and don't worry about the impedance of your pickup loop. When tuning the radio to antenna system for a transmitter, you have to impedance match the load to minimize reflections because power loss is typically a big deal. Reflections lead to high ...


5

700-900MHz is "blocked" by law in the US and a number of other countries due to a (now antiquated) law that was designed to prevent wideband communications receivers from eaves dropping on old analog cellphones, which broadcast in the clear. These days it's entirely unnecessary, but it's still on the books because regulators are lazy like that. As for ...


5

See SigiWiki: This wiki is intended to help identify radio signals through example sounds and waterfall images. Most signals are received and recorded using a software defined radio such as the RTL-SDR, Airspy, SDRPlay, HackRF, BladeRF, Funcube Dongle, USRP or others. https://www.sigidwiki.com/


5

This is caused by the RTL-SDR I was using to view my signal. Here's a quote from the guide at https://www.rtl-sdr.com/rtl-sdr-blog-v-3-dongles-user-guide/: Note that this feature makes use of direct sampling and so aliasing will occur. The RTL-SDR samples at 28.8 MHz, thus you may see mirrors of strong signals from 0 - 14.4 MHz while tuning to 14.4 - 28.8 ...


4

Check out AISMon; I've never used it, but it looks like it fits the bill. Here's a thorough tutorial: http://www.rtl-sdr.com/rtl-sdr-tutorial-cheap-ais-ship-tracking/


4

Sounds like the receiver is being overloaded by a strong station. Those inexpensive RTL receivers work by feeding the RF into a quadrature mixer to downconvert the frequency, and then into a pair of analog to digital converters (ADCs). At that point the data are processed by computer software that does all the filtering and demodulation. Those ADCs have a ...


4

So, a teaser first: With GNU Radio, you can transmit various DVB standards. Here's Alexandru Csete demoing his DVB-S2 transmission: As you can see from the FFT Plot generated by the flow graph in the upper picture, some 7MHz of bandwidth would be sufficient for this transmission mode – the HackRF should, like the device Alexandru used, be able to deal with ...


4

While the other answer is quite detailed, this one is much shorter and specific: I purchased and installed an aluminum RF shield for the dongle. The noise floor is significantly lower as a result! I was able to drop it even further by adding a ferrite bead to the USB cable. (I was already using a 10' extension to get it away from the computer.) Update: ...


4

Controllers and ATIS are transmitting from the ground, so they're always harder to receive because terrain, trees, and buildings are in the way. The best thing you can do to improve your reception is move your antenna as high off the ground as you can, to reduce the density of line-of-sight obstacles. If you can't put it on the roof, at least hang it from ...


4

This will be an incomplete answer to begin with, hoping that collectively we can come up with a workable solution: Freq 156.425 Mhz -f 156.425e6 Filter: Wide I did not find "Filter" in any online-reference of rtl_fm, unless this is "sample rate", which can be set by: -s 12k for narrow FM Mode: Narrow FM -M fm AGC: Fast I did not find AGC in ...


4

Yes, with some important qualifications, you can "directly" generate SSB on the Raspberry Pi. The rpitx software you mention does that, and in combination with an RTL-SDR type dongle you would indeed have a sort of half-duplex transceiver. So in short: yes (Q1) and yes (Q2). Based on that exact combination, qtcsdr is an app that aims to make your Raspberry ...


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