Monday, November 25, 2013

"RTL" Dongle with HDSDR

Well, having played with both of the dongles, and most of the available free software, I've come to a few conclusions, so far.

The FUNCube dongles have a restricted bandwidth, with around 80 kHz of useful display.

This is both good, and bad.

It limits you to how much spectrum can be observed at any one time, which makes it easier to spot a signal in the relatively narrow transponder passband of a satellite (50 to 100 kHz, depending on the satellite), but it sucks for using it as an ersatz spectrum analyzer to see what's popping up in the section of spectrum that you want to monitor.

It also helps cut down "image" frequencies as far as I can observe. (It's probably what's called "aliasing" in digital sampling and Digital Signal Processing, but I'm a Radio Guy, so I'm calling it an "Image"!).

It's also NOT enough to decode Broadcast FM Stereo by a long shot, or even Broadcast Mono, if that's important to you, but then it was never designed to do that, so I'm sure not crying about it now that I know the limitation.

The "$20 RTL" dongles do about 2 Mhz of bandwidth (mine does about 2.15 MHz), which makes sense, as they were designed to receive Digital Television signals.

This is almost twenty-seven times the bandwidth!

It receives FM Stereo very nicely using SDR#, and not so well using HDSDR. HDSDR was coded for "Communications" use, so the demodulators, even the "FM Wide" can't cope very well with Broadcast FM.

BUT....it dows a bang-up job of observing a 2 MHz slice of spectrum, as seen in this screenshot:



The display is centered on 162.550 MHz, which is the strongest of the NOAA Weather Radio broadcasts here.

To the left are the other NOAA broadcasts at 162.400, .425, .450, .475, .500, .525, and ,550.

Off to both the left and right, you'll see some broad, blue bands in the waterfall (top) display. From a casual glance, considering their bandwidth (over 100 kHz), I suspect these are FM "images" caused by the poor filtering in the front-end of these little receivers. Somewhere around here I have an FM band-reject filter, and if I can find it, I'll stick it in and see if this goes away.

Almost all the way to the left, you'll see some activity, which turns out to be the railroad frequencies. There's  quite a few "RailFan" websites out there, and some people are fascinated listening to trains. I prefer to listen to aircraft, but to each his own.

Above the NOAA channels are various services listed as "Mobile", but since I don't usually explore this part of the spectrum, I'm not sure what's up there. I just picked this frequency to center on as I knew NOAA was there.

Here's a shot of 2 MHz worth of the FM band out here in L.A.:



And here's what 2 MHz wirth of the 2 Meter Amateur radio band look like:



So, to wrap up tonight's "experiments", I'm getting more comfortable using these little guys, and learning more about each of them, and the different software available for them. They both have their pluses and minuses, as does the software.

I suspect, and I've read, that to get the best performance out of them they both need some front-end filters for whatever band you want to use them for, but then you can say that about almost any receiver made.

More to come......

4 comments:

  1. I'm surprised you haven't just gone ahead and gotten a spec amp :-)

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  2. Thanks for link on HDSDR - I think I need to play with that. It has been a week sucked up all spare time, but maybe this week will be better.

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  3. Yeah, it's a pretty neatt program.

    I'm working on getting the M2 EB-144 2 Meter Eggbeater with my "spare" SSB Electronic preamp installed today.

    I cleaned it up, treated the connections with some Penetrox, and swept it with and without the radial kit, and man, is that thing flat over the entire 2 Meter band.

    Almost made me think something was wrong with it until I swept it outside the band, where it looked more normal.

    It also looks like it will work very well for the 137 MHz NOAA APT weather satellites.

    The preamp has helical bandpass filters built in to it, so I'll probably have to turn it off for wxsat use.

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Keep it civil, please....