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Building a spectrum analyzer

Discussion in 'Technical' started by gunbunny, Apr 16, 2015.

  1. gunbunny

    gunbunny Never Trust A Bunny

    I had the D7 apart tonight to see what IC chips I could find. Note the elusive RTL 2832 looks like this:
    The little chip to the right of the antenna connection that looks to have 8 pins per side. While the case was apart on my D7, I could not identify one of the chips. It had 10 pins on each side, so it's not an RTL2832U. It was marked Silead GSL1680. With a websearch for Silead, I identified it as the D7's touchscreen driver chip.

    Better luck next time. I would have taken a picture of the innards of the D7, but Mrs Gunbunny has the camera tonight. There really isn't much to see in there, most of the board is shielded, and I'm not taking that apart. It seems I'm going to have to contact DigiLand and ask them what they have in the tablet.
  2. gunbunny

    gunbunny Never Trust A Bunny

    I just got a reply from the email I sent out over the weekend. The results are:

    "Sorry for the delayed response.

    Your tablet has an analogue FM radio. This is not an SDR chip. The way that these chips work is completely different from the hardware required by an FM radio. They can't be used for SDR.

    The SDR is more similar to an AM radio. Actually it is in fact two AM radios stuck together with a phase locked IF (intermediate frequency). Without going into further detail, once data from SDR chip is digitised, a software algorithm can be used to perform any demodulation. That's why SDR Touch can demodulate FM.

    However, since FM chips built into phone and tablets are doing similar processing in analogue circuitry, their output can't be used for SDR. Therefore you need to connect an external USB SDR receiver.

    Feel free to ask more questions,
    Hope this was useful.


    So, it looks like I'll be buying a dongel off of Amazon to get it to work. Cool, now I know. I also just downloaded an app for Android that shows all the hardware and capabilities. When it came to the audio chip, it recognized there was a radio chip there, but it couldn't tell me what it was. !! :( !!

    At least now I know and can continue on.
  3. fmhuff

    fmhuff Monkey+++

    Many smartphones have the computing power to do just about anything from RF to IR cameras as long as they have the sensors. Much like Arduino "shields" and others I would imagine that our smartphones or at least tablets will have this capability soon. They might not yet be the Star Trek Tricorder but I predict they will rock the world as much as the PC did.
    gunbunny and Ganado like this.
  4. Airtime

    Airtime Monkey+++

    gunbunny likes this.
  5. gunbunny

    gunbunny Never Trust A Bunny

    I received the $12 dongle from amazon yesterday, and combined with the $49 Android tablet with a $9.99 app (spectrum touch), I was scanning the electromagnetic waves from 45 MHz to 450+ (I was tired after work and didn't go any higher) by the tenth of a MHz. It was enlightening to say the least.

    I used the old 2003 FCC frequency map as a guide, but it didn't help much:
    frequency map.pdf

    Things didn't quite look like I would expect them to- FM radio stations were wide and bulbous. I looked up the local AWOS and got the local weather observation. I then looked up the VORs in my area and could get a signal, but couldn't hear the station identifiers. Did they go digital? A lot of aircraft nav radios are not digital, and you should ID your VOR if using it.

    The rest of the spectrum was dotted with really high spikes that only emanated a single tone. Some of the spikes were silent, but obviously something was there. I listened to a spike that sounded like a fax machine. Sometimes I would see a line of dots of perfectly timed short transmissions, looking like a vertical line of periods. I saw a really high spike that had a geometric pattern to it, and repeat it seamlessly and perfectly.

    Very confusing for someone who doesn't deal with this sort of thing everyday. I need an encyclopedia that can give me a picture of the wave and what it is. Almost EVERY frequency has something on it in the range I stated above.

    Attached Files:

  6. gunbunny

    gunbunny Never Trust A Bunny

    Building your own SDR-based Passive Radar on a Shoestring | Hackaday

    I want to build one! I need to get another SDR dongle and reread the article thoroughly. I understand that you will not get anything if there are no radio towers transmitting, but it would be awesome to give it a try. The worst case scenario would be I would have to get a new dongle to replace the two I mess up.

    Learning how to interpret the data would be another thing. I'm still working on trying to completely understand what I'm looking at with the regular spectrum analyser.
  7. gunbunny

    gunbunny Never Trust A Bunny

    What, nobody else wants radar?
    vegasrandall likes this.
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