Frequency finder required

I’m looking for a little help with a project that has been on the back burner for a while.

I have a commercial product that may (or may not) transmit data in bursts in the 433MHz band. Is they a way, or an inexpensive device that I can obtain, that can tell me the exact channel it is transmitting on? I do have a cheap receiver but the likelyhood is that I would be listening to the correct channel when it wasn’t transmitting.

Any guidance would be appreciated.

Yes, nice little toy called a Spectrum Analyzer. Maybe rent one with recording capability.

Google "software defined radio" (SDR). You can buy USB dongles for ~$20 that turn your computer into a radio and spectrum analyser. ~50Mhz to ~2Ghz. The one I tried had "#@%!" software, but eventually I got it working. Leo..

One of the many... http://www.adafruit.com/product/1497 https://learn.adafruit.com/freq-show-raspberry-pi-rtl-sdr-scanner

Wow! http://www.kerrywong.com/2014/11/16/testing-an-rtl-sdr-spectrum-analyzer/

Googling "software defined radio spectrum analyzer" brings up some interesting, inexpensive stuff!

Thank you, that gives me a term to search for!

I've got a couple. The 28MHz oscillator on the board is crap, it drifts everywhere. Not an issue when using it as a TV with AFC, but for my uses, I'd like something better.

I found some 14MHz +-0.5ppm TCXO modules pretty cheap. I also got some toroids and dual diode modules to make a frequency doubler.

Sure, but for $20 you can’t complain.
I suppose if you let it warm up, and use a reference like a (ham) radio station.
And enter the offset in the software.
Then they are quite good to get a general picture of what’s going on in the band.
You can always use it to listen to the cops, or as a digital TV receiver.
Leo…

I got a couple recently (mainly because the first one didn't work). Cheap, works more-or-less as advertised. For the price, pretty good.

To give them credit, the eBay suppliers replaced the non-working one with one that worked, after I opened it up and sent them a photo that proved that the chip on board was slightly different from the advertised one.

Oh, I -can- complain. ;)

One of mine drifts like crazy, up and down, all day. Never settles down.

So I found that oscillator and thought, why not?

Up until I read this thread I thought a spectrum analyzer was out of my price range. Now I'm really interested in where this thread is going...

That is one of the reasons why I want my SDR to have a stable oscillator.

I also bought an upconverter to use it on HF and lower frequencies:

http://www.amazon.com/NooElec-Ham-Up-v1-2-Upconverter/dp/B009LQT3G6/ref=sr_1_1?ie=UTF8&qid=1426189652&sr=8-1&keywords=nooelec+upconverter

I got the box for it, too.

http://www.amazon.com/NooElec-Extruded-Aluminum-Enclosure-Upconverter/dp/B00J7EY1Z0/ref=sr_1_2?ie=UTF8&qid=1426189652&sr=8-2&keywords=nooelec+upconverter

And put my SDR in a metal case (a mint case) with room to add the oscillator circuits.

Hah! Twisted minds think alike. Nooelect now sells an RTL SDR modified in just the way I'm discussing.

http://www.amazon.com/NooElec-NESDR-Mini-Compatible-Guaranteed/dp/B00NW3OBVG/ref=sr_1_9?ie=UTF8&qid=1426189652&sr=8-9&keywords=nooelec+upconverter

See http://hackaday.com/2014/12/11/over-engineering-ding-dong-ditch/

Also http://www.windytan.com/ - page maintained by Oona Räisänen.

In particular her page http://www.windytan.com/2013/09/the-burger-pager.html about decoding the workings of the "pager" you get given in cafés to let you know when your food is ready, is amazing.

For this kind of situations I often carry my RTL2832U-based television receiver dongle with me (the so-called rtl-sdr). Luckily this was one of those days! I couldn't help but tune in to 450.2500 MHz and see what's going on there.

She goes out for a burger, and gets interested in how the pagers work, fishes out her RTL-SDR dongle (and laptop) which she just happened to have on her, and starts decoding the signals!

polymorph: That is one of the reasons why I want my SDR to have a stable oscillator.

I must admit that the one(s) I got don't seem to agree with the frequency of the local radio stations, and I presume they are more correct than my $20 dongle.

Hah! Twisted minds think alike. Nooelect now sells an RTL SDR modified in just the way I'm discussing.

That's what I would have expected from the start. Better late than never.

Yeah, it will cost me a lot less than $75 to build that, it won't look as purty but it'll be more satisfying.

Oops, I meant to say 14.4MHz doubled to get 28.8MHz. $3.50 each.

http://www.digikey.com/product-detail/en/ASVTX-09-14.400MHZ-T/535-9335-1-ND/675756

Their 28.8MHz oscillators are +-25ppm, not really better than just a crystal.

It is a 3V clipped sine wave, but since I'm doubling and buffering it anyway, that won't matter.

If you don't mind paying more, I see some on eBay for $18 to $20, but in through hole packages.

http://www.ebay.com/itm/1pcs-TCXO-28-8MHz-28-800M-Ultra-precision-Crystal-Oscillators-DIP-0-5ppm-NT-/171354014902?pt=LH_DefaultDomain_0&hash=item27e57ec4b6

This one claims +-0.1ppm for $20, I'm a bit skeptical.

http://www.ebay.com/itm/1PCS-TCXO-28-8M-28-800MHz-Ultra-precision-Crystal-Oscillators-DIP-0-1ppm-NT-/221703874521?pt=LH_DefaultDomain_0&hash=item339e94a7d9

In particular her page http://www.windytan.com/2013/09/the-burger-pager.html about decoding the workings of the "pager" you get given in cafés to let you know when your food is ready, is amazing.

If you think that is impressive, take a look at how she decoded a broadcast signal containing the real-time GPS location signals of a news helicopter: http://www.windytan.com/2014/02/mystery-signal-from-helicopter.html

She's a clever woman. I'm tempted to send a message "Wow, just wow" but I'm afraid it will come out lame.

Segueing onto this (from a link on that page): https://www.youtube.com/watch?v=uhIEfxRLiPI

The security guy on the clip was trying to restrict information so the scientist (Jodie Foster) says "you want to classify Prime numbers now?" to which he replies: "OK, they've got the primes". Classic.

A Spectrum analyser may help here, but it depends on how long the unknown transmissions are , and whether its possible to screen out all other transmissions in the same band whilst doing the measurement.
Whats the device thats doing the transmitting?