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using xtal other than 28.8 MHz

Posted: Tue Jul 14, 2015 5:43 pm
by scottrf
I would like to use a 26 MHz TXCO with the RTL-SDR. I see where the software allows selection of a different tuner XTAL, but when I try using a frequency much different than 28.8 MHz, the USB does not register. I drove the clock input with a function generator, and it works with 28.8 MHz, but deviating much from that the USB doesn't work.

I bought a cheap 28.8 MHz TXCO on eBay: ... 5d5a16d41b
but the phase noise was HORRIBLE. It was on frequency, but the noise is so bad it is totally useless for my application (1.4 MHz OFDM). With the standard 28.8 MHz crystal, my receiver works, but the sampling and frequency drift are problematic. My receiver tracks and compensates for frequency error in software, but it doesn't compensate for sampling error (I don't have time to resample in software).

26 MHz TCXOs are easy to find at low prices. I have 1ppm devices that are less than $4 US. Does anyone know how to get the USB driver to work with a non-28.8MHz clock?

Re: using xtal other than 28.8 MHz

Posted: Tue Jul 14, 2015 10:26 pm
by rtlsdrblog
I don't think it's possible to use an oscillator of a different frequency. A few people have discussed this before, and the reason it can't be done is that the RTL2832U needs the 28.8 MHz clock for USB operation, no other frequency will work.

This might help? ... local.html

Out of interest, what would you estimate the phase noise specs of that TCXO were?

Re: using xtal other than 28.8 MHz

Posted: Wed Jul 15, 2015 4:14 am
by scottrf
Thanks for the link.

I didn't measure the phase noise, I just looked at the spectrum on an analyzer, and it was lousy. When I use this TCXO with the dongle, if I put in an offset carrier I see quadrature sine waves on I and Q, but they are swamped with noise. It isn't pretty. At first I thought I might be over-driving the chip, or that the supply was noisy (I tapped off the 3.3v supply), but it was neither of these things. With a clean supply, and just going straight into a spectrum analyzer it looked bad. I tried DC block, different loading... Too bad to bother quantifying how bad.

I wish 28.8 MHz was a standard frequency that you could order on Digikey, Mouser, etc. Anyone know of a source that comes with specs? I'm reluctant to try another eBay source. I'll probably ask some reps.

Re: using xtal other than 28.8 MHz

Posted: Thu Jul 16, 2015 8:26 am
by rtlsdrblog
Yeah 28.8 MHz is pretty rare, you need to usually buy in bulk. I think some companies like Golledge will supply low quantity custom oscillators, a few people have gotten 28.8 MHz TCXO's from them before.