Some time ago I bought some cheap RTL-SDR dongles from ebay. They were supposedly 1ppm with a TXCO built in.
They turned up and I plugged them in, and using Gqrx tuned them to a local P25 beacon that I know. surprise, surprise, the SDR dongle was a mile (12 odd khz) off frequency. Upon pulling the thing apart, I found that there was a standard rock installed. Also this board is sold as a cheaper unit, and still has the IR receiver installed… direct ripoff. I turned the board over, as there was another board sandwiched on the back. On this board is an sma connector, a few inductors, a small transistor amp and WOW, a 28.8Mhz TXCO sitting there not hooked up.. Circled on the following photo, is the pad it connects to, however this has not been soldered as the standard crystal is still installed. Checking, the TXCO is powered by 5V, Oscillates quite happily and doesn't go anywhere. Well, me being me, I couldnt help but smash the standard crystal off the board and throw a bit of solder down the hole: And well, for 5 mins of work, and a $30 chinese dongle, I now have an SDR Dongle that is on frequency, and stays there!