# USRP1 Clock drift **URL:** \ **Category:** GNU Radio\ **Created:** [January 7, 2011, 12:02am UTC](https://www.ruby-forum.com/t/usrp1-clock-drift/201418 "2011-01-07T00:02:01Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![dubstep](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/cdc98d/32.png) [@dubstep](https://www.ruby-forum.com/u/dubstep)\ **Post date:** [January 7, 2011, 12:02am UTC](https://www.ruby-forum.com/t/usrp1-clock-drift/201418/1 "2011-01-07T00:02:01Z")
Hi Guys, I ran Kalibrate against my pair of USRP1’s using GSM900 towers (around 940MHz) and found my first unit to me 5kHz off and the second was 13kHz off. Is this within limits for the built in oscillator? I have been using them to transmit P25 frames around 435Mhz and found that my commercial P25 radios will not decode anything coming from the second USRP. The first USRP works OK. I have swapped around all daughterboards between both USRPs and the only common factor is the second USRP motherboard so I am sure its due to the clock in that unit. Perhaps clocktamer or a secondhand GPSDO is needed for my Tx applications. Cheers, Matt ---
**Author:** ![hazmatt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/b2d939/32.png) [@hazmatt](https://www.ruby-forum.com/u/hazmatt)\ **Post date:** [January 7, 2011, 12:43am UTC](https://www.ruby-forum.com/t/usrp1-clock-drift/201418/2 "2011-01-07T00:43:24Z")
On Fri, 2011-01-07 at 10:01 +1100, Matt R. wrote: > I am sure its due to the clock in that unit. This is within spec. It doesn’t tend to drift that much so if you characterize how far it’s off it’ll stay reasonably constant (to a few ppm) unless temperature changes drastically. > Perhaps clocktamer or a secondhand GPSDO is needed for my Tx applications. You will need an oscillator with a ~64MHz output to run the USRP1. There is no 10MHz reference input on USRP1. I used a GPSDO and a homebrew PLL based around an Si570 VCXO to keep mine synchronized, but it wasn’t a quick hack. Unfortunately there don’t seem to be many 64MHz TCXOs in a reasonable form factor out there. –n ---
**Author:** ![hazmatt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/b2d939/32.png) [@hazmatt](https://www.ruby-forum.com/u/hazmatt)\ **Post date:** [January 7, 2011, 4:15am UTC](https://www.ruby-forum.com/t/usrp1-clock-drift/201418/3 "2011-01-07T04:15:22Z")
Hi Matt; Is it possible to include in your application a monitor which would receive a transmission which you could use as a reference in order to correct your transmitter’s local oscillator? It seems to me that this would be a better solution to your problem than modifying the hardware in order to inject an external clock. On Fri, 7 Jan 2011, Matt R. wrote: > I am sure its due to the clock in that unit. > > Perhaps clocktamer or a secondhand GPSDO is needed for my Tx applications. > > Cheers, > Matt Cheers – Peter F Bradshaw: [http://www.exadios.com](http://www.exadios.com) (public keys avaliable there). Personal site: [http://personal.exadios.com](http://personal.exadios.com) “I love truth, and the way the government still uses it occasionally to keep us guessing.” - Sam Kekovich. ---
**Author:** ![hazmatt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/b2d939/32.png) [@hazmatt](https://www.ruby-forum.com/u/hazmatt)\ **Post date:** [January 8, 2011, 9:29pm UTC](https://www.ruby-forum.com/t/usrp1-clock-drift/201418/4 "2011-01-08T21:29:04Z")
Hi Mike/Nick, Thanks for your help, I tried offsetting the transmitter by a couple of kHz and it all came to life (isn’t software radio great like that!) The error of 13kHz was only at 940.2MHz, which is definitely within spec. As Mike explained this correlates to an error of about 6kHz @ 435MHz. Seems that the ppm unit of measurement had caught me out - once I figured out that it is a _ratio_ and not a specific value everything suddenly became clear. Cheers, Matt