different waveforms from same transmitter?

[Insiderbe]

Hi all,

i'm transmitting from my PMR446 to my USRP N210

sometimes a receive a peak signal, other times something weird? How can i resolve this? 

the Peak in the first image is different everytime, the signal is sent on 446.00625 MHz and yet the usrp receives 446.00561MHz

--> calibration / synchronisation error? how can i fix this? :S

thanks in advance

[TomTr]

Hi Insiderbe,

I'm just wondering if you could give some further information about the "something weird" you get.

Is the signal you receive always off by the same amount? If this is the case then there could be some sort of Calibration error. 

Could this also be caused by noise interfering with the signal?

Kind regards,

Tom

[vvkakade]

Hi,

If the transmitter and receiver do not have the same ref clock driving them there can be some frequency offset between what you transmit and what you receive as far as I know.

If the Tx and Rx are both locked to same ref clock which is in turn used by the LO (local oscillator) chips you would see a tone at the exact same spot on both receiver and transmitter.

[alynch]

PMR446 is frequency modulated, so at any instant, the actual frequency will be within the 12.5 kHz channel centered on the transmit frequency. Your measurement is within that range, so that is not a sign of any problem.

[Insiderbe]

Hi everyone,

Thank you for your responses,

the 'something weird" is visible in the right image attached to my first post. is has no peak, just a lot of them 😮 There is no interference, because next to the USRP, I use a spectrum analyzer which doesn't receive something else.

There is no "fixed amount" of offset on the USRP, but there is a fixed amount of offset on the expensive spectrum analyzer. I measered it (see annex) after different times the USRP was started (10, 20 and 30 minutes)
So i think the USRP can't measure that precise... ?


@alynch; is it maybe because a PMR446 is cheap and has simple hardware that it's not that precise?