Re: [Discuss-gnuradio] Recording continuous I-Q stream and frequency offset with an external reference clock

[Nazmul Islam]

John,

Thanks a lot for your email. My previous USRP2 modules had some old daughterboards. I am currently working with USRP N210. I am not sure even this one is getting the external reference, either. Couple of things:

1. I have the tools to probe the 10 MHz reference signal. But I did not find R523 :S. I can see R521, R522, R524, C523, etc. But I did not see R523 in USRO N210.

2. I have kept the J510 jumper at 1-2. Therefore, it should take signal from external reference clock instead of GPS.

3. When I use the "external" clock source option in UHD:USRP Sink block of gnuradio-companion, the E led turns off. On the other hand, if I transmit with "default" clock source option in UHD: USRP sink block, the E led turns on !!! It's very surprising ! 

4. I did another experiment to check if the USRP is locked to the external reference. At first, I gave an input of 10 MHz sine wave as an external source and transmitted a signal at 700 MHz carrier frequency. The carrier frequency option was given at gnuradio-companion software. Thereafter, I varied the reference clock frequency slightly (from 10 to 10.3, 10.4 MHz, etc.). During this period, I observed the signal at my laboratory spectrum analyzer. If there was a fixed gain that converted the 10 MHz reference source to 700 MHz carrier frequency, the transmitted signal's center frequency would have shifted from 700 MHz because the fixed gain would multiply 10.1 instead of 10 now. However, the center frequency did not change.

I suspect that my USRP is still locked to its internal clock source. Since this happened to two different USRP's, the issue might be in software. Has anyone used external clock source from a gnuradio-companion generated python code? I am changing the external clock reference source in GRC. Do I need to make any other change?

Thanks a lot for your help.

Nazmul

On Fri, Jun 8, 2012 at 12:53 PM, John Malsbury <address@hidden> wrote:

Nazmul,

Do you have the tools(o-scope) and capacity to probe the 10 MHz reference signal at various places?  Looking at the schematic, it looks like R523 is a good place to determine if the 10 MHz reference is a good place.  If you don't see the external reference across this resistor, there may be a problem with the reference input or conditioning circuitry.  If we eliminate this as a possibility we can investigate some others...

-John

-- 
Muhammad Nazmul Islam

Graduate Student
Electrical & Computer Engineering
Wireless Information & Networking Laboratory
Rutgers, USA.