How to switch central frequency fastly with labview and USRP?

Question

Hello everybody,
         Recently I was making a frequency hopping receiver with labview and USRP N210 and SBX. The transmitter works on the 2.4G band, and it changes from one central frequency to another every 65536us. As I think, if I start receiving at the correct time, and  change the central frequency of the USRP every 65536us, I should be able to receive the data.
    But I find that, when usrp switch form one central frequency to a different one, it spends nearly 48ms and when it switch from one frequency to the same one, it only spends 3ms(I test the performance by modifying the niUSRP EX Rx Continuous Async Reconfig on the Fly.vi example). So if I want to follow the hopping sequency, I may can only receive 17ms data because I have to save time for switching central frequency. 
     My problem is that: why usrp changes central frequency so slow,  how can I make the process faster? If I want to follow the hopping sequency, can anybody give me some good advice? Thanks

Answer 1

There is an attribute "LO Frequency" that you can use to lock down the frequency of the local oscillator.  Then, when you set the "Carrier Frequency" attribute, the difference between the carrier frequency value and the LO frequency value will be realized by frequency shift in the DSP in the FPGA.  Changing this "digital" frequency shift value is much faster, since you don't have to re-tune the local oscillator.
Let's say you're sweeping from 2.44 GHz to 2.46 GHz.  Set the "LO Frequency" to 2.45 GHz then Commit.  Then, in your loop, program the "Carrier Frequency" attribute from 2.44 GHz to 2.46 GHz.  In the loop, it will only program the digital frequency shift, which will happen much faster.
Another tip is to use the property node to program only the attributes you want to change, rather than the niUSRP Configure Signal.vi (which sets many at once).
For more details, see the "Configuration:Advanced:LO Frequency Property" in the NI-USRP Help, and look at this example:
<LabVIEW dir>\examples\instr\niUSRP\niUSRP EX Fine-Tune LO Frequency.vi