Hi,
I am facing a strange problem in synchronizing and recording data from multiple (4 or 8) N210s.
The problem:
After each data writing to the files on disk, it seems that each N210 re-tunes/ re-locks to the frequency references. It changes the fixed phase offset from channel to channel. It kills the purpose of providing a calibrating signal to ascertain the fixed channel-channel phase differences. Why?
Setup:
Eight N210+WBX devices are synchronized with external 10MHz and 1PPS signal. The data is routed from all channels through a gigabit switch to computer where GRC gnuradio-companion is being used on Ubuntu. Here a multi-usrp block is created, each channel tuned to the same frequency (900 MHz) with same sampling rate (1 MS/s). USRP source is connected to either a scope or file sink. The type of sink is chosen in runtime using a switch block in GRC.
The purpose is to apply and record a calibrating signal which is essentially a tone at the carrier frequency to find out the fixed phase offsets from channel to channel. This is used to apply the correction phase errors later to the actual signals of interest.
The problem occurs when I apply a calibrating signal and push the button to switch from scope sink to file sink for a couple of seconds. After I switch it back to scope, the channels have now a different phase offsets than before. It seems that channels have re-tuned and re-locked. So, this way, I am unable to know the phase offsets from channel-channel and hence the system cannot be calibrated.
The attached file shows a four channel setup. I have a similar eight channel one.
Any suggestions on why it is happening (re-tuning after file write) and how to calibrate the multi-channel system.
Thanks,
Khalid.
4Channel.png