Hi Matt, I'll try hooking up my ttl line on Monday (I have done that but not with our current configuration, so far only when I was playing around with building custom firmware).
Here's the official description of what we are doing (I hope this helps!): We have two USRP2s sharing common clocks via the MIMO cable. The first USRP2 receives a 20MHz input signal and outputs an amplitude and phase shifted copy of this same signal. Analog subtraction is performed between the input and output signals of the first USRP2. The result of this subtraction is an error signal that is used as an input to the second USRP2. The host implements a signal processing algorithm that adaptively estimates the amplitude and phase shift to apply to the 20MHz signal such that the resulting error signal at the input to the second USPR2 is driven to zero. This processing forms the foundation of a real-time adaptive closed-loop control system that has a myriad of diverse applications. The system described above appears to work well for relatively low throughput bandwidths (i.e. for USRP2 sample rates below about 3.1MHz, decim = 32). In addition, because of apparent latencies between the time at which the host updates its calculations and the time at which these calculations are reflected in the output of the first USRP2, we must use additional decimation in the host by a significant number of samples (something like 512 is typical). So the effective closed-loop throughput bandwidth is approximately 100MHz divided by decim * 512 or only about 6kHz. We would, of course, like to run this system with as much throughput bandwidth as possible and we are not sure at this point where the biggest bottleneck is coming from. I would only add that what is referred to as the "first" USPR2 above does input and output through eth0 (the controller on the motherboard - this is on a quad-core Lenovo running the latest 64-bit Ubuntu). The second USRP2 (the slave, set up to get its clock through the MIMO cable from the first USRP2) is receiving date through eth1. My code (the signal processing algorithm that drives the error signal to zero) seems to "blow up" after about 30 seconds unless I set RX to OFF on eth0. -Tom On Fri, Jan 15, 2010 at 9:21 PM, Matt Ettus <m...@ettus.com> wrote: > On 01/15/2010 03:53 PM, Tom Gross wrote: >> >> Thanks Matt, Eric and Jonathan (hope I didn't forget anyone. :-) ). >> >> We greatly appreciate the information and need to think about stuff on >> our end. I've been deliberately vague about our application (not that >> I could really explain it even if I felt authorized discuss it). The >> thing to remember is that we believe (maybe we are fooling ourselves) >> that we see easily reproducible problems when RX is ON but not when RX >> is OFF. > > It is very hard to help when we don't have information about what you are > trying to do. The important piece of information is that you are > transmitting and receiving at the same time when you see this problem. This > indicates that there may be flow control tuning issues. > > Is the RX stream ok and the TX has a problem? Or is it that the TX is ok > and the RX has a problem? Or is it both? > > Do you have a TTL serial port hooked up to J305? Do you see characters > there? Do you see "S" characters on the receive application window? > > Are you trying to use 2 separate programs (1 tx, 1 rx) to talk the the > USRP2, or are they in the same app? > >> One more question was just sent to me to pass on, while I was >> composing this email: >> >> crazy idea: is there any way to "clock" the host, i.e. keep track of a >> time stamp in the host and gate/trigger the host outputs at a constant >> sample rate that is consistent with the sample rate of the USRP2? > > No, for 2 reasons: > - Even if the host had a clock, clocks drift relative to each other > - The USRP2 might need to hold off on sending for some reason. > > This is a system that requires feedback and there is no way around it. On > the USRP1 the feedback is done by the flow control built into the USB > protocol. On the USRP2 the feedback is done by the flow control built into > ethernet. You could imagine doing a different feedback mechanism using your > own protocol, but it would still involve the device telling the computer > when to go and when to stop. > > Actually there is one way around it -- have infinitely large buffers in the > USRP2, but that would add to the cost :) > > Matt > _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
