[Discuss-gnuradio] Large RX Delay

[Thomas Schmid]

I am currently investigating different USRP delays. Some of them I can
explain, others not. For example, I see an average delay of 6.2ms
while receiving data from the USRP at a sample rate of 8MHz (short
real samples, i.e. I am using the usrp.source_s).
Here is my setup:
- I have a function generator which generates a 1Hz square wave. This
signal is fed into a LFRX on the USRP and into CH1 of an oscilloscope.
- On the PC side, I modified the null sink to check for a pos/neg
signal (with some thresholding). When the signal is high, I output 1
on the parallel port, when it is low, I output 0. The parallel port is
also hooked up to the Oscilloscope. This allows me to measure the
delay between the two signals.
- I usefusb_block_size=512, fusb_nblocks=1

Here are some numbers I got:
- decimation 8: mean delay 6.1ms
- decimation 32: mean delay 6.4ms
- decimation 64: mean delay 4.3ms
- decimation 256: mean delay 14.7ms

First of all, I don't understand why we have such a high delay.
Shouldn't it be more in the hundreds of /mu s instead of in the ms
range? Second, why is the delay shorter for decimation 64, and again
larger for a decimation of 256?

Thomas