Thank you Marcus for the feedback. Indeed I realized just after sending the post that I had omitted to use the external 10 MHz reference for clocking the FPGA. Although I had suspicions on the result, I have waited to return to the lab and run the experiment again with the correct settings before replying.
> Looking at the script, it looks like you've set the *time* source to > "external", but are still relying on the internal > clock for the actual reference clock that is used to drive everything > internally--including the ADCs, FPGA > DSP bits, etc. Corrected now, although I believe for the short measurement duration (3 seconds for each run) the TCXO should be close enough to the White Rabbit grand master reference to drift by a few samples at most (a 1 ppm frequency fluctuation would lead to 3 us fluctuation after 3 s at most, or 15 samples at the end of the record or 5 samples after 1 s when timestamping the first 1-PPS rising edge). > The "1PPS" on USRPs is used ONLY as a 1-time synchronization point for > resetting the timestamp clocks to a known value > *at a known time* (the 1PPS pulse). That timestamp register "ticks > over" based on either the internal reference clock, > or an external one, but you haven't requested external reference clock. As I understand using the internal reference instead of the external means I loose the alignement of the 1PPS edge with the clock driving the FPGA and hence might be off by +/-1 sample or +/-200 ns when sampling at 5 MS/s. Here I observe again 4 ms fluctuations of the 1PPS edge detection (bottom chart) overnight, the full experiment duration being 11 hours here. Another thing I fail to understand is that when I look at the timestamp (stat -c %y) of the file last access time (top chart) I do get the sawtooth shape of the timer control by NTP and I do get the sawtooth amplitude as inverse of the linux kernel timer rate so I'd say that the record starts at the right time with respect to the 1-PPS rising edge (otherwise the file timestamp would be randomly distributed, which is not the case, see inset) but I fail to understand how the 1PPS edge detection in the IQ stream (bottom chart) can fluctuate so much if the IQ file last storage time is consistent. Thanks. _______________________________________________ USRP-users mailing list -- [email protected] To unsubscribe send an email to [email protected]
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
#
# SPDX-License-Identifier: GPL-3.0
#
# GNU Radio Python Flow Graph
# Title: cm4_x310
# GNU Radio version: 3.10.5.1
from gnuradio import blocks
from gnuradio import gr
from gnuradio.filter import firdes
from gnuradio.fft import window
import sys
import signal
from argparse import ArgumentParser
from gnuradio.eng_arg import eng_float, intx
from gnuradio import eng_notation
from gnuradio import uhd
import time
class cm4_x310(gr.top_block):
def __init__(self):
gr.top_block.__init__(self, "cm4_x310", catch_exceptions=True)
##################################################
# Variables
##################################################
self.samp_rate = samp_rate = 5e6
##################################################
# Blocks
##################################################
self.uhd_usrp_source_1 = uhd.usrp_source(
",".join(("", '')),
uhd.stream_args(
cpu_format="fc32",
args='',
channels=list(range(0,2)),
),
)
self.uhd_usrp_source_1.set_clock_source('external', 0)
self.uhd_usrp_source_1.set_time_source('external', 0)
self.uhd_usrp_source_1.set_subdev_spec('A:0 B:0', 0)
self.uhd_usrp_source_1.set_samp_rate(samp_rate)
self.uhd_usrp_source_1.set_time_unknown_pps(uhd.time_spec(0))
self.uhd_usrp_source_1.set_center_freq(0, 0)
self.uhd_usrp_source_1.set_gain(0, 0)
self.uhd_usrp_source_1.set_center_freq(0, 1)
self.uhd_usrp_source_1.set_gain(0, 1)
self.blocks_interleave_0 = blocks.interleave(gr.sizeof_gr_complex*1, 1)
self.blocks_head_0 = blocks.head(gr.sizeof_gr_complex*1,
(int(6*samp_rate)))
self.blocks_file_sink_0 = blocks.file_sink(gr.sizeof_gr_complex*1,
'/tmp/data.bin', False)
self.blocks_file_sink_0.set_unbuffered(False)
##################################################
# Connections
##################################################
self.connect((self.blocks_head_0, 0), (self.blocks_file_sink_0, 0))
self.connect((self.blocks_interleave_0, 0), (self.blocks_head_0, 0))
self.connect((self.uhd_usrp_source_1, 1), (self.blocks_interleave_0, 1))
self.connect((self.uhd_usrp_source_1, 0), (self.blocks_interleave_0, 0))
def get_samp_rate(self):
return self.samp_rate
def set_samp_rate(self, samp_rate):
self.samp_rate = samp_rate
self.blocks_head_0.set_length((int(6*self.samp_rate)))
self.uhd_usrp_source_1.set_samp_rate(self.samp_rate)
def main(top_block_cls=cm4_x310, options=None):
tb = top_block_cls()
def sig_handler(sig=None, frame=None):
tb.stop()
tb.wait()
sys.exit(0)
signal.signal(signal.SIGINT, sig_handler)
signal.signal(signal.SIGTERM, sig_handler)
tb.start()
tb.wait()
if __name__ == '__main__':
main()
230720.pdf
Description: Adobe PDF document
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