Empathy List Archives

SJ

Sherman, Jeffrey A. (Fed)

Wed, Jun 1, 2016 3:45 PM

Jim Lux:
If you pick the right USRP models, you can lock the sampling clocks
together or distribute the clock. I don't know if that distribution is
sufficiently high quality for time-nuts kinds of applications.

A bit of extra detail related to this but not reported in print... The N210 has two means of locking the sampling clocks of two SDR units together. First, a 10 MHz reference signal can be split and input into the two units' reference ports. Second, a "MIMO link" can be made between the two units with a SFP-style "direct-attach" cable. I don't know the details of this digital link, but it supports data transfer (so only one of the two SDR units requires an Ethernet connection) and reference frequency/time transfer.

Is the digital MIMO-link method any worse than the analog splitter method? Over short averaging intervals (1 us through ~30 us), we resolved no degradation in a 1-channel measurement. This is consistent with the advertised bandwidth of the PLL ~3 kHz. Over intervals 1 ms through 10 ms, the MIMO-link reference method resulted in about a factor-of-2 worse time deviation (in this test, the NCO was turned for a heterodyne frequency of approximately 8 Hz, which also leads to an oscillation peak in this range). Beyond 10 ms, both methods showed the same ~150 fs flicker floor that we've attributed to the ADC aperture jitter.

Finally, the SDR also has a PPS input, which can be used to "name" a 100 MHz master clock edge as an epoch (with 10 ns resolution). Although I didn't test this, I think this epoch can be synchronized over the MIMO link.

Best wishes,
-js

Jim Lux: If you pick the right USRP models, you can lock the sampling clocks together or distribute the clock. I don't know if that distribution is sufficiently high quality for time-nuts kinds of applications. A bit of extra detail related to this but not reported in print... The N210 has two means of locking the sampling clocks of two SDR units together. First, a 10 MHz reference signal can be split and input into the two units' reference ports. Second, a "MIMO link" can be made between the two units with a SFP-style "direct-attach" cable. I don't know the details of this digital link, but it supports data transfer (so only one of the two SDR units requires an Ethernet connection) and reference frequency/time transfer. Is the digital MIMO-link method any worse than the analog splitter method? Over short averaging intervals (1 us through ~30 us), we resolved no degradation in a 1-channel measurement. This is consistent with the advertised bandwidth of the PLL ~3 kHz. Over intervals 1 ms through 10 ms, the MIMO-link reference method resulted in about a factor-of-2 worse time deviation (in this test, the NCO was turned for a heterodyne frequency of approximately 8 Hz, which also leads to an oscillation peak in this range). Beyond 10 ms, both methods showed the same ~150 fs flicker floor that we've attributed to the ADC aperture jitter. Finally, the SDR also has a PPS input, which can be used to "name" a 100 MHz master clock edge as an epoch (with 10 ns resolution). Although I didn't test this, I think this epoch can be synchronized over the MIMO link. Best wishes, -js

J

jimlux

Wed, Jun 1, 2016 6:07 PM

On 6/1/16 8:45 AM, Sherman, Jeffrey A. (Fed) wrote:

Jim Lux:
If you pick the right USRP models, you can lock the sampling clocks
together or distribute the clock. I don't know if that distribution is
sufficiently high quality for time-nuts kinds of applications.

A bit of extra detail related to this but not reported in print... The N210 has two means of locking the sampling clocks of two SDR units together. First, a 10 MHz reference signal can be split and input into the two units' reference ports. Second, a "MIMO link" can be made between the two units with a SFP-style "direct-attach" cable. I don't know the details of this digital link, but it supports data transfer (so only one of the two SDR units requires an Ethernet connection) and reference frequency/time transfer.

Is the digital MIMO-link method any worse than the analog splitter method? Over short averaging intervals (1 us through ~30 us), we resolved no degradation in a 1-channel measurement. This is consistent with the advertised bandwidth of the PLL ~3 kHz. Over intervals 1 ms through 10 ms, the MIMO-link reference method resulted in about a factor-of-2 worse time deviation (in this test, the NCO was turned for a heterodyne frequency of approximately 8 Hz, which also leads to an oscillation peak in this range). Beyond 10 ms, both methods showed the same ~150 fs flicker floor that we've attributed to the ADC aperture jitter.

Finally, the SDR also has a PPS input, which can be used to "name" a 100 MHz master clock edge as an epoch (with 10 ns resolution). Although I didn't test this, I think this epoch can be synchronized over the MIMO link.

Interesting..

We were doing some work with the earlier USRPs and wanted to actually
run the ADC tied to an external clock at a peculiar rate - which it
turns out the USRP doesn't support: you can lock the internal clock to
an external reference (or to a signal from another USRP), but there's
that PLL in the mix, so you have the usual issues. I think the MIMO
link actually transfers the same clock around (so you can guarantee
synchronous sampling).

But ultimately, we wound up going another direction - we needed the
"external clock" input.

I think the MIMO transfer doesn't go through the PLL

On 6/1/16 8:45 AM, Sherman, Jeffrey A. (Fed) wrote: > Jim Lux: > If you pick the right USRP models, you can lock the sampling clocks > together or distribute the clock. I don't know if that distribution is > sufficiently high quality for time-nuts kinds of applications. > > A bit of extra detail related to this but not reported in print... The N210 has two means of locking the sampling clocks of two SDR units together. First, a 10 MHz reference signal can be split and input into the two units' reference ports. Second, a "MIMO link" can be made between the two units with a SFP-style "direct-attach" cable. I don't know the details of this digital link, but it supports data transfer (so only one of the two SDR units requires an Ethernet connection) and reference frequency/time transfer. > > Is the digital MIMO-link method any worse than the analog splitter method? Over short averaging intervals (1 us through ~30 us), we resolved no degradation in a 1-channel measurement. This is consistent with the advertised bandwidth of the PLL ~3 kHz. Over intervals 1 ms through 10 ms, the MIMO-link reference method resulted in about a factor-of-2 worse time deviation (in this test, the NCO was turned for a heterodyne frequency of approximately 8 Hz, which also leads to an oscillation peak in this range). Beyond 10 ms, both methods showed the same ~150 fs flicker floor that we've attributed to the ADC aperture jitter. > > Finally, the SDR also has a PPS input, which can be used to "name" a 100 MHz master clock edge as an epoch (with 10 ns resolution). Although I didn't test this, I think this epoch can be synchronized over the MIMO link. > > Interesting.. We were doing some work with the earlier USRPs and wanted to actually run the ADC tied to an external clock at a peculiar rate - which it turns out the USRP doesn't support: you can lock the internal clock to an external reference (or to a signal from another USRP), but there's that PLL in the mix, so you have the usual issues. I think the MIMO link actually transfers the same clock around (so you can guarantee synchronous sampling). But ultimately, we wound up going another direction - we needed the "external clock" input. I think the MIMO transfer doesn't go through the PLL

KR

Kevin Rosenberg

Fri, Jul 29, 2016 9:05 PM

Jeff,

Thanks for your very useful paper Oscillator Metrology with SDRs[1]. I created a C++ program and checked residuals using a 10 MHz clock split to the A and B channels of a LFRX and BasicRX boards and sampled at 1 Mhz. Using boxcar averaging of 1000 samples at 1 kHz, I was impressed by the low noise floor approaching that of my Timepod which was several times the cost. I included the Allan Deviation without averaging showing the sqrt(1000) increase in noise floor without the averaging[2].

I had a question about your experience. You mentioned using a input signal near the maximum of the USRP’s ADC to get the best SNR. I reviewed the schematics and application notes. I found a maximum Vpp mentioned of 3.3V. I was wondering what voltage you were using to drive the USRPs. When I go above 1.5-2 Vpp, I start getting signal distortions and not much increase in the amplitude.

Many thanks for publishing your work in this area.

Kevin

[1]
https://arxiv.org/abs/1605.03505

[2]

Jeff, Thanks for your very useful paper Oscillator Metrology with SDRs[1]. I created a C++ program and checked residuals using a 10 MHz clock split to the A and B channels of a LFRX and BasicRX boards and sampled at 1 Mhz. Using boxcar averaging of 1000 samples at 1 kHz, I was impressed by the low noise floor approaching that of my Timepod which was several times the cost. I included the Allan Deviation without averaging showing the sqrt(1000) increase in noise floor without the averaging[2]. I had a question about your experience. You mentioned using a input signal near the maximum of the USRP’s ADC to get the best SNR. I reviewed the schematics and application notes. I found a maximum Vpp mentioned of 3.3V. I was wondering what voltage you were using to drive the USRPs. When I go above 1.5-2 Vpp, I start getting signal distortions and not much increase in the amplitude. Many thanks for publishing your work in this area. Kevin [1] https://arxiv.org/abs/1605.03505 [2]

time-nuts@lists.febo.com

Re: [time-nuts] Commercial software defined radio for clock metrology