RE: SDR transceiver having independent mixer and ADC sampling clocks

[Heikki Laamanen]

> So you want to produce something like a stable 10MHz reference and a 1PPS which is synchronous to UTC using broadcast/terrestrial signals such as DVB-T, ATSC, LTE, etc?  And your edges can be within +/-50ns of actual UTC?

 

Yes. All those signals (not sure about ATSC) are synchronized to UTC directly (via GPS) or indirectly (via PTP over fiber). Thus, the signals carry inherent timing information. There are naturally lots of issues related with radio wave propagation in each case like multipath propagation caused by reflections, groundwave/skywave phenomena concerning LF radio wave propagation, e.g. DCF-77 and the ancient C-Loran. The most ideal cases are line-of-sight wave paths like GPS satellite – GPS receiver.

 

I have one more question. I have not found any GNU Radio object utilizing the possibility of controlling the ADC sampling frequency via controllable PLLs on the RF HW. All sync-related objects seem to use – e.g., polyphase filters, for adjusting sampling phase. Am I right?

 

Heikki

 

 

From: Brian Padalino <address@hidden>
Sent: 25. helmikuutata 2020 21:37
To: Heikki Laamanen <address@hidden>
Cc: GNURadio Discussion List <address@hidden>
Subject: Re: SDR transceiver having independent mixer and ADC sampling clocks

 

On Tue, Feb 25, 2020 at 2:00 PM Heikki Laamanen <address@hidden> wrote:

Thanks Brian. You are right, there exists three VCOs and PLLs in the AD9361 which can be controlled independently.

 

AD9361 Reference Manual UG-570 shows a more detailed diagram in Figure 7. on page 20. The baseband PLL comprises an N/N+1 divider controlled by a high-precision sigma-delta modulator. AD9361 Register Map Reference Manual UG-671 describes that the control word of the sigma-delta modulator consists of 29 bits giving a relative frequency resolution of 1.9 ppb (parts per billion).

 

A brief description of the timing system.  The objective is a system for protecting or replacing GPS-based timing. In addition to navigation, the GPS system is used for generating an accurate clock which is in synchronism with the Coordinated Universal Time (UTC). A clock synchronized with the UTC, can be for example a 1 Hz physical clock signal whose rising edge goes up exactly at the start of an UTC second. The accuracy of an GPS-based timing system is about +-50 ns. Thus, we can say that the phase accuracy of GPS-based timing is about 50 ns.  The objective is to generate such clock by receiving a broadcast radio signal which is known to be in synchronism with the UTC.

 

You mentioned that time is completely relative. If we think of the UTC, we could say that it gives an absolute time which is defined by several atomic clocks around the globe. A copy of the UTC can be generated, with a certain accuracy, by receiving the GPS or other similar satellite signal (Galileo, Glonass, Beidou) and synchronizing to it.

 

So you want to produce something like a stable 10MHz reference and  a 1PPS which is synchronous to UTC using broadcast/terrestrial signals such as DVB-T, ATSC, LTE, etc?  And your edges can be within +/-50ns of actual UTC?

 

I guess I'm still unsure if you do or do not believe the radios out there allow for this to happen?  I'm not sure where you believe you have a limitation.

 

Brian