Cross posting this from the Yahoo group mailing list so that it does not get lost in the sands of time, as it is valuable discussion worthy of preserving:
John Ackermann N8UR firstname.lastname@example.org
8:28 AM (3 minutes ago)
Rob, you're correct that the ANAN and other HPSDR radios use a PLL to
lock their internal reference to an external source. More precisely,
there is a very low phase noise 122.88 MHz VCXO that is locked either to
an internal 10 MHz TCXO or external reference.
But an important note on external references with the HPSDR radios. A
few of us have tried to use Hermes-based radios for precise frequency
measurements and have discovered that there is an issue in the PLL
circuit that results in phase jumps at random intervals (usually in the
10s of seconds apart). These are severe enough to destroy sensitive
measurements. I think they are inherent to all the radios that derive
from the Hermes schematic and firmware.
After experimenting we've found one fairly simple fix that at some point
Phil will put into the production firmware. But even after that fix
there is a problem (though of much smaller magnitude) that I haven't
been able to track down yet. As I have time I'm continuing to work on that.
This problem doesn't affect phase noise or real-world frequency
stability, but does show up in sensitive measurements as random "ticks"
in the transmitted phase.
On 08/31/2018 01:35 AM, Rob Sherwood. email@example.com
> Hi Scott,
> I don’t know anything about the oscillators or GPSDOs you mentioned.
> My main Denver Rubidium is an Efratom M-100 military unit I purchased
> NOS at least 15 years ago that has been running 25/7 ever since. The
> backup Rb in Denver and my only Rb at Ault are Lucent branded with
> Efratom physics packages. My only GPSDO is a rack mount Trak Systems in
> Denver I use to verify my Rubidiums are happy. They all test very clean
> with noise about -135 dBm / Hz at 10 kHz. Not like my Wenzel, but
> reasonable house standards from a frequency standpoint. The Austron
> distribution amp feeds all the HP test equipment except the 8662As and
> the 8642As in Denver.
> I will be locking the 7000DLE to the Rb at Ault. I have two 8642As at
> Ault, so one of them will be my signal source. I have no idea which
> clock is in the 7000. Has that changed over time? Warren said an
> external reference locks to the external reference as opposed to
> replacing it.
> As a second test I’ll feed in the other 8642A in as the reference. The
> 8642A noise at 10 kHz offset is about -153 dBm/Hz.
> What level is optimum for feeding in the 10 MHz reference to the 7000DLE?
> Rob, NC0B
> Sent from my iPad
> On Aug 30, 2018, at 4:03 PM, Scott Traurig firstname.lastname@example.org
> <mailto:email@example.com> [apache-labs]
> <firstname.lastname@example.org <mailto:email@example.com>> wrote:
>> That will be very interesting, Rob. Do you have BG7TBL unit there and,
>> if so, which OCXO is it equipped with? That makes a big difference, of
>> course. The one I have here is equipped with the excellent Russian
>> Morian MV89 OCXO..
>> On August 30, 2018 5:51:28 PM EDT, "Rob Sherwood. firstname.lastname@example.org
>> <mailto:email@example.com> [apache-labs]" <apache-labs@yahoogroups..com>
>> I’ll test this, too, on a 7000DLE on Sunday or Monday. Rob, NC0B
>> Sent from my iPad
>> On Aug 30, 2018, at 2:56 PM, firstname.lastname@example.org
>> <mailto:email@example.com> [apache-labs] <firstname.lastname@example.org
>> <mailto:email@example.com>> wrote:
>>> I have a GPS Disciplined Oscillator, Model BG7TBL that I got on
>>> Ebay from China that I used to use with my 200D. I now am using
>>> a 7000DLE and I'm wondering if this unit will degrade my receiver
>>> in regard to phase noise. I believe Scott you are using or were
>>> using this model of GPSDO
>>> Frank, W2SDR