Hi Jerry,
Thanks for that update on the FA-2. I had the opportunity to test one
last week. Comments:
> the unit isn’t competitive with the TICC2 and TADD combo
They are quite different instruments: one is a timestamping or time
interval counter, and the other is a frequency counter. Using a
frequency counter as a LF time interval counter or using a time interval
counter as a RF frequency counter will not often yield best results.
Note that both instruments can achieve on the order of 11 digits/second
of resolution in ideal cases.
One significant difference is that the FA-2 has a large dead time, the
tau is not actually 0.1 or 1 or 10 s, and it reports only frequency and
not phase. This may have implications for using it for long-term
monitoring. I'm doing some tests to quantify this effect. It wouldn't be
the first time a BG7TBL product struggled with the concept of phase vs.
frequency measurement.
> the returned values are (around 1.000 000 000) 1.000 000 005 and
0.999 999 995
Correct. The FA-2 does not do well with LF signals; for that its best to
use a time interval counter, such as the TAPR/TICC (which has ~0.1 ns
single-shot resolution). The FA-2 manual implies the core counter has
only 10 ns resolution so for slow signals (such as 1PPS) you're limited
to 8 digits/second resolution. The PLL-based (?) enhanced resolution
kicks in above 100 kHz.
> never seen anything come back from it other than 10:9 or 10.9 format
Right. Counter resolution is often quoted as "digits per second". This
counts *all* significant digits, not just *decimal places* (digits to
the right of the decimal point). For example, a reading of 9 999 999.976
has ~10 digits of resolution. And a reading like 10 000 000.000 32 has
~12 digits of resolution. I can explain more if you want and how this
ties to tau and ADEV, etc.
> the 1PPS out of an HP 5061B Cs
The 5061 is known for an unusually powerful 1PPS output. Beware. It can
fry the front-end of some counters.
> someone should start a group to move this off list
Probably not. Discussion of frequency counters is about as on-topic as
one can get here on this list. If we started a new group every time a
shiny new GPSDO or counter showed up on eBay the list would have a
hundred subs by now. What is helpful are postings like yours that
combine multiple issues and discuss them in depth. By contrast, short
stream of consciousness postings tend to fill up the list and push busy
people away.
/tvb
On 10/4/2019 10:38 AM, Jerry Hancock wrote:
I noticed you can change the baud rate to for instance, 115200, but I can’t get
it to stay persistent after power off. You send it a command of $B11520* to
change it to 115,200, but since it only sends like once every 300msecs, it
doesn’t really matter the baud.
I found the unit isn’t competitive with the TICC2 and TADD combo. When
measuring a 1PPS signal, the returned values are (around 1.000 000 000) 1.000
000 005 and 0.999 999 995. Note the ‘5’ as the last digit. When measuring
signals greater than a certain frequency that escapes me, the last digit will
then add more resolution switching from units of 5 to 1.
Also, in the doc they talk about digits returned from 8 - 12. I’ve never seen
anything come back from it other than 10:9 or 10.9 format depending on your
coding skills. I tried putting it into ‘High precision” mode by sending it a
$E3434* command.
You can change the gate on the fly using the $G* command. It cycles through
the gate times opposed to being able to set it directly. There are a number of
persistent commands that enable default startups, etc.
And one other issue, though it has a somewhat high input level, I noticed I had
to put an attenuator on it for measuring the 1PPS out of an HP 5061B Cs.
Otherwise it returned a time of 3hz instead of 1hz for some reason. All done
in 50 ohm mode.
Otherwise a nice unit for the money. I’m thinking someone should start a group
to move this off list.
Regards,
Jerry
On Oct 3, 2019, at 8:59 PM, Mark Sims <hol...@hotmail.com> wrote:
I did a test of the FA2 internal OCXO warm up time. I powered mine down for 4
fours and then powered it up and measured the frequency as it warmed up. It
looks like it takes 45 minutes to stabilize.
Note that it took a week of constant power for the oscillator drift rate to
settle down. The drift on mine seems to be around 0.00025 Hz/day. I need to
do a longer test to better weed out temperature sensitivity.
BG7TBL internal oscillator warm up time:
Minutes Freq error
0 221 Hz
1 17 HZ
2 0.2760 Hz
3 0.0500 Hz
4 0.0350 Hz
5 0.0240 Hz
10 0.0130 Hz
15 0.0080 Hz
20 0.0060 Hz
25 0.0050 Hz
30 0.0042 Hz
35 0.0040 Hz
40 0.0034 Hz
45 0.0021 Hz <--- stable
50 0.0025 Hz
55 0.0022 Hz
60 0.0020 Hz
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
