[time-nuts] RE: phase locking Rb to GPS (was time-nuts Digest, Vol 14, Issue 30)
Richard H McCorkle
mccorkle at ptialaska.net
Sat Oct 22 22:53:01 EDT 2005
While the Brooks Shera (http://www.rt66.com/~shera/index_fs.htm) GPS OCXO
Controller design is excellent for use with an HP 10811, there are
challenges to using the standard design with low sensitivity oscillators.
The Shera controller uses a 24MHz phase counter design with the gain set for
a 7.5e-9 / volt sensitivity over a +/- 3 volt span, or a 4.5e-8 controller
span. To interface the controller to a typical HP 10811 the output is fed
through a divider to the bottom of a frequency trim pot. A precision
reference voltage is fed to the top of the pot, and the oscillator EFC input
is driven from the wiper. The frequency trim pot provides a fixed offset
voltage to set the frequency, and this voltage is "modulated" by the
controller output to maintain GPS lock. This design works well with
oscillators capable of frequency spans of 1e-7, and sensitivities of >
1.5e-8/volt. Less sensitive oscillators require different methods to match
the oscillator sensitivity to the controller span.
In interfacing an Isotemp mil spec version of the HP 10811B with a
sensitivity of 9.7e-9/volt and a span of 4.85e-8 to the Shera controller
even with RA and RB removed and the DAC feeding the frequency trim pot
directly the pot acts as a divide by 2 to the controller output; and
additional gain would have been required to match the controller span.
Adding an additional gain stage after the DAC output results in amplifying
the DAC noise, which can adversely affect the short-term stability of the
oscillator. By feeding an inverting summing amplifier and inverter with the
offset and controller outputs and driving the oscillator from the inverter
output, an effective gain of 2 was realized by removing the frequency trim
pot from the controller path without adding additional gain to amplify the
DAC noise. Using this arrangement allowed the lower sensitivity Isotemp
oscillator to be matched to the 4.5e-8 controller span without adding
additional gain.
Rubidium oscillators can be used with the standard Shera design, but
the low sensitivity (1e-9/volt for my LPRO, 4.5e-10/volt for my FRS-C)
requires the gain to be 9x - 20x normal to match the controller span. (Up to
40x for the FRS-C with the original frequency pot arrangement!) Multiplying
the DAC noise by a factor of 9x - 20x is not a good approach for maximizing
short-term stability in a precision oscillator. The available span for the
LPRO is 5e-9, so it can only use 12% of the available 4.5e-8 controller span
if matched with an external gain stage. The FRS-C has a span of 2.25e-9, and
can use only 5% of the controller span. Brooks will share his source code if
you ask him nicely, so modifications to the controller software are
possible. But just increasing the gain 2x in the controller software
requires changing the limiter and restricting the span in the lower filter
modes, and higher gains result in other design issues making an additional
gain of 2x the practical limit, and only with additional work on the
limiter.
There are other options to achieve the required gain, and for rubidium
oscillator control they are appropriate. One method to increase the gain is
to use a faster clock and a shorter sample time. Operation of a similar
design using 74F163 counters at speeds up to 125MHz is possible. Due to path
delays encountered in high-speed operation, layout becomes a major factor
for reliable operation at speeds above 50MHz. Standard perf-board techniques
work well up to 50MHz, so a design was tested using two 74F163 counters, a
74HC165 shift register, and a 50MHz oscillator in the phase detector. The
counter is read and reset each second and the results accumulated in the
PIC. The result is scaled for the sample time chosen (divide by 2 for 60
sec) and fed to the process. The sample divider used was an ST 74HC393 dual
binary counter, with one divider used to divide the 10MHz oscillator down to
625KHz to feed the phase detector and the second divider used to divide the
50MHz clock by 4 to supply a 12.5MHz clock for the PIC. The faster clock
reduces the controller span from 4.5e-8 to 2.16e-8, still well beyond the
span of a rubidium oscillator. It also increases the controller sensitivity
to 3.6e-9/volt giving an effective gain of 2.08 over the original 24 MHz
design.
Another approach to increasing the effective gain is to increase the
filter constant. This requires more stability in the oscillator to be
effective, which is just what a rubidium oscillator offers. Each doubling of
the filter constant effectively increases the gain by 2. Increasing the
filter constant becomes the best way to get the gain required for a rubidium
oscillator controller without adding an additional gain stage. Increasing
the sample time to 60 sec results in a controller sensitivity of 9e-10/volt
with a 5.4e-9 span using a 50 MHz clock. This configuration will properly
drive an LPRO with 1e-9 / volt sensitivity using 93% of the available
controller span. For the lower sensitivity FRS-C, the F1 filter constant is
adjusted 1 step to scale the filter by an additional factor of 2. This
results in a controller sensitivity of 4.5e-10 / volt and a span of 2.7e-9.
This provides a proper match to the FRS-C sensitivity, using 83% of the
available controller span.
Because a rubidium oscillator has poorer short-term stability when
compared to a good OCXO, short-term variations in EFC should be suppressed
when using a rubidium oscillator, and the Shera design has an Alpha filter
available to do just that. Once stable in your selected mode, the Alpha
filter should be employed to maximize short-term stability. Keep in mind
that the filter constant is now 2x or 4x longer than the stock controller so
you are effectively starting the IIR filter in mode 3 or 4 and going up from
there to effectively mode 8 or 9. The filter settle time for the highest
modes become 1 to 2 days, and this can be too long to correct for daily
variations in the environment. Settle times of about 12 hours produced the
best long-term stability, with typical 1 day stability of < 5e-13 being
achieved using a combination of these techniques on an LPRO.
----- Original Message -----
From: "Christopher Hoover" <ch at murgatroid.com>
To: <time-nuts at febo.com>
Sent: Sunday, September 18, 2005 10:40 AM
Subject: [time-nuts] RE: phase locking Rb to GPS (was time-nuts Digest, Vol
14, Issue 30)
>
> On 9/18 "Tom Clark, W3IWI" <w3iwi at toad.net> wrote:
>
> The frequency knob that you tweak to correct the Rb frequency
> passes some tens of ma thru a coil surrounding the RF interaction
> region. If you try to phase lock a Rb to GPS, you need to develop
> a current source error signal.
>
> Hmmm ... can you say more about this current source error signal?
>
> I've been thinking of locking my Rb standard to GPS. I was simply going
to
> use the same circuit (1 pps, 10 MHz in; analog EFC voltage out) that I
have
> running with my OXCO but with different filter and EFC DAC coefficients.
>
> So is this not sufficient to phase lock GPS to a Rb standard?
>
> Naively,
> -- Christopher.
>
>
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