Andy
Maybe I am missing something here, (and I am certainly not trying
to detract from an excellent piece of work) but couldn't you also take
the one second markers from MSF using an AM receiver, and drive
your circuit with that?
Related to that you might also be able to source the 1 sec pulses
from a partially dissassembled MSF (or DCF77) clock.
Thanks & 73
Stewart G3YSX
Talbot Andrew wrote:
Looking at frequency standards for LF, the requirements are slightly
different than for the higher frequencies. For microwaves, where
instantaneous frequency (over a few seconds) needs to be very good to avoid
chirp on SSB or CW, a high stability oscillator has to be used as part of
the Phase Locked Loop, locked to a master reference source. The master
source can be off air such as Droitwich or TV Sync pulses in which case loop
bandwidths can be made wide enough that lockup to a few parts in 10^-9 is
possible in minutes. Both these are very good in the short term, but may
have glitches or anomalies if relied on for hours at a time. Another
standard in use is that by Brooks Shera that locks a high quality VCXO to
1PPS from a GPS receiver - requiring hours to lock up and a very good VCXO.
In all cases long term accuracy is that of the standard used - typically
parts in 10^-10 or better. Designs for all these have appeared in Amateur
publications over the last few years.
For LF, however, particularly where we are integrating over many seconds
worth of data, the requirement for short term stability goes away, provided
this period is significantly shorter than the signalling interval; long term
stability is now even more important. So the requirement for the high
stability VCXO has gone, and all we need is a locking scheme that can
maintain phase to within a few degrees over a few seconds, and in the long
term remain perfectly locked to the master reference without cycle slippage.
Here a GPS receiver really excells itself. Rather than try to phase lock an
oscilltor at a 1Hz reference frequency which would lead to inordinately long
lockup times, I have used a frequency locked loop, based very roughly on the
old Huff & Puff stabiliser published in the 1970s. A sort of H & P
stabiliser Mark 3.
The idea is this :
A VCXO runs at any frequency that is an exact multiple of 1Hz (I use
4.194304MHz ). This directly clocks an 8 bit synchronous counter made up of
74HC161 chips. The outputs of this are connected to an 8 bit latch,
74HC374, and the 1 Pulse per Second signal from a GPS receiver module
latches the count once per second. The latch outputs then contain the
counter contents, updated very second. For frequencies that are an exact
multiple of 256Hz, the reading should therefore not change. For frequencies
that are not an exact multiple of 256, the count will increment each second
by (Frequency MOD 2565). If the frequency deviates slightly from its
correct value, the count will increment each second by 1 for every 1Hz in
error. By not resetting the counter, as is done in normal frequency
counters, the effect is more of a phase detector than a frequency counter as
any error leads to a cumulatively increasing count.
A PIC interrupted by the 1 PPS signal then reads this latched figure, and
calculates the error from a nominal mid value of 128. Using a PIC here
allows a calculation to be made for any frequency, not just a multiple of
256Hz. The direction and magnitude of the error count is then used to
drive a charge pump, which in turn drives the varicap diode of the VCXO.
The effect is to keep the VCXO precisely locked in the long term to the GPS
signal, although in the short term it's instantaneous phase is jittering,
and therefore the frequency is shifting by a Hz or two every second. By
apropriate choice of charge pump R/C values, the jitter can be minimised.
When this source is subsequently divided down to LF, the phase shift is
reduced by the division factor. The PIC includes an initiallisation
routine to force the charge pump to a mid voltage, which is close to that
needed for zero frequency error, so the loop can lock up in less than five
minutes. In comparison, a conventional PLL with 1Hz reference would needs
over 20 minutes even if the capacitor can be precharged AND the two pulse
edges forced into synchronisation by allowing the GPS to reset the divider.
Results so far are encouraging. The residual phase blip when divided down
from 4.194..MHz to 137kHz is about 10 - 20 degrees over a 1s period, and
when averaged out over a typical 30s signalling period amounts to less than
1 degree overall. Long term, when compared locally to other frequency
standards available(Caesium, Droitwich, TV Sync) there is no overall phase
shift of the 137kHz signal visible after many hours of monitoring, other
than the propagation effects of the latter two standards themselves.
A GPS receiver may seem an extravagance, but its value for LF signalling
will be immense ! As well as providing the ultimate long term accuracy for
frequency, by timing PSK signalling to GPS pulses as well, the requirement
for data clock recovery is removed, so gaining many potential dB's in S/N
capability. By defining the starting phase as being at particular time,
even the requirement for differential coding has gone, immediately giving a
factor of two reduction in error rate and removing the threshold effect wrt.
S/N seen with differential coding.
A GPS receiver also makes an ideal instrument for general purpose frequency
measurements (use it to drive a frequency counter) and a time standard as
well as giving your location !
For anyone who wants to have a go and duplicate the design, I will supply a
copy of the circuit, a PCB layout and the PIC software on request. There
may be a bit of a delay however, as the design was only 'frozen' this
weekend and easy-to-read documentation is almost non existant at the moment
! TAPR still market the Garmin GPS25 receiver module as far as I know, see
their web at www.tapr.org
(4.194304 MHz was used as it allows a DDS to generate any frequency that is
an exact multiple of 1 Hz without any rounding errors. Which is not the
case for 5 or 10MHz references !)
Andy G4JNT
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