Message-ID: <001c01c1a82c$ad32cea0$239e01d4@g4jnt>
From: "Andrew Talbot" <G4JNT@thersgb.net>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Frequency standards for LF.  The next g	eneration
Date: Mon, 28 Jan 2002 18:50:30 -0000
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

Yes, possibly.  But reception of MSF here is marred by local interference
which kills the ability to keep to much better than 10^-9 even with a very
narrow PLL.  Trying to use the 1PPS output, which requires reception in a
significantly wider bandwidth would give quite bad jitter which would kill the
frequency locked loop I am using.   A conventional PLL would do this a lot
better, but we would necessarily have to go to ultra narrow bandwidths, so
requiring lock up times measured in days.

Also, the delay inherent in any commercial MSF receiver will upset its use for
accurate UTC determination for signalling.   With GPS you know that the rising
edge of the pulse is within a microsecond of UTC anywhere in the world.

A GPS receiver is such a  universally useful piece of kit to have in the
shack, that once you have one you'll wonder how you ever managed time and
freqeuncy calibration before !

Andy  G4JNT

-----Original Message-----
From: Stewart Bryant <stewart.bryant@virgin.net>
To: rsgb_lf_group@blacksheep.org <rsgb_lf_group@blacksheep.org>
Date: 28 January 2002 17:23
Subject: Re: LF: Frequency standards for LF. The next g eneration


>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
>>
>> --
>> The Information contained in this E-Mail and any subsequent correspondence
>> is private and is intended solely for the intended recipient(s).
>> For those other than the recipient any disclosure, copying, distribution,
>> or any action taken or omitted to be taken in reliance on such information
is
>> prohibited and may be unlawful.
>
>
>
>
>