Service manual

Model 2A Operation And Service Manual
2A Manual A5 21 January 2005 Page 9
Even for a FSD temperature change, the rate of internal temperature rise, and
hence the phase rate (=frequency error) is limited, partly by the thermal mass and
its inertia and by the low drift coefficient.
Such receivers do still suffer from several effects, which cause degradation of the
accuracy of the output frequency, these include the following effects:
a. Possible transmitter outages: maintenance, severe weather, and power failure.
b. Temporary local
RFI. There is always the possibility of an arc welder, or similar
being next door, or perhaps an electric railway or transit system.
PCs and other
modern electronic equipment are also regular offenders. Toroidal transformers,
used in power supplies, with their high transient switch-on currents, electric
kettles etc. also contribute to the overall level of the local
RFI
c. Natural
RFI: Auroras, thunderstorms, atmospherics, and aircraft reflections….
d. Night - time sky wave. This degrades the accuracy of the outputs of the models
2A-X and 2A-Y, as they try to follow the phase (distortion) of the receiver
signal, in the medium term, i.e. at their loop bandwidths. It is impractical to make
a simple receiver/
PLL system with a very slow loop- not the least because the
customer does not want to wait forever! In order to reduce this effect, these
interfering effects at
VLF are also very slow and small (typically much greater
than 100 seconds), yielding perhaps 1 part in 10
9
or at worst 1 part in 10
8
errors
at the receiver's input frequency. They are therefore only partially amenable to
slow
PLL’s as a solution, although they may improve matters by a decade or
two.
However, neither the ionosphere, nor the receiver is capable of accumulating net
phase (forever) and the frequency accuracy is hence only degraded at a given gate
time, and is always able to be restored by using longer gate times - providing the
receiver does not lose lock altogether. This may be verified over a 24 hour run,
showing that the output phase always returns to the same place and that during
each daytime period no cycles have been slipped. In fact, even if the receiver does
loose lock (a very rare occurrence), the phase will still return exactly, once
reacquired and settled. There will however, have been a region of phase error,
across which the average frequency will be in error. Both Models
2A-X and 2A-Y
take less than one minute for the phase to settle.
Model
2A-Y does alleviate the sky wave effects somewhat, due to its lower phase
locked loop time constants. Closer to the transmitter, this will not be a problem,
either due to the high
LUF, at high incident angles, or due to the sheer strength of
the ground wave signal. This
SC - cut OCXO also confers excellent phase noise
above the loop bandwidth.