Service manual

Model 2A Operation And Service Manual
2A Manual A5 21 January 2005 Page 8
General description
1. Receivers
These frequency standards are intended for indoor use and operate on LF or VLF
frequencies, within the reception range of suitably stable, high-power, long wave
transmitters. This range often exceeds 2500 km. Many such transmitters are locked
and/or are traceable to well recognised national frequency standards via published
data "post facto".
This series of standards yields a choice of price/performance trade-offs and is
normally able to suit many medium to high frequency and (relative) time
applications without resorting to the expensive acquisition and maintenance of
atomic frequency and time standards. Within reasonable bounds, the results
obtained are fully traceable to the appropriate (primary) reference source.
All models contain a dedicated integrated - circuit receiver system with a very
sharp band pass crystal filter to extract the transmitter carrier (and reject the
sidebands).
The
2A-X uses a VCXO, running at 10 MHz, to provide phase locked loop (PLL)
inputs and also to provide output signals-after appropriate processing. The Model
2A-Y has the VCXO replaced with a fast warm-up, directly heated crystal
OCXO. This results in a lower phase noise - beyond the much reduced loop
bandwidth, hence improving all of the noise performance figures.
2A-X and 2A-Y deliver 10 MHZ, 5 MHz and 1 MHz square waves and sine
waves. These sine wave outputs are of exceptional purity and are derived from an
internal, 10 MHz crystal oscillator. Output impedances are all at the standard, 50Ω
level. Furthermore, they are able to drive any load, including
TTL, HCMOS and
misterminated loads (using 50Ω coaxial cable) with high predictability and
minimum effort.
A high-pitched beeper alerts the user to a loss of lock. To monitor loss of lock
during non-presence of the operator, the
2A-X and 2A-Y also have a “past loss of
lock” red
LED. Both LEDs operate on positive logic.
All of these models may have a small phase-temperature coefficient, but must have
a zero frequency-temperature coefficient, due to their basic operating principles.
Once the temperature settles down, even after a step change, the average output
phase will tend to a constant value, and hence the average output frequency
accuracy will tend to that of the transmitter - with zero frequency error.