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Dear LF Group,
One of the traders at the Stevenage rally a few
weeks back had a stack of these frequency standards for sale - I have finally
got round to looking at one, and it turns out to have some very interesting
features for LF or other radio use. I don't have any technical data for these
units, but it is possible to figure quite a lot out by peeking under the
covers.
It consists of two parts - the receiver part has a
10MHz crystal oscillator phase-locked to the 198kHz Droitwich signal - it would
be difficult to use any other signal, because the RX has a 198kHz crystal, and
the synthesiser logic is inside a programmable logic IC. The unit comes with a
remote ferrite rod antenna connected via 50ohm coax. There is an S-meter, and
lock indicator light, which comes on within a minute or so of a signal being
connected. On the back, there are 1MHz, 5MHz and 10MHz outputs from the
receiver.
The other part is the actual frequency standard -
it takes the 10MHz output from the receiver (or another 10MHz standard), and
uses this to lock a 20MHz ovened reference (HCD71 -see http://www.hcdresearch.co.uk/71.htm
) When no 10MHz signal is present, the OCXO provides the frequency standard,
which can be trimmed from the front panel. The 20MHz signal is available at the
front panel "monitor" socket, but the main output is from a DDS synthesiser,
which is set by a row of PCB rotary switches, which can only be accessed with
the cover off. The synthesiser is interesting, because it seems to use
binary-coded decimal logic instead of straightforward binary like the AD dds
chips. This means the frequencies come out as exact decimal numbers, rather than
the nearest binary fraction approximation, which will inevitably have a small,
built-in error. The frequency can be set from 0 - 15.999... MHz in steps of only
10e-4Hz. The output starts to roll off below a few kHz. Since my LF TX has a
divide-by-100 stage built in, I could use one of these standards at 13.6MHz to
give tuning steps of 1 micro-hertz, should a need arise... There seem to be
pads on the PCB for a connector to external switches, so I guess it woud not be
hard to add a computer interface if you wanted to. The whole thing runs off a
nominal 12V at several hundred of mA, so would be quite useful for portable
operation, or battery back-up. It is in a 19" rack case about 45mm thick and
200mm deep.
I have the phone number of the guy who was selling
these if anyone is interested - the asking price at Stevenage was 50GBP, and he
said he had about 60 available at that time.
Cheers, Jim Moritz
73 de M0BMU
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