[time-nuts] Newbie question - what use is a Ruby?

Murray Greenman Murray.Greenman at rakon.com
Mon Oct 5 18:12:01 UTC 2009


Joseph,

There are several uses for a Rb unit (even if you already have a GPSDO).

1. The Rb unit is very portable, and is on frequency from cold in 5
minutes. Great for tests away from home. Easily battery operated. No GPS
antenna required either!

2. With fast warm-up, they are ideal as a reference for gear you don't
use too often, and can't justify the power consumption (or added space)
of a GPSDO and its backup supply, which must run continuously.

3. Some units (such as the FEI FE-5650A and FE-5680A) are frequency
programmable, and so you can use them as a stable signal generator or
(if you're a Radio Amateur) as an Exciter for experiments. I have
software which can transmit an on/off or frequency shifted pattern for
ID purposes.

4. They are a handy adjunct to the GPSDO. Use the latter to calibrate
the former, and then you've a reference even if GPS is unavailable. The
Rb units also have very good short-term stability.

5. If you are interested in making phase noise or frequency measurements
requiring a mix-down technique, the programmability of the FEI units is
very useful. Phase noise is good enough for most applications.

I have two units (FE-5650A and FE-5680A), from China. They started life
providing only 1pps (1Hz) and required reasonably simple modification to
provide 1kHz to 15MHz operation in ~10mHz steps. When thus modified, the
output level is about +9dBm. The units use a 50.255MHz oscillator in the
physics package, and this acts as reference for a built-in DDS
synthesizer, hence the frequency flexibility.

Disadvantages?

1. You can have any frequency you like, but only one with sub-ppb
accuracy (because of the DDS step size).

2. The power consumption (not that it's as high as the GPSDO).

3. Ageing, and the need for occasional calibration.

Regards,
Murray Greenman




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