Efratom Modular Frequency Standard

(Click on the image to get the full-size version)

The Efratom MFS (Modular Frequeny Standard) is a configurable frequency reference that I think was sold into telecom and land mobile radio systems. I recently acquired one at a hamfest and had a chance to explore it.

The unit I have is a model MFS-300U-01 and has a 1996 date stamp. It has an MGPS GPS receiver with LCD display (MGPS documentation), an MRK module which contains an FRK Rubidium frequency standard (FRK documentation), an MXO crystal oscillator module (no documentation available), and an MDC module which I believe provides fail-over from the Rb to the XO oscillator (no documentation available).

(Click on the image to get the full-size version)

On the rear panel, there are five MBF modules that each provide four 5 MHz outputs, one MDB module that provides four PPS outputs, two MFM fault monitor modules, and two MPS power supply modules. I don't have documentation for any of these modules.

I do have some general documentation that describes the system and the modules: a 17-page brochure, and a 13-page description.

The GPS appears to discipline the Rb frequency and derived PPS signal by applying corrections at a rate that can be set from the front panel. On this unit, frequency corrections are set to occur every 3 hours, and time corrections every hour. Here are some photos of the MGPS display:

Here is a frequency stability plot measuring the output over 8 days, compared to an HP 5065A Rubidium standard:

The slight upward hook at the longest tau represents the noise floor of the 5065A; at this point, the MFS is assuming the long-term stability of the GPS.

And here is the phase noise, referenced to a Wenzel ULN oscillator:

This is an odd looking response. I suspect that what we are seeing is a follow-on oscillator (part of the MDC module?) that is locked to the Rb signal and not particularly well filtered from digital hash.