Thoughts on my new frequency standard

I have an Efratom 10MHz Rubidium standard I found on ebay, it runs 24/7 on a Meanwell switchmode PSU, which feeds a pair of 12V 4AH gel-cells and a low-drop-out 24V regulator. The Meanwell is set to 28.4V to keep the gel-cells in top condition. The output of the Efratom is fed to 8 port and 4 port distribution amplifiers from

In the same rackmount chassis, I have a 10MHz Morion MV89A double-ovened OCXO, which I can switch to if the Rubidium source ever dies. That is encased in two inches of polystyrene foam, along with a 22-turn trimpot and series resistors, which are all glued to the case of the Morion to keep them at a constant temp.

I also have a G3RUH 10MHz GPSDO, which uses an external GPS powered antenna up on the roof.

I have an AD8302 monitoring the output levels and phase difference between the GPSDO output and the Efratom, and a second AD8302 looking at the difference between the GPSDO and the Morion. Those drive simple analogue meters at present.

I feed the sinewave 10MHz from the Efratom/DA to both of my Elad FDM-DUOs through SMA 6dB attenuators, and to both spectrum analysers, the 20GHz Racal frequency counter, my 3GHz HP signal genny, the 2m, 23cm and 13cm transverters and to a set of ADF4531 synth LOs for the pair of 144MHz G4DDK Anglian transverters I use for SHF IFs into the DUOs. The 4351 LOs run at 2556MHz for 3cm and at the fundamental for 9cm, and I have others for 70cm and 4m.

Also in the frequency standard chassis is a Valpey-Fisher 100MHz OCXO. I am just working on a PLL board to keep that locked to the 10MHz Rb source, using an ECL /20 prescaler, and a single-gate SOT23 XOR chip. The 10MHz Rb ref feeds a comparator and a /2 divider into the other input of the XOR. The output is filtered and has a time-constant of around 5 seconds. I will need to build another 4-port distribution amp for that, it will be used to lock some SHF local oscillators based on a set of YIGs for 3GHz up to 26.5GHz fundamental, to give me stable low-noise sources for 24GHz, 47GHz, 76GHz and higher, but that’s another story.

The key elements for locking the DUO are just the Efratom and the DA. I have been measuring the stability of this free-running MV89A against the long-term ref of the G3RUH GPSDO and it is stunningly good.

I used the MV89A to generate a test signal from an ADF4351 pll chip on 2,300,000,000.0Hz, after the Morion had been running for a week. Receiving that on the DUO and the locked transverters, after a careful check of the Efratom calibration, there was less than 3Hz difference. That is about 0.02Hz off on 20m, which I think is good enough, especially as the DUO only does 1Hz steps….

For the fine calibration of the Efraton and MV89As, I am looking at a beat frequency of less than 1mHz, or a phase shift of less than 18 degrees per minute, so the 3802s and a high-stability voltmeter are needed, or a dual-beam scope and a lot of patience.

Once I have the 100MHz OCXO locking circuit finished, I am going to make a new GPSDO based on a UBlox LEA-M8F with Galileo support and proper direct integration with an MV89A. I’ll also run the AD8302 boards into a PIC with A/D and Flash to monitor the long-term stability and variance, with USB monitoring. Next will be another full frequency standard for my motorhome, with a Ublox, MV89A and this PTI 100MHz OCXO

If I wasn’t so totally obsessive about frequency precision, I would just use the MV89A in a block of foam and set that to zero beat with a decent standard frequency transmission. It is never more than about 12Hz out at 10GHz, so really the Efratom is a bit redundant. I guess I just liked the idea of having that Rubidium thing running here in my lab racks.

Leave a Reply

Your email address will not be published. Required fields are marked *