Magnus is correct that many Rb's have a voltage input EFC control. But in case yours needs a current source, the note below, which is an excerpt from some notes on how the drive an Efratom FRK-L from a gps controller, might be a start. The FRK-L has one end of the C-coil grounded and the other fed by a resistor network. The resistor Rc mentioned below is the piece of the network that connects to the C-coil.
If you happen to have an FRK-L I can send you the entire notes file. In the case of the FRK-L the sensitivity was df/f = 2.2x10-9 per ma, so the current change needed to swing the frequency over a reasonable range was quite modest. Modifications for locking to GPS The DAC in my gps controller board (see QST, July '98 or www.rt66.com/~shera) can operate in either a voltage (+/- 3v) or a current mode (+/- 1ma), however the PCB distributed by A&A is wired to use it in a voltage mode. A voltage source can be used as current source if it drives a low impedance through a relatively larger series impedance. With this in mind, and to conveniently match the FRK-L to the sensitivity presently programmed into the digital PLL in the controller, I have chosen to control the C-coil current by attaching a resistor at the Rc - C coil junction. The DAC is connected directly to the other end of this resistor. The value needed for the resistor turns out be 250 ohms (I used a 270). I wired the resistor inside the unit so if I accidentally applied a large voltage, like +5v, to the C-coil connector pin I wouldn't burnout the C coil. Regards, Brooks ----- Original Message ----- From: "Magnus Danielson" <[EMAIL PROTECTED]> To: <[email protected]>; <[EMAIL PROTECTED]> Sent: Sunday, September 18, 2005 12:45 Subject: Re: [time-nuts] RE: phase locking Rb to GPS > From: "Christopher Hoover" <[EMAIL PROTECTED]> > Subject: [time-nuts] RE: phase locking Rb to GPS (was time-nuts Digest, Vol 14, Issue 30) > Date: Sun, 18 Sep 2005 11:40:36 -0700 > Message-ID: <[EMAIL PROTECTED]> > > > > > On 9/18 "Tom Clark, W3IWI" <[EMAIL PROTECTED]> wrote: > > > > The frequency knob that you tweak to correct the Rb frequency > > passes some tens of ma thru a coil surrounding the RF interaction > > region. If you try to phase lock a Rb to GPS, you need to develop > > a current source error signal. > > > > Hmmm ... can you say more about this current source error signal? > > > > I've been thinking of locking my Rb standard to GPS. I was simply going to > > use the same circuit (1 pps, 10 MHz in; analog EFC voltage out) that I have > > running with my OXCO but with different filter and EFC DAC coefficients. > > > > So is this not sufficient to phase lock GPS to a Rb standard? > > In practice you usually see a voltage input on Rb standard. I have one on mine, > it is directly converted into current which drives the C-field coil. > > Cheers, > Magnus > > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
