[time-nuts] RE: phase locking Rb to GPS

Brooks Shera ebs at starband.net
Sun Sep 18 17:11:48 EDT 2005

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
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" <cfmd at bredband.net>
To: <time-nuts at febo.com>; <ch at murgatroid.com>
Sent: Sunday, September 18, 2005 12:45
Subject: Re: [time-nuts] RE: phase locking Rb to

> From: "Christopher Hoover" <ch at murgatroid.com>
> 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:
<20050918184006.D6ACEA00031 at mail.murgatroid.com>
> >
> > On 9/18 "Tom Clark, W3IWI" <w3iwi at toad.net>
> >
> >    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
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> time-nuts mailing list
> time-nuts at febo.com

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