[time-nuts] Lucent RFTG-m-XO 15MHz -> 10MHz

Charles S. Osborne k4cso at charter.net
Mon Apr 14 00:28:11 EDT 2008

I've done three 15 MHz to 10 MHz conversions now, an RFTGm-II-Rb, RFTG-m-XO,
and an RFTGm-II-XO. So I'll mention the quick and dirty mod that worked. The
circuit boards are virtually identical between all three.

I pulled the 15 MHz metal cased filter out. Backtrack from the +23dBm 15 MHz
output, thru the buffer amplifier, and you'll run into the metal bandpass

I used an oscilloscope to find the 10 MHz output on the OCXO. Using a 470pf
ceramic monolithic capacitor I ac coupled off a sample of the RF via a small
coaxial cable. Then route it over to a resistive power divider into the
missing filter's output socket.

The resistive divider is a very small 1K ohm 10turn pot in series with the
coax center conductor. I used a 51 ohm from the filter output to ground to
form the other part of the divider and help terminate the buffer amp input.

This allows all the level alarm circuitry or whatever is under the daughter
board to continue functioning. The potentiometer is adjusted to drive the
output buffer amp to +23 dBm at 10 MHz output just like it was with 15 MHz

I also have the original 15 MHz passive external distribution divider for
the system. So the +23 dbm drives the splitter to give six 10 MHz outputs at
+9 dBm each. It's an interesting divider. I'll have to take it apart
sometime. It has two inputs to allow the Rubidium RFTGm-II-Rb unit's output
to become the backup source. When the XO output fails it's muted, and the Rb
takes over as online source.

By the way I modified the RFTGm-II-Rb the same way to use the buffer amp to
output 10 MHz from the LPRO. The associated boards are identical between the
Rb and the XO, just stuffed with different parts. In fact there's a spot for
the GPS receiver, its just missing parts. Maybe when I learn a lot more
about the loop constants and how they interact I'll get ambitious and try
grafting the LPRO into the XO system to see if that results in a GPS locked

My RFTGm-II-Rb however still has problems. Thanks to the help of several on
the time-nuts list, I did get both units functioning redundantly again after
the lightning damage several years ago. But after about six or eight hours
the Rb side turns itself off. I'm thinking a DC-DC converter is probably
overheating inside and shutting down. Over the next month I hope to open it
up and confirm that. I may just use an external power supply and remove the
suspect 5v dc-dc converter as a test of the theory. Always hard to test a
thermal problem when the only way to do it is to open the box confining the

Charles Osborne, K4CSO

----- Original Message ----- 
From: "Bo Granlund" <granbo at tonnikala.org>
To: <time-nuts at febo.com>
Sent: Sunday, April 13, 2008 4:08 PM
Subject: [time-nuts] Lucent RFTG-m-XO 15MHz -> 10MHz

> Hi everybody,
> Sorry for asking this kind of stupid question, but I've tried to do my
> with google and it hasn't revealed a lot of decisive results.
> The thing is, I have a Lucent RFTG-m-XO that outputs 1PPS and 15MHz
> I was wondering, how do you go ahead and convert the 15MHz output to a
> 10MHz output? What kind of equipment do I need in between there to make
> this happen?
> I'm really sorry, because I do realize this is a really stupid question,
> I'm kindof out of ideas, and I'm more of a software guy than an
> guy. :)
> Oh, there is a 10MHz input to the RFTG-m-XO. What is the purpose of this
> input?
> kind regards,
> Bo Granlund
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
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