Hi Bruce, I wonder how the analog switches' tempco would affect the performance. They usually do have 10 - 100 Ohms series resistance if I remember correctly. The Temex OCXO is achieving STS of parts in E-013, it would be unfortunate to lose that performance due to external circuitry. At these extremely small values, I would guess that the finite off-resistance of the switch, and the leakage currents through the switches' transistors would also degrade the performance. Maybe a mechanical relay would work better? But that would cause a mechanical vibration that could jerk the crystal. Also, I guess one would have to add a small RS-232 micro to decode the Validity bit from the GPS and do the actual switchover. Here is an interesting factoid: Magnus mentioned that many GPS receivers do strange things when they go into holdover. I can only concur to that. The M12+ and M12M are really excellent in this, but other receivers are not and may give you a couple of 1PPS pulses that are way off, say 200ns or much more before they shut up. Not so nice. bye, Said In a message dated 2/13/2008 15:09:54 Pacific Standard Time, bruce.griffiths@xtra.co.nz writes: It would also be necessary to use an analog switch in series with the PI filter input resistor and for extended holdover keep the PI filter feedback cap charged at the DAC voltage to minimise transients on reverting to normal operation. Bruce **************The year's hottest artists on the red carpet at the Grammy Awards. Go to AOL Music. (http://music.aol.com/grammys?NCID=aolcmp00300000002565)

SAIDJACK@aol.com wrote: > Hi Bruce, > > I wonder how the analog switches' tempco would affect the performance. They > usually do have 10 - 100 Ohms series resistance if I remember correctly. > > The Temex OCXO is achieving STS of parts in E-013, it would be unfortunate > to lose that performance due to external circuitry. > > At these extremely small values, I would guess that the finite > off-resistance of the switch, and the leakage currents through the switches' transistors > would also degrade the performance. Maybe a mechanical relay would work > better? But that would cause a mechanical vibration that could jerk the crystal. > > Also, I guess one would have to add a small RS-232 micro to decode the > Validity bit from the GPS and do the actual switchover. > > Here is an interesting factoid: Magnus mentioned that many GPS receivers do > strange things when they go into holdover. I can only concur to that. The M12+ > and M12M are really excellent in this, but other receivers are not and may > give you a couple of 1PPS pulses that are way off, say 200ns or much more > before they shut up. Not so nice. > > bye, > Said > > Said If your worried about analog switch tempco then the classic solution of using an noninverting buffer after the selection switch works well. The tempco of the analog switch in series with the PI filter elements is relatively insignificant in that it only affects the PLL natural frequency and damping and not the filter dc output. Leakage current shouldn't be a problem in the selector switches, unless the switches get really warm. A custom discrete switch could be used to eliminate leakage problems by ensuring that the voltage across the last off switch in the switch chain is very small. One could always use the output of the tracking ADC to drive the EFC input, disabling tracking during holdover and reenabling when the GPS reference is again stable. If one monitors the message containing the estimated variance of the PPS output one may be able to circumvent such effects. Bruce