[time-nuts] Lars GPSDO on EEVblog

Jim Harman j99harman at gmail.com
Sat Sep 8 22:17:45 EDT 2018

Charles wrote,
> According to my tests, the B-C junction of a high-quality 2N3904 has
> about 50pA of leakage at 20vDC reverse voltage.

That's good to know, but the problem I had with this circuit was with the
forward current at a low forward voltage. With this phase detector, the
HC4046 makes a 0 to 1 usec pulse representing the time difference between
the 1 pps from the GPS and the 1 MHz derived from the OCXO. This charges
the capacitor to a voltage proportional to the width of the pulse, the ADC
measures the capacitor voltage, and the cap discharges through the 1 Meg
resistor during the rest of the second.

When the HC4046 output goes low at the end of the pulse, the voltage on the
other side of the 1N5711 diode goes down to about 0.4 V. With a 1N4148 or
similar diode instead of the transistor, the forward current after the
capacitor is mostly discharged through the 1 meg resistor is enough to
prevent the capacitor from discharging all the way and the minimum voltage
at the ADC is about 0.3 V. or about 120 counts on the ADC. I was able to
get this below 90 counts by using the transistor. It might be possible to
reduce the 1 Meg resistor, but then we risk significantly discharging the
capacitor in the short time between the end of the pulse and the A/D

I would certainly be interested in any suggestions on improving this


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