[time-nuts] Lars GPSDO on EEVblog

Bob Martin aphid1 at comcast.net
Sat Sep 8 23:08:15 EDT 2018


Charles,

  Attached is another way to do it. A higher charging voltage 
increases capacitor voltage linearity over the range of the ADC 
making curve compensation easier.

The bus switch has a very low on resistance to pretty much discharge 
the cap. The design is pretty old - I'm sure there are better parts 
available now. I believe there was also a calibration cycle that 
occurred fairly often which is why U21 looks so busy.

Cheers,

Bob Martin

On 9/8/2018 8:17 PM, Jim Harman wrote:
> 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
> reading.
> 
> I would certainly be interested in any suggestions on improving this
> circuit.
> 
>>
>>
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