The role of the diode is to break the current path to the cap when S1 shorts the current to ground when PPS 2 occurs.
With the diode, S1 does not short the cap to ground. Bill Hawkins -----Original Message----- From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Alex Pummer Sent: Thursday, September 24, 2015 3:58 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] algorithms and hardware for comparing clock pulses the role of the diode is just to have a voltage drop? 73 KJ6UHN Alex On 9/23/2015 11:31 PM, Bill Hawkins wrote: > Perhaps I can do this in words, as I have no schematic software. > > Start with the input to your favorite microprocessor's A/D converter. > Connect it to a suitable (more later) capacitor to analog ground. > Connect a cmos switch across the cap and call it S2. When S2 is on, it > discharges the cap. > > Now build or buy a constant current generator connected from a > suitable positive voltage to another cmos switch called S1. > When S1 is on, all of the current generated flows to analog ground. > > To make it all work, connect the anode of a diode from the junction of > the current source and S1 to the cap and analog input. > > When S1 is on, no current gets to the cap. When S1 is off, all of the > current gets to the cap, if S2 is off. This causes a linear buildup of > voltage across the cap, for a suitable time. > > When 1 PPS pulses are compared, suitable means one second to charge to > almost the maximum that the micro A/D supports. > The value of I is chosen to overwhelm diode leakage and A/D input > current. The value of C follows. > > All that remains for a working system is a pair of flip-flops to > control > S1 and S2. > FF 1 is set by PPS 1 and cleared by PPS 2, and by power on reset. When > FF1 is on, S1 is off. > FF 2 is set by PPS 1 and cleared by an output from the micro when the > A/D conversion is done. When FF2 is on, S2 is off. > > And so C will charge from PPS 1 to PPS 2, hold the value while the A/D > conversion occurs, and be reset to zero volts when the micro is done > processing the input. > > This gives the micro a linear conversion of pulse difference time > rather than an RC exponential value. > > Feedback controllers do better with linear error signals. > > But all of this is wasted if the PPS signals are not accurate due to > things that affect pulse rise and fall times. > > If the above was not adequately clear, please ask for clarification. > Or do a schematic and ask for corrections. > > Bill Hawkins > > P.S. This will not work well for small differences between PPS 1 and 2. > It will work if the goal is 50% difference, or 90 degrees phase shift. > > > -----Original Message----- > From: Can Altineller > Sent: Wednesday, September 23, 2015 2:56 AM > > --------%< ------ > > 4. I think an analog solution like Bill Hawkins described, would be > best suited for this task. But I have not understood it enough to build it. > > Best Regards, > C.A. > > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
