[time-nuts] algorithms and hardware for comparing clock pulses

[Bill Hawkins]

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-bounces at 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.
>
>