[time-nuts] J06 HP-59992A time interval calibrator for HP-531xx counters

[Bruce Griffiths]

With a sinewave input LF feedback from the 74AC04 output to the shaper inputs could be used to regulate the output duty cycle by adjusting the switching threshold. However ensuring that the input amplitude is sufficiently large to override potential LF oscillation. The LF feedback will also compensate for delay asymmetry in the 74AC04 as well.

The output transition times of ACMOS is relatively slow compared to modern LVCMOS parts.  

Bruce 
> On 09 July 2017 at 12:15 Mark Sims <holrum at hotmail.com> wrote:
> 
> 
> I think the way the fine cal works by checking the the intervals between four different edges that a lot of asymmetries in the signals are nulled out in the software.
> 
> How good are 1:2 180 degree phase shifters at exactly shifting by 180 degrees?   At what cost?   
> 
> Also coax and RF relays cost a lot.   Pretty soon your BOM cost is over what a 59992A will run... assuming you can find one.
> 
> So far my design is tending towards:  10MHz ref input -> Minicircuits doubler -> Wenzel squarer -> 74AC74 divider -> 74AC04 buffer -> level shifter.  The doubler/divider might not be needed,  but I think it will give a more symmetric output.  I might include a space for a 10 MHz TTL oscillator for non time-nut users... hopefully it might be stable enough over the short time interval for a cal measurement cycle.
> 
> For the gain cal an 5V LM4040 (or other) reference.   Routing signals to the output BNC's via 2P4T slide switches.  The slide switches might be a weak link... but they are less than 50 cents each.  Whatever, it should beat the pants off doing just the "quick TI cal"
> 
> The reason I started looking into this is that I want to upgrade the old firmware to a much newer version that allows the comma separators in the decimal digits.   Upgrading the firmware requires a new calibration.
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