[time-nuts] Fine print on HP 5334B

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Feb 23 19:46:54 EST 2007


SAIDJACK at aol.com wrote:
> Hi Bruce,
>  
> would you have some sample schematics for a more modern,  faster version of 
> this than let's say the one used in the 5334A etc that  you could share?
>  
> thanks,
> Said
>   
Said

The Attached GIF file illustrates one possible approach to a TAC using 
discrete components.
The opamp is a high speed op amp with a low bias current. The reset 
switch Q4 is normally on as is Q2.
Just before the beginning of the pulse width to be measured Q4 is turned 
off, then after a short fixed delay Q1 is turned on until the end of the 
pulse.
The following ADC which has been tracking the output of the opamp then 
begins a conversion at the end of which Q4 is turned on resetting the 
integrator.
Connecting Q2 collector to the output of the opamp reduces the transient 
current load seen by the opamp output stage.

The linearity is affected by:

1) The voltage dependence of the drain source capacitance of Q4 has to 
be reduced, replacing Q4 a T-switch configuration will significantly 
reduce this effect. The

2) The Early effect will change the effective turnoff switch threshold 
of the longtailed pair. Fast transition times on the input signals at 
the bases of Q1 And Q2 will reduce this effect.

3) The opamp input bias current will contribute some error, however this 
effect is alleviated by always starting the ADC sample to hold 
transition a fixed time after Q4 turns off.

4) The finite gain of the opamp will contribute some error, this can be 
reduced significantly by using a composite amplifier using a pair of opamps.

5) The drift in the transistor common base current gain with temperature 
will affect the TAC gain TC.

There are also a host of other small effects (e.g. dielectric 
absorption) that contribute to TAC nonlinearity and tempco.

Bruce
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