[time-nuts] Temperature and signal amp for 'Bay FE-5680A?

[John Ackermann N8UR]

The TADD-3 output circuit with 3 74AC04 output stages paralleled is 
stolen from Tom Clark's design in the original TAC GPS interface board. 
  The 47 ohm resistors aren't intended as back terminations; if you 
assume a low output impedance at the chip, the three resistors are 
effectively in parallel, so more together they are more like 16 ohms 
than 50.

The value was a compromise between output voltage swing (trying to make 
sure that TTL levels are maintained at the far end of the cable) and 
protection against one '04 stage backfeeding another if their switching 
times aren't identical.  There's been discussion that lower values, 
maybe 27 or 33 ohms, might be OK and would deliver more swing at the far 
end.

Bruce is correct about the crosstalk problem -- in a desire to provide 
lots of flexibility, I designed the TADD-3 so that each of the six 
output channels could be jumper-set to a different pulse rate.  Each 
output uses 1/2 of a 74AC04, and between trace radiation and Vcc bounce, 
there is cross-mod if two channels sharing an output chip are set to 
different rates.  The simple answer is to keep each pair (channels 1 and 
2, 3 and 4, 5 and 6) set to the same rate.

John
----
Bruce Griffiths said the following on 01/10/2012 03:57 AM:

>> The TADD-3 uses 3 AC drivers in parallel, each going through a 51 ohm
>> resistor. Changing those resistors to 150 ohms should work. Maybe a
>> bit lower to account for the impedance in the drivers. I'd probably
>> check it with a scope.
>>
>>
> That approach doesn't do anything for the Vcc and GND bounce exhibited
> by the driver chip.
> GND and Vcc bounce is the cause of the high frequency ringing exhibited
> by the TADD-3 outputs.
> This ringing can even be observed at the outputs of inverters whose
> inputs are tied low or high in the same package
>
> Damping the crossover current induced transient in the supply leads
> (bondwire and lead frame) inductance is one way to minimise this.
> A small resistor in series with the Vcc pin often works well, the
> resistor value being chosen for near critical damping.
>
> Another problem with the TADD-3 is the sharing of a driver chip by
> different input frequencies which leads to intermodulation between the 2
> outputs.
>
> Bruce
>
>
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