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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jan 10 08:57:49 UTC 2012


Hal Murray wrote:
> davidwhess at gmail.com said:
>    
>> I looked at the TADD-3 design and it sacrifices back termination impedance
>> for signal swing which results in ringing but I presume not too much if
>> people were using it successfully.
>>      
> If the far end (receiver) is terminated to match the coax, there will be no
> ringing and no reflection so the mismatch in back termination won't be a
> problem.  (I'm being sloppy when I say "coax".  It also works for PCB traces
> or any other transmission line.)
>
>
> There is an approach that gets full height at the receiver and good back termination (and low power).  Digital geeks call it series termination.  The idea is to leave the far end (receiver) open and use good back termination to catch the reflection.
>
> When switching, the wave will go down the coax at half height.  When it gets to the far end (open circuit), it bounces back.  The far end sees a clean switch to full height.  That's the sum of the original wave and the reflection.  When the reflection gets back to the transmitter, it sees a clean termination and doesn't generate any more reflections.
>
> Note that this only works if you have a single receiver at the far end of the transmission line.  If you have a receiver half way down the line, it sees a half-height signal between the time the outgoing wave goes past and the time the reflection gets back.
>
>
> 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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