Hal Murray wrote:
[email protected] 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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