What happens when you input a blocking signal into a TTL input
Introduction
There is a peculiarity about the TTL input which sparked when we were stress-testing the CONV-TTL-BLO PCB, while inputting a blocking signal into a TTL-BAR input.
We noticed that the output at the TTL-BAR was delayed by an unexpected amount of time (approx. two pulse cycles).
Note that this is not a problem in the TTL input stage, hence this page is not an issue. A blocking signal should normally not be input to a TTL input, when a normal TTL signal is input as seen below, this waveform does not appear. This page only documents an observation and it might be useful in the future.
Measurement points in the schematics
Measurement with TTL signal on the input
When a normal TTL-level signal is input, the signals at the measurement points look as below:
Measurement with the blocking signal on the input
When the blocking signal is input, a delay is noticed between the time that the blocking signal goes back low and the time when the input at the Schmitt trigger buffer goes low, which at the beginning might seem strange:
Reason for the delay
The delay which appears as seen above is due to the fact that the BAR66 diode* used to protect the Schmitt trigger buffer is a PIN diode. Since PIN diodes have an intrinsic semiconductor region sandwiched in between the P and N regions of the diode, they have a higher carrier charge recombination time, which means that when the diode turns on, it takes a lot longer than normal diodes until it turns off.
In the case of the BAR66 diode, the charge carrier lifetime is 0.7 seconds:
If we look at the waveform above, the interval between the falling edge of the blocking output and the time the BAR66 starts going off is just about 0.7 seconds, which accounts for the observed delay.
Measurement after replacing the BAR66 PIN diode with a HSMS-2802 Schottky diode
To test the hypothesis, we replaced the BAR66 with a HSMS-2802 Schottky diode. The waveforms at the measurement points after this replacement can be seen below.
Theodor-Adrian Stana, May 2014