Fiberoptic receiver IF-D95T uneven pulse width

According to the data sheet for the IF-D95T, the high to low and low to high should be identical.The signal from the IF-E97transmitter is a 8.68uS square wave. From the receiver I’m getting an average of 10.5 uS positive pulse and 6.86uS negative pulse for a 115,200 baud RS-232 communication. We are getting a lot of comm faults when 600kW PWM controller is activated. I believe that this pulse unevenness is the problem. The IF-D95T from Industrial Fiber Optics data sheet

shows a 100nF cap from Vcc (5V in our case) to ground. The design engineer used a 1uF instead. Could this cause the problem? The out put goes directly to a microprocessor MC9S08DN60AMLFN.
Thanks in advance for any assistance.

Welcome to the Technical Forum. Yes. I would say this could be the issue. I would switch thta capacitor to what thed data sheet recommends. I would say it should fix the issue.

I just changed to a 0.1uF as per the data sheet. The positive pulse width decreased from 10.4uF to 10.2uF. We still have a long way to go. What would cause all 12 receivers to respond in this way to a perfect 8.68 square wave input? Is there any way to send a screenshot from the scope to you?

Hello @RickWoodbury,
The datasheet specifies a typical value of 6 μs for propagation delay, low-high and high-low. It does not state they are identical so it’s possible they are slightly different which could possibly account for the differences you are seeing.

Tim Huie suggested that we might need a bias on the transmitter to even the signal, as there is no way to bias the receiver. He suggested that bias circuitry with an op amp or transistor might work. Could you suggest such a circuit? Here is the signal that we’re dealing with. It should be 8.68uS square.

The ~8us pulse width spoken of is on the same order as the IF-D95T’s 6us (typical) propagation delay, and some pulse width distortion due to factors such as detector saturation should probably be expected at such speeds. Measurements become dicey when the errors in the measurement tool are nearly as large as the thing being measured.

Use of an IF-E97 transmitter was mentioned, which is somewhat mis-matched in terms of speed potential to the 'D95 receiver. I suspect switching to the faster IF-D97 detector would help address the pulse width distortion issue. Pre-distorting the transmitted signal by shortening the positive pulse width might be another means of compensating.

It’s conceivable that this may have something to do with the issue also, in the sense that proximity to orbital rocket launches can make carrying on a cellphone conversation difficult.

Thanks to all of you for your valuable input.
Rick Woodbury