I need to send a 40 MHz clock (18V PP) 30 to 45 cm (12 to 18 inches). What kind of cable should I use (i.e., does it need to be shielded and how should it be terminated)?

A question on a different matter, I need a reset signal for multiple devices with a total cable length of about 2 to 3 meters. Will regular cable work (24 AWG) or do I need a shielded cable or perhaps a differential signal? If it does not need to be a differential signal does it make a difference if it is active high or active low? A reset occurs only about once every few seconds at the most. I know a differential signal would be best, but I am trying to keep the pin count on my connectors as low as possible.

DSG

 

In order to answer your question it would help to know the characteristics of the driving source and the characteristics of the receiver.

As for the second question it will help to know the characteristics of the driver and the number and type of receivers.

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PB,
For question one, the application is for a remote switch using an ultrasonic transmitter and receiver. The 40MHz transmitter signal is driven by a couple of CMOS inverters in parallel on one pin of the transmitter and another (inverted) pair on the other pin. A 9V battery powers the transmitter circuit. Only a few pulses (at 40KHz) will be sent at a time for a transmission. Will a pair of wires work or do I need to do something more?

For the other question, there can be perhaps 5 to 10 devices connected together. I'm just looking to make sure I don't get any false reset signals. I haven't selected the line drivers/receivers yet, but it will be normal digital logic to interface to a PIC. I would even consider using RS-232 line drivers/receivers if it would allow a single line instead of a differential pair.

The reason for these questions is that I don't know enough about the risks of noise interference on reliable operation.

DSG

 

You've used both 40KHz and 40MHz, which do you mean?.

40KHz is the ultrasonic one, and plain cable will be fine.

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Oops, I did mean KHz. I guess I just have MHz on the mind.

Thanks,
DSG

 

I would start with ordinary wires of say 22-24 guage for your 40 kHz. clock and your reset signal. I would then use several ground wires of the same guage connecting the subsystems together. So we are up to four wires. There are two primary methods by which noise is coupled into a system. Capacitive coupling is how electric fields influence signal integrity. Inductive coupling is how magnetic fields influence signal integrity. Shielding is the primary means of dealing with electric fields, and twisting pairs of wires is the primary means of dealing with magnetic fields. Of course this is grossly oversimplified, but it will get you thinking about cause and effect and allow you to evaluate different strategies. Note - twisting a signal wire with a ground wire is not the same thing as using a differential signal.

The next thing to consider is to use CMOS Schmitt Trigger Inputs for the receivers of the RESET signal. This will give you some additional noise immunity AND hysteresis.

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Thanks for the help.