Interference problem using external timebase

Hi All,
We're using the NI 6602 as part of a wireless communications receiver for pulse generation and rotary encoder interfacing. For precise time synchronization of data acquisition, an external timebase must be used with the pulse generation task, so we've been using a 10MHz (or 1 MHz) square wave derived from a master oscillator as external timebase. The 10MHz TTL square wave is injected into the unshielded terminal block (CB-68LPR) on PFI39 (CTR0 SRC) and travels down the shielded cable (SH68-68-D1) to the PCI 6602. A problem which has encountered is that harmonics from this 10MHz (or 1MHz) timebase are appearing in our measured signal. Note that we're measuring radio frequencies in the range of 30MHz to 80MHz. As you might know, signal levels at the front-end of a receiver are typically very low (order of microvolts), so even a small amount of interference can cause serious problems. In this case it seems some of the square wave energy is being radiated and picked up by the receive antenna. If the square wave is disconnected, i.e. the shielded cable is disconnected from the terminal block or computer, the problem goes away.
If anyone has any ideas on how to mitigate this problem, I'd like to hear them. The obvious solution of moving the antenna further away from the computer is not an option since it is mounted on top of the vehicle. One question is what type of shielding the SH68-68-D1 has, and how good is it? One possibility I've been considering is trying to shield the cable even more by using copper tape or a metal braid wrapped around the outside and grounded at the terminal block and/or computer? Could the level of interference have something to do with the standing wave resulting from impedance mismatch (what is the characteristic impedance of this cable anyway)? Any help would be much appreciated.

Hello Axiom,
Your system sounds pretty complicated so I didn't completely understand
your setup; however, I do have a couple suggestions for you.
First, I would recommend replacing the unshielded CB-68LPR terminal block with a shielded one, such as the SCB-68.
Having a shielded connector block may fix the problem depending on how
close your connector block is to your antenna.
Second, you might want to take a look at the Field Wiring and Noise Considerations for Analog Signal Tutorial, there are quite a few noise considerations you might want to take into account.
Finally, the characteristic impedance for the SH68-68-D1 cable is 125
ohms differential @ TDR and 79 ohm single ended @TDR. I doubt
that the interference you see is because of an impedance mismatch
however.
I hope that helps,
Micaela N
National Instruments

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