Another Version of a Coax Probe

John Fisher of Cisco Systems ( writes:

I love your book, every time I open it (anywhere in the book) there is always some good stuff begging to be read again!

We are looking at noise (less than 100 mV to 300 mV range) on the various ground pins of some gigabit transceivers which fail (lose link) under high traffic rates.

We are trying different scopes (DAS 601 1 Ghz BW, TDS 744) but primarily using a single ended P6243 active FET probe w/ short ground.

I think a differential 21:1 semi-rigid coax probe into a Tek 11801B w/SD24 set as shown on page 107 of your book would be better.

Any suggestions, what is your favorite reference for measurements??

Thanks in advance.

Thanks for your interest in High-Speed Digital Design.

When measuring noise on ground pins, remember that the effective impedance of your source is extremely low. Therefore, you do not need to use a high-impedance probe for this measurement. I'd use a plain old piece of 50-ohm RG-174 (or hardline coax, if you wish), and run it no more than three feet to the scope. Crimp an appropriate connector on the scope end of the probe, and use the 50-ohm built-in terminator at your scope input.

At the other end, I usually trim the coax to expose a short section of center conductor and about .125" of shield. I then wrap a piece of bus wire around the shield and tack solder that to the shield and a nearby ground via. If I intend to probe a number of connections, I'll crimp the center-pin from a BNC connector onto the signal wire of the coax. This makes a nice, sharp, gold-plated contact on your signal wire. Use plastic or wooden tweezers to move the center contact into place. The total exposed loop area of this probe is very small, as is the ground wire length.

Touch the signal pin to the coax ground point to make sure you aren't picking up any spurious noise before you do your measurement.

Regarding good books on measurement technique, don't miss this one: Doug Smith, "High Frequency Measurements and Noise in Electronic Circuits", Van Nostrand Reinhold, 1993.

Best regards,
Dr. Howard Johnson