Extending BNC probe's reach - Page 1
Forum section: electronics / beginners

author: ksat44
author_rank: Contributor
timestamp: April 01, 2022, 02:51:55 am
content: Hi, folks.I wanted to extend the reach of my scope's standard probe leads about a meter (3ft). I have a 50 ohm BNC cable of with male ends of that length and thought I'd use that along with a female to female adapter to connect it all up.Would I be sacrificing much in the way of performance (signal loss) by setting it up that way? I use the scope primarily for automotive diagnosis. One concern I have is when using an amp clamp where readings get quite small (in the mV range).

author: bdunham7
author_rank: Super Contributor
timestamp: April 01, 2022, 03:29:14 am
content: 1X or 10X? What frequencies?For most automotive uses that might be a OK, especially at 1X. Fluke actually shipped extension cables with their automotive ScopeMeter kits.

author: ksat44
author_rank: Contributor
timestamp: April 01, 2022, 04:44:50 am
content: Mostly 1x. Most of the frequencies encountered in automotive testing are quite low. Though I don't know the figure offhand, I would guess sub kHz range.

author: tggzzz
author_rank: Super Contributor
timestamp: April 01, 2022, 07:24:57 am
content: The highest frequency is determined by the waveform's rise/fall time,notthe repetition period."High" impedance scope probe cables aren't traditional coax, they are a special lossy coax.To see if your suggestion is adequate for your purposes, inspect a known representative signal with and without the modification.

author: CaptDon
author_rank: Super Contributor
timestamp: April 01, 2022, 02:04:18 pm
content: If your probe is 1X/10X combination probe then if set at 1X there will most likely be no noticeable effect. At 10X the probe has a compensation adjustment and expects to see an exact 'input' capacitance at the scope. The capacitive trimmer inside the probe is adjusted to make 1khz square waves look square. That bit of extension cable would probably need the probe compensation to be adjusted, but sadly re-adjusted when not using the extension. My experience with the Tektronix clamps that have the built-in 9v battery is that they can drive 25ft of coax directly with very little degradation. They are however only 100khz probes. I have also found the more sophisticated Tektronix probes that use the external amplifier module (not the AM502 or AM5002 modules) to be capable of also driving 25ft of coax from the amplifier output to the scope input and MUST have the 50 ohm terminator activated. We use the Tektronix clamps (Looks like a huge overgrown clothespin) that are basically self contained with the 9v battery supply to examine the fuel injector pulses on diesel locomotives. Our peak is 26 amps during boost phase, 19 amps for hold1 allowable time period and 13 amps during hold2 phase for really long pulses at high HP (4500+). The clamps are rated to 70 amps peak and work well for injector diagnostics. Sadly the probes have a high price. Tektronix......If you have to ask, you can't afford it. That should be their motto!!!

author: macboy
author_rank: Super Contributor
timestamp: April 01, 2022, 02:34:26 pm
content: Passive High-Z (common 10 MOhm) probes will NOT work with regular BNC (RG-59/whatever) cable extension. The probe cable is very special, it has a super fine conductor in order to both reduce capacitance (smaller surface area) and to provide resistance (on the order of 100 Ohm / ft) which helps prevent effects of reflections from the scope input, which is obviously not impedance matched/terminated. Even swapping a 6 ft probe cable onto a 3 ft probe, which is techincally possible, would not be likely to work. The compensation network in the BNC box (and/or probe head) needs to be specially tailored to the cable in use.You can however extend Low-Z probes (such as Tek P6158 or homemade) using the correct impedance of cable. They are usually designed for regular 50 Ohm cable. If you build your own, just build them long enough in the first place. If you don't know, a Low-Z probe has a 500 Ohm (for 10x) or 1000 Ohm (for 20x) input impedance, usually with around 1 pF (or less) of capacitance, and require a 50 Ohm termination at the scope. At frequencies above 100 MHz or so, the higher capacitance of a "High-Z" probe makes the input impedance actuallylowerthan the Low-Z probe, and therefore loads the circuit node even more than the Low-Z probe.Some Tektronix probes were available with a 2 m / 6 ft cable. If you want to DIY something for little cost, try making a Low-Z probe. You might be surprised how well they work.

author: alm
author_rank: Super Contributor
timestamp: April 01, 2022, 02:37:36 pm
content: Anything designed for driving 50 Ohms would be fine with extending the cable with more 50 Ohm coax (within reasonable limits re attenuation and skin effect loss).For a 1x probe into a 1 MOhm input, I would expect the bandwidth to be degraded due to the extra capacitance, but more importantly the frequency response will be uneven, and the pulse response will be degraded, for any frequency content (not just fundamental frequency) with a wave length of less than 10x the length of your extension cable. So maybe 6 MHz for a 3m extension.For a 10x probe I'd expect it to be unable to compensate for any significant amount of extension.If this is a scope with isolated BNC connectors, you'd want to make sure to use isolated f-f adapters and cables for safety.

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 01, 2022, 10:15:26 pm
content: You can get longer reach probes up to about 3 meters if you go looking.