Probe Advice Needed (Oscilloscope, function/signal generator, frequency counter) - Page 1
Forum section: electronics / beginners

author: 6E5
author_rank: Regular Contributor
timestamp: December 17, 2014, 03:10:32 am
content: I have a 100Mhz oscilloscope, what is a good brand name passive probe set to buy?With 50ohm impedance devices such as signal generators, function generators, and frequency counters, what kind of probes should I buy for them? BNC to BNC, BNC to alligator, those kinds of things? What brands are good? Do they make some that look like oscilloscope probes?Finally, whats a good assortment of impedance matching terminators to have? I've heard 50ohm T-terminators are common, what else? (For the record my oscilloscope has a 50 ohm setting, so I don't need one to connect my signal generator to the scope, right?)

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2014, 03:18:04 pm
content: Quote from: 6E5 on December 17, 2014, 03:10:32 amThe key point is to understand thetypeof probe, and each type's limitation. This is best done by looking at the many publications by Tektronix and/or HP/Agilent/KeySight, and by other threads on this forum.In particular understandthe probe's tip capacitance, and what the corresponding impedance is at 100MHz (hint: <1kohm!)the inductance of any grounding lead (hint: 1nH/mm)the resonant frequency of those twoThen, for digital signals you will see why "low impedance Z0" probes with ground bayonet/spear connections are the best passive probes. Fortunately you can make these yourself, unlike other probes.Even for ordinary probes, a ground bayoner/spear is preferable; see picture for one example.Expect a decent probe to be eye-wateringly expensive; professional ones can cost >$10k!The 50ohm terminations in scopes are typically low power - understand the scopes' and probes' input voltage ratings at each frequency. Through termination "3dB pads" or higher are often used to protect scopes.

author: nfmax
author_rank: Super Contributor
timestamp: December 17, 2014, 04:35:06 pm
content: Generally speaking, if you want to connect a signal generator to a breadboard it is best to solder a BNC socket to the breadboard and use a BNC-BNC cable, if only because it is much less likely to fall off than a BNC to alligator clip lead. At frequencies above around 1MHz you shouldn't use alligator clips, and you probably want to 50 ohm terminate the BNC-BNC cable at the breadboard: this can be a 50 ohm (or 51 ohm) resistor on the breadboard itself, or maybe the component on the breadboard you are driving has a load impedance which is significant compared with 50 ohms and you need a matching network between the two.A good trick here is to use a BNC-BNC attenuator adapter (6dB or 20dB) right at the breadboard input: the attenuation makes the cable termination relatively insensitive to the actual input impedance of the breadboard circuit, so it will always be close to 50 ohms. Obviously you need to increase the signal level from the generator to compensate for the loss. Higher loss gives better matching.BNC 'T-pieces' are always useful to connect multiple cables together. By themselves they don't provide any matching: for that you would want a similar looking but very much more expensive thing called a 'power divider' - you probably don't need one (yet).One or two 50-ohm BNC terminators (plug only) or through terminators (plug and socket) are handy for providing cable termination. Given your scope has switchable 50 ohm input termination you won't need to use one in that position, but it can still be handy for use with breadboards.In general (given your scope has 100MHz bandwidth) you can happily use a conventional x10 passive probe to monitor breadboard outputs as well as internal signals. If the breadboard circuit is supposed to drive a 50 ohm load, again solder a BNC socket and use a BNC-BNC cable to the scope, selecting the 50 ohm termination on the scope input.AlsoDon't use a scope probe (even a x1 scope probe) to connect a generator output, as they have built-in resistance and you will wonder where the signal wentFrequency counters may have either 50 ohm inputs, high-impedance inputs (like 'scope inputs), or switchable 50 ohm termination. Use BNC-BNC cables with 50 ohm inputs and ordinary scope probes (x1 or x10 as appropriate) with high impedance inputs. Alligator clip leads and x1 probes are generally good only up to 10MHz or soWARNINGYou will find yourself accumulating a large number of cables, adapters, terminators, and probes of different types. You will wonder where they all came from. And you still won't be able to find the one you need