BNC Connections with Test Equipment

[TheDriver]

I just got a frequency counter and soon will have a function generator. Does any special care need to be taken when making any of the following connections?

Function generator => Scope
Function generator => Device under test
Function generator => Frequency counter
Device under test => Frequency counter

When I got my scope, I was advised on selecting the correct probes. For the above connections can I use just any BNC to BNC or BNC to Clip connector?

Can and should I use my scope probes for any of the connections. For example;

Function generator => Device under test
Device under test => Frequency counter

Thanks,
Matt

[oshifis]

Matt,

First thing you should be aware of: there are two versions of BNC: 50 ohm and 75 ohm (and 91 ohm for old computer networks). The 75 ohm version has thicker prong, which can destroy the 50 ohm socket.

Apart from this, the scope probes are only for the scope inputs. They can not be used for anything else. Connect the frequency meter to the trigger output of the function generator (if there is any such output). Alternatively, many scopes have a signal output at the rear side.

In general, BNC is not absolutely mandatory for audio frequencies. They were developed for high frequency, where impedance matching (50 ohm source to 50 ohm termination) is important, and the coaxial cable itself also has 50 ohm wave impedance, in order to avoid reflections. Coaxial cable is used mainly for its characteristic impedance, not for shielding. At audio frequencies the effect of reflection (due to impedance mismatch) is not significant, but shielding is, especially above a few kohm termination impedance.

[jackinnj]

BNC are good up to a few hundred MHz

You can make quite good BNC cables on your own...if you buy them, though, look for high quality cables from Pomona.

[Elvee]

Originally posted by oshifis
Matt,

First thing you should be aware of: there are two versions of BNC: 50 ohm and 75 ohm (and 91 ohm for old computer networks). The 75 ohm version has thicker prong, which can destroy the 50 ohm socket.

It is the opposite.
But anyway, you don't have to worry too much about it, all garden varieties of BNC are 50ohm or undefined.
75 ohms are used in principle in video and telecom equipements, but in practice, it is rare they are actually fitted with true 75ohm connectors: only precision instruments such as VNAs, etc do have them.
Moreover, most female 75ohm BNCs have a flexible enough orifice (well well, we are on a slippery slope here) to accept 50ohm males without sustaining permanent damage, unilke N connectors f.e.

[unmibh]

Hi Matt,

Let me add that you will need a proper oscilloscope probe as they have a calibration fuction for the frequency response.
IIRC a small variable capacitor.

If you build your own cable for the counter,just use a quality coaxial cable, Belden for example and use the right tools like crimping tool for crimp type connector and also use the same connector type for the same type of coaxial. 75 ohms to 75 ohms cable.
Don't mix 75 and 50 ohms cable and connector.

Cheers
Rom

[TheDriver]

Thank you everyone for your advice!

What is the difference between RG58C/U, RG174/U, and RG223/U?

These are the three types of coaxial cable that Pomona offers for BNC cables. I thought about making my own but decided the cost is worth it this time.

Is the "Trigger Output" on a function generator the same as the "Sync Output"?

I checked my scope, tek 2235, and it doesn't have a signal out

Why is it that a DUT => Scope connection needs to have an adjustment device on the probe but a DUT => Frequency Counter connection doesn't need a adjustment device?

Along the same thought, I see people connecting a function generator to a scope with a BNC to BNC cable with no adjustment device. Why is a adjustment device not used in this connection? Is it because the output impedance of the function generator is the same as the input impedance of the scope? ( I use the word impedance here without a complete understanding of what it is. Maybe a better question is what is impedance matching?)

Thanks,
Matt

[unmibh]

Hi Matt,

I may be wrong here but here are my thought.

The oscilloscope is an instrument where your prime requirement is to see what the signal looks like and from there you can determine what's going on if you have under shoot or over shoot of your signal or even clipping.
With this your probe need to be calibrated and so as not to give you a wrong analysis, You scope should have a cal point normally if memory serves me 1 Khz square wave.

Counter are for measuring frequencies , oscillators, it's prime use is to count the interval of the signal. Like for example to give you your sine wave are 10 Khz.

Cables are design for a particular job/performance and cost.

From your questions , I got the feeling that your just starting to learn so I encourange you to read more.

Cheers

Rom

[oshifis]

Originally posted by TheDriver
What is the difference between RG58C/U, RG174/U, and RG223/U?

These are the three types of coaxial cable that Pomona offers for BNC cables. I thought about making my own but decided the cost is worth it this time.

Is the "Trigger Output" on a function generator the same as the "Sync Output"?

I checked my scope, tek 2235, and it doesn't have a signal out

Why is it that a DUT => Scope connection needs to have an adjustment device on the probe but a DUT => Frequency Counter connection doesn't need a adjustment device?

Along the same thought, I see people connecting a function generator to a scope with a BNC to BNC cable with no adjustment device. Why is a adjustment device not used in this connection? Is it because the output impedance of the function generator is the same as the input impedance of the scope? ( I use the word impedance here without a complete understanding of what it is. Maybe a better question is what is impedance matching?)

Thanks,
Matt

Rg-58 is the most common standard 0.2" outer diameter cable. Rg-174 is about half diameter, used for low power and Rg-223 is double diameter, used for high power. This is a rough guide. Loss is proportional to diameter, but at audio this is not an issue.

I just connect the Sync output of my signal generator to the External Trigger input of my scope with a piece of coax cable (and sometimes also to the frequency counter using a T-piece), this way the measured signal is rock stable regardless of the level. The impedances are not matched, but as I said at audio this is not a problem.

The adjustment on the scope probe is for calibrating the square wave response to a known good square wave (using the cal. output of the scope). No overshoot and flat top is the goal. Then you can believe what you see on the scope when measuring the DUT. Calibration has to be done in the 10:1 divider setting.