# How to read frequency on an oscilloscope?

#### Tonyr1084

Joined Sep 24, 2015
7,358
Have an old Techtronix scope. Trying to understand the sweep: Sec/Div.

I have it set to 20µ and the sine wave is two major divisions. Is the 20µS per major division or per the minor divisions? If minor - would that be 4000 Hz?

How would I calculate the Hz?

The frequency I'm trying to understand is supposed to be ultrasonic, so 4kHz would be WELL within normal human hearing range. It's GOT to be greater than 20kHz. A dog barking device that's supposed to distract the dog and stop them from barking. I want to know what frequency it's operating on.

#### AlbertHall

Joined Jun 4, 2014
12,233
It is per major division. So the period is 2 x 20uS = 40uS = 25kHz.

#### Tonyr1084

Joined Sep 24, 2015
7,358
Thanks Al. How do we come to 25kHz?

#### dl324

Joined Mar 30, 2015
15,536
How do we come to 25kHz?
$$\small f = \frac{1}{T} = \frac{1}{40uS} = 25kHz$$

#### LesJones

Joined Jan 8, 2017
4,007
I am assuming that you mean that it is set to 20 uS per major division. (That is the normal way the calibration is marked.) So as the sine wave is two major divisons that is 40 uS. One second is 1000000 uS so 1000000/40 = 25000 Hz (Or 25 KHz)

Les.

#### Tonyr1084

Joined Sep 24, 2015
7,358
$\small f = \frac{1}{T} = \frac{1}{40uS} = 25kHz$
Thanks. Makes sense to me now. Expected something greater than 20kHz. Dog hearing range.

I am assuming that you mean that it is set to 20 uS per major division.
There's no other markings on the dial other than 20µ and Sec/Div. That's partly why I'm wondering if it's major or minor divisions. The minor divisions are 5 to one major division. (four small marks between the big marks)

#### dl324

Joined Mar 30, 2015
15,536
There's no other markings on the dial other than 20µ and Sec/Div.
Always major division; just like vertical sensitivity.

#### Tonyr1084

Joined Sep 24, 2015
7,358
Thanks all.

#### dl324

Joined Mar 30, 2015
15,536
Thanks all.
I've always liked Tek scopes; even when I worked at HP. I probably have a dozen of them. What scope do you have?

#### Tonyr1084

Joined Sep 24, 2015
7,358
It's a T921. Single trace. Been kicked around a bit before I got ahold of it. Knobs need some cleaning and probably a new solder joint or two. Sometimes the sweep doesn't behave properly. Also not very confident in its calibration.

For dual trace I have a PC based Hantek. Does the job but I'd have to hook up the computer and I don't feel like doing that. I'm on a Mac right now and the Hantek operates on Windows. The Tektronix is just a plug away from being switched on. And the Hantek does the math for me. Keeps me dumb.

#### dl324

Joined Mar 30, 2015
15,536
It's a T921. Single trace.
My 15MHz scope is an SC502 like the one below; fits in a TM500 series mainframe:

#### Tonyr1084

Joined Sep 24, 2015
7,358
Also have a Tektronix 561A. Four trace (3S74) but the darn thing wanders constantly. It needs serious help. I'd sell it if I could find a buyer for it. I also have a couple other plug in modules I know nothing about other than they slip in and out of the chassis. A 3S76 and a 3T77A. They have the old connectors (before BNC connectors). I think I even have a connector or two for it around somewhere along with two 10X Attenuators.

There is a test lead as well (BNC connector type).

#### dl324

Joined Mar 30, 2015
15,536
Those were the first Tek scopes I used at school. We coveted the newer ones, but the older ones were more available.

You probably have some weak tubes.

#### AlbertHall

Joined Jun 4, 2014
12,233
You probably have some weak tubes.
I understand that happens when you get older.