Frequency Measurement with a DMM

Discussion in 'Electronic Basics' started by Joseph Sroka-10.2.8, Apr 27, 2005.

1. Joseph Sroka-10.2.8Guest

I have a Radio Shack (actually Metex) DMM that measures frequency, among
other things.

When measuring the 60 Hz line frequency with this meter, it seems that
several items in the user's manual contradict each other or at least are
somewhat confusing.

In the Measuring Frequency section of the user's manual, it has the following:
************
(My Item A)
Warning: If you try to measure the the frequency of a signal that exceeds
250 volts AC rms, you might damage your meter and expose yourself to a
severe shock hazard.

(My Item B)
Notes:
Input limit voltage: AC 20 V max.
Input sensitivity:
1 Hz up to 1 MHz - 150 mV rms.
1 MHz up to 20 MHz - 300 mV rms.

(My Item C)
Warning: When you connect the test probes to an AC outlet, do not turn
the function selector switch to another range. It could damage the
meter's internal componens or injure you.
*********

Items A and C seem to imply that it is okay to measure the frequency of
the power from the 120 V ac household outlet. But what is this "Input
limit voltage: AC 20 V max." in Item B?

Also, what does the following mean?
"Input sensitivity:
1 Hz up to 1 MHz - 150 mV rms.
1 MHz up to 20 MHz - 300 mV rms."

Thanks.

--- Joe

2. John BokmaGuest

A) just says: if you do this, you might damage your meter *and* expose
yourself. My best guess: if you go over AC 20V max, you just damage
your meter. If you go over 250 V, you not only damage your meter, but
you risk more.

C) sounds to talk about if you just measure AC in the correct range:
don't change the function (for example to frequency measurement).
My best guess: < 150 mV rms -> frequency can not be measured accurately
(if at al) in the 1 Hz .. 1 MHz range.

3. ChrisGuest

Good morning, Joe. I kind of hoped you would have given the Radio Shack
part number -- they've got manuals for many of the meters they sell or
sold online.

There are two basic things they're trying to tell you here in A and B
-- the signal amplitude at which the meter is made to work, and the
amplitude at which damage will occur.

I read what you've copied as saying:

"This meter was made to give accurate readings of AC signals between
150mV and 20V if the signal is 1MHz or less, and between 300mV and 20V
if the signal is over 1MHz to the limit of the meter.

"Input protection circuitry kicks in above 20V. But it only works to
250V, so be careful, you might damage the meter. If you try to measure
signals above 20V, we can't guarantee the meter accuracy."

"If you try to measure voltages over 250VAC you will damage the meter
for sure."

Item C is warning separately that it's a very bad idea to turn the
switch while you've got the probes on line voltage, and will damage the
meter.

You know what line frequency is anyway -- just don't bother measuring
frequency over 20VAC. Line voltage isn't a very good way of checking
meter accuracy in measuring frequency, anyway (although everybody seems
to want to do it). 50/60 Hz is right at the bottom of the measurement
range.

And you shouldn't ever switch a meter switch while measuring anything
with any meter, just as a matter of principle. Remove the probes
first.

Good luck
Chris

4. Joseph Sroka-10.2.8Guest

Thanks, Chris.

The box and Owner's Manual are marked "Cat. No. 22-168A".

--- Joe