digital multimeter doubt?
Posted on Dec 29, 2009 under Digital news and reports |a multimeter can read ac voltage till 750V,but my doubt is can a multimeter detect RF waves,for example when the probe is touched to the output of a FM transmitter,(multimeter set to read AC voltage),will it show and value maybe (-) or (+) any value even 0.02. or will it not give any readings.
and doing this does the RF wave damage the digital multimeter?
December 29th, 2009 at 7:39 am
It should be able to give readings, but make sure you set the multimeter to a setting where it is sensitive to smaller voltages such as in mV and make sure you have set it to detect AC.
It should not damage your multimeter either!
Hope it helped!
December 29th, 2009 at 7:39 am
A multimeter is not designed to read RF. At the frequency of a FM transmitter ( roughly 100 MHz) the DVMs shunt capacitance would be too high. Also the electronics isn’t set up to read an AC frequency greater than a few thousand hertz.
December 29th, 2009 at 7:39 am
Multi-meter AC voltage input was designed only able to measure up to 1000Hz signal. Any higher frequency is filter out from the input. Therefore,it even cannot measure lower RF frequency like AM radio carrier 640kHz. FM transmitter carrier is above 110mHz ,FM radio frequencies are 98 to 108MHz. that are much higher than AM signal.
A simple device called RF signal diode can convert RF singal into DC then feeds it to a sensity DC current meter (100uA range) can do the job like what you expected.
If you measure a high output over 100 to 1000 watts RF transmitter antenna with your digital AC meter directly. The meter input shall be burnt away.
December 29th, 2009 at 7:39 am
A multimeter will not read any AC voltages above AC line frequencies. That is all it is designed for.
And the leads and the meter has so much capacitance that it would disrupt the circuit.
You need a good oscilloscope that will handle several hundred MHz with a good 10:1 probe. Probably you need one that responds to 300 MHz.
And a good spectrum analyzer is also essential, that will go up to 1 GHz, to check for harmonics and such.
.
December 29th, 2009 at 7:39 am
The DMM will NOT respond to FM transmitter RF frequencies.
Unless the power exceeds a few Watts your meter will
not be damaged if set to voltage function.
In the absence of appropriate equipment, such as an
RF Voltmeter, 100MHz + Oscilloscope, Frequency Meter;
I suggest two LOW COST alternatives.
1. Use the equipment that you intend to use with the
. transmitter, tune TX for reception, Modulated or unmodulated.
. Tune for Freq. and Signal Strength.
. or
. Use a common FM radio to tune across the FM band,
. while listening for the FM carrier. If you can follow the
. FM carrier on the radio, as the transmitter frequency is
. gradually changed, you will be ‘ON’ Frequency.
. A radio with digital frequency display gives the TX Freq.
. However it may be the Fundamental or a Harmonic Frequency!
. This activity relates to FM transmitter frequency determination.
2. Build, borrow, or purchase a VHF Probe. The probe
. elements are located very close to the VHF source.
. A half wave rectifier low capacitance diode plus a
. capacitor, pass a DC voltage to YOUR DMM for
. measurement and display. See LINK for DIY info.
. This activity relates to determining the approximate
. VHF V rms at various nodes of the TX.
. Beware, there be regulations, licences, or laws to be .
. .observed regarding the above transmissions.
December 29th, 2009 at 7:39 am
I hate DMM’s when trying to measure RF voltage. I have a great meter that is a VTVM called an RCA Sr VoltOhmist. I made a RF detector probe, and I work on all of my transmitters using it.
I would suggest that you go buy a book called the ARRL Handbook For Radio Amateurs by the ARRL and if you are really serious, add the RSGB Handbook. The Brits sometimes have a different take on things.
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