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TekScope® "dB" and "dBm" Measurements

Using the DMM's dB and dBm Scales

In some circuits, reference values may be specified in dB or dBm, rather than volts. The TekScope DMM can display voltage measurements in volts, dB (relative to a specified reference voltage), or dBm (into a specified load).

 

To determine the gain of an audio-frequency amplifier, measure the voltage at its input. In this case, the input is 300 mV.

 Input voltage is 300 mV

To see results in dB, bring up the vertical menu, then use the fourth bezel button to set the "Volts Scale" to "dB".

 Set Volts Scale to dB

Use the ± rocker to set the reference voltage. Since the input is 300 mV, that is what we set the reference voltage to.

 Set reference to 300 mV

While the probes are still connected to the 300 mV input signal, the measurement is 0 dB.

 The input is 0 dB

Move the probes to the output of the amplifier. The meter shows the ouput is about 20 dB relative to the input, so the gain of the amplifier is 20 dB (10x).

 Output is 20 dB

If we need to know the output power of the amplifier, we can set the "Volts Units" to "dBm", and set the impedance to match that of the circuit. For a 50-ohm load the power is 22.5 dBm (about 180 mW).

 Output is 22.5 dBm

By setting the "Volts Scale" back to "Volts", we see the output voltage is 3 V. As we deduced from the dB measurement, the amplifier has a voltage gain of 10x.

 Output is 3 V

 

Return to the index page to learn more about TekScope features.

 

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