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Power measurements, what am I doing wrong?

May 15th, 04:40

ON4DD

Joined: Aug 2nd 2014, 09:58
Total Topics: 0
Total Posts: 0
I have tried to combine a few measurement methods on a signal coming from a signal generator.
They don't really seem to agree on the measured power, so I guess I might be making some measurement/thought mistakes.

Source:
Signal generator: TTi TGR2050, 100MHz, 0dBm out
Cable: know good Ecoflex 10 cable

Measurements:
1. Tektronix TDS 5054 500MHz scope
Terminated in 50 ohms, measured Vpp=0.261V
Power: P=U²/R, adds up to 1.36mW or 1.34dBm

2. Rohde&Schwarz FSL Spectrum analyser
RBW and VBW set to 300Hz, span 10kHz
Peak power: -7.63dBm

3. Rohde&Schwarz URY millivoltmeter with RF sensing head
Terminated in 50ohms load
-7.67dBm
PEP: -7.53dBm

Seems I am not measuring correctly, or my understanding isn't deep enough.
What am I doing wrong?

Regards,

Dieter
ON4DD
May 15th, 10:13

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
0.261 volts/2=0.13 volts peak
0.13 volts peak/1.414=0.092 volts rms
(0.092volts)^2/50ohms=0.17 x 10^-3 watts or 0.17 mW
10*log(0.17)dBm =-7.7 dBm

Zack Lau W1VT
ARRL Senior Lab Engineer
May 15th, 10:40

ON4DD

Joined: Aug 2nd 2014, 09:58
Total Topics: 0
Total Posts: 0
Dear Zack,

Thank you very much.
That makes sense and confirms the other measurements.
So... I should have my doubts about the signal generator then, correct?

Regards,
Dieter
May 15th, 12:04

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
It may be a bad connector.

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