please help in RF power Signal...
please can any one help me to know how i calculate power Rf (27 MHz) signal from oscilloscope...
i adjusted the attenuate switch in probe to x 10 and the amplitude of signal is (2 x 4 square) as shown in oscilloscope.
note: i don't know if the signal is 8 volt or 80 volt after i switched to x 10 in probe...
thanks in advance
You can use the reference signal of oscilloscope to calibrate.
The RF power should be (Vpp^2)/Z0/8.
thank's tony, but my question is when i switched the attenuate probe to x10, is the signal peak to peak value x 10 or x 1?
because when i switched the probe,signal (27 MHz) is decrease only 0,5 or 1 time from original value...
thank u.
A typical oscilloscope with a probe shows the voltage pattern across the high-impedance input, 1 MegaOhm, plus some capacitance. If your scope has a 50-Ohm input impedance, then you should NOT use the high-impedance probe. Use a direct 50-Ohm cable connection; then the RF power is the squared peak-to-peak voltage divided by 50 Ohms.
If your scope has only the high-impedance input, use a 50-Ohm through termination at input.
thank's for answer.
my oscilloscope model is (GWinstek GOS-6050), i think there is no 50 ohm input impedance.
i read in a lot of articles, when you want to operate on hi freq as (27 MHz) you should switched the prob to x 10 for divide the parallel capacitor in probe.
please focus on my question, when i show the signal on oscilloscope for example on x 1 on the probe, the signal value from peak to peak is 8v. and when i switched to x 10 in the probe the signal value from peak to peak is 5v. there is no change ten time of 2 state,
and when you work on 100 KHz freq you see the Difference ten time of 2 state...
i don't know what is the correct read...
Please read again what I wrote. You can observe a RF voltage using the high-impedance probe but you cannot MEASURE the power as you do not know the oscilloscope-input impedance with the probe at 27 MHz. The right method is as I described, with a 50-Ohm load. No probe.
There is another method but for this you need a calibrated signal source at 27 MHz. If you know its output power (across any impedance but best with 50 Ohms), you can measure the voltage with your probe, then compare the observed voltage on the scope with the other you want to measure. Best if your calibrated signal generator can adjust the output so the probe can be used.
The calibrator in the scope typically generates a 1 kHz or 100 kHz square-wave signal but this is too low frequency compared to 27 MHz. It is best to adjust the probe response for pulsed signals.
To measure a power by a voltage, you must know the impedance across which the voltage is measured. A typical scope probe has not a calibrated impedance at 27 MHz.
There is an easy way for you to determine this for yourself. Set up a signal generator to drive into a matched load (if it's an RF sig gen with a 50 ohm system impedance, then drive the signal into a 50 ohm load and probe the voltage across the load). The power on the sig gen readout should directly match the power you calculate using the oscilloscope voltage measurement (with Z0 = 50 ohms, since you are measuring power inside a 50 ohm system, in this example). If your calculations do not work out correctly, scale the o-scope measured voltage by 10x or /10 to see if you get a matching answer. This will help you to better understand what your hardware is giving you.
(at 27 MHz or below, you can probably solder a 50 ohm resistor, with short leads, onto the end of a coax cable and get pretty accurate results)
Using simple experiments to validate your test setup is a good way to learn to diagnose and troubleshoot your own problems in the design/development environment. Asking questions can be very time consuming, and learning to use the tools you have at hand will make you a better designer. Try some simple, known circuits to see if the measures meet your expected outcome. You'll find that setup errors can be some of your worst nightmares, and simple validation before starting your experiments can save you lots of headaches.