Power Meter: Calibrator

2013/7/21

Before start building the AD8307 or other Power meter, a calibrator is need to be ready for it.

Bob Kopski's RF Power calibrator

QEX 2004 Jan/Feb published the Bob Kopski's RF Power calibrator.   very suitable for AD8307, other real RMS Power Meter and SA calibration.

1) connect a 50R load

2) adjust 200R R2 to get 158mV DC on Test Point

3) AD8307 power meter should  show -20dBm

4) true RMS power meter should show -23dBm

5) SA:

        fundamental frequency: 10Mhz,  -24dBm

                                                    50Mhz, -37.9dBm  (0.2dB error)

                                                    90Mhz,  -43dBm ( 2dB error on this frequency )

Note: none asymmetry by 5% generate only 0.5dB error.   This instrument provide about 0.5dB accuracy.

qrp.pops.net version by Jane

Jane build  a Bob's calibrator with a little change to the attenuation pad. and also a sinusoid version calibrator.

go to his excellent website: http://www.qrp.pops.net/RF-workbench-5.asp

 Jane also suggestion use -10dBm as calibrator power level.  and calibration go on as following step:

1) make a sine wave meter,

2) get same lever as your calibrator.

3) insert a step attenuator to check the linearity.

Calculator： given a test point voltage V in mVolt, what's it's  corresponding AD8307 RMS value(NO Attenuator)?

                       square wave: V*2  Peak to Peak

                       equivalent sine wave:  V*2*2 =4V

                       AD8307 RMS: 4V/2.83

                       AD8307 dBm:  10*log[ (4V/2.83)^2/50]  = 10*log(16*V*V/(2.83*2.83*50*1000)=10*log(V*V/25027.8125)         

Note:

        1) replace 20dB attenuator to 10dB, how get accuracy resistor from 5%: refer to  Attenuator: 50ohm/81dB 1dB step

        2) connect a step attenuator is acceptable.

        3) I chose a local version CMOS oscillator which similar performance, but raise time ~= 10nS.

Checking the Power Level

 AD8307 power meter should  show -10dBm

 True RMS power meter should show -13dBm

  SA result should be:

         10Mhz,  -14dBm

          50Mhz, -27.9dBm

          90Mhz,  -23dBm