RF Power Meter Calibration reworked

I reworked the calibration procedures again for the 1N34A based power meter and the 1N4148 based power meters. The full calibration results are included on this Blog page.

For the 1N34A I used a DC calibration procedure but this time I used my commercial attenuators to set the input DC Voltage levels. I then compared the measured voltages with the expected value. The correlation as can be seen from the table below is exactly the same. In other words the measured values = the expected values over the measurement range.

For the 1N34A meter I also injected RF power from my K2 rig into the RF power meter at FSD. I then inserted the attenuators using the same procedure as the DC readings. The Meter readings for the DC input and for the RF input at 7020KHz were exactly the same. This increases my level of confidence that the DC calibration method is valid, at least to within a single decimal place of a dBm measurement.

I also reworked the 1N4148 calibration. I reworked these due to the upgrade of the terminator to a newly built dummy load using 9X 2 Watt 470 Ohm resistors and 1X 1000 Ohm 2 Watt resistor (refer to the pics below). This allowed me to increase the calibration range up to 25 VDC. The voltage ranges used were set by varying the input DC Voltage from the variable voltage power supply unit.

Thus the RF Power measurement range now attainable from this power meter is summarized as follows:

1N34A
Vpk (V)  0.3 - 3
P (mW)   0.92 - 93
P (dBm) 0 - 19

1N4148
Vpk (V) 1.5 - 10
P (mW)  20 - 960
P (dBm) 13.5 - 29

Vpk (V) 1.8 - 15
P (mW)  30 - 2280
P (dBm)  15 - 33

Vpk (V) 2.91 - 25
P (mW) 80 - 6240
P (dBm) 19 - 38

Refer to the calibration curves below

1	Heading	1N34A RF Power Meter Calibration
2	Label	RF Power Meter Calibration
3	Date	12/23/2013.
4	Acknowledgements	1.      Author : ZS6RSH. 2.      Reference: EMRFD Section 7 paragraph 7.3.
5	Revision	Rev 2
6	Revision History	This is a repeat of the previous measurement but aimed at improved accuracy.
7	Scope	Calibrate the 1N34A RF Power meter using measured input DC voltages that are derived using calibrated commercial attenuators.
8	History	Reference 6 above
9	Configuration	Refer to Figure 2 below.
10	Test equipment specifications	1.      Keithley model voltmeter. 2.      Homebrew variable voltage current limited, power supply 2VDC-15VDC. 3.      3dB, 6dB, 10dB, 20dB attenuators. 4.      Connection leads.
11	DUT specifications	1. Homebrew RF Power meter using a 1N34A diode.
12	Workbench process	1.      Set the DC Voltage level and the variable pot to read 3VDC at FSD 2.      Insert the attenuators one at a time and record the DC input voltage and the Meter Reading.
13	Expected Results	1.    The recorded input voltages should be very close to the expected voltages when the attenuators are inserted.
14	Uncertainties	1. Keithly meter inaccuracies. No difference to 2 decimal points is seen between the Keithly and the Fluke voltmeters. 2. The RF meter may respond differently to RF input as opposed to DC input. This calibration method fundamentally assumes that at least at 7MHz, that there is no measurable difference between the DC and RF meter response. This was confirmed to 2 decimal places by measuring the same meter readings for RF power input against the expected attenuator values. 3. The commercial attenuators are assumed to be accurate. The theoretical voltage drops aligned with the measured voltage drops. Refer to the measurement table. 4. The termination load in the RF power meter is measured as 51.6 Ohms using the Keithly Ohmmeter
15	Preparation	Start with the DC voltage on the variable power supply set to minimum.
16	Perform validation measurements	Both DC and RF at 7020KHz were injected as separate measurements. Starting from a baseline of approximately 3VDC FSD and 3VAC FSD the meter readings were exactly the same between RF and DC for each inserted attenuator pad.
17	Perform the full measurement plan	Carried out as per 12 above down to approximately 0dBm
18	Observations	The results were consistent with the expectation.
19	Change Control	None
20	Computation	Calculate the RF Power for each meter reading as P = Vdc^2/2R. (where R = 50 Ohms)
21	Analysis	The plotted curve shows the transfer characteristic of the RF Power meter over the measurement range.
22	Conclusions	Using the calibration curve, the Power Meter will provide acceptable results. However this should ideally be confirmed against a calibrated RF signal generator in order to determine the accuracy with higher confidence. This procedure fundamentally assumes that a valid result can be obtained by using DC Voltage levels.
23	Documentation	Done as shown on the Blog.

1	Heading	1N4148 RF Power Meter Calibration
2	Label	RF Power Meter Calibration
3	Date	12/23/2013.
4	Acknowledgements	1.      Author : ZS6RSH. 2.      Reference: EMRFD Section 7 paragraph 7.3.
5	Revision	Rev 2
6	Revision History	This is a repeat of the previous measurement but aimed at improved accuracy. The 50 Ohm load termination was changed by using 2 Watt carbon resistors. This gives a 20 Watt power dissipation capability.
7	Scope	Calibrate the 1N4148 RF Power meter using measured input DC voltages. Calibrate the meter using 2 FSD values of 25VDC, 15VDC, 10VDC
8	History	Reference 6 above
9	Configuration	Refer to Figure 2 below.
10	Test equipment specifications	1.      Keithley model voltmeter. 2.      Homebrew variable voltage current limited, power supply 2VDC-15VDC. 3.      Connection leads.
11	DUT specifications	1. Homebrew RF Power meter using a 1N4148 diode and a 20 Watt 50 Ohm Terminator.
12	Workbench process	1.      Set the DC Voltage level and the variable pot to read 25VDC, 15VDC & 10VDC at FSD (Full Scale Deflection) 2.      Reduce the input DC voltage and take an input voltage reading for every 0.1 Ma meter reading as close as possible. Ie aim for 10 readings.
13	Expected Results	1.    The results should show a ‘fairly’ linear transfer characteristic.
14	Uncertainties	1. Keithly meter inaccuracies. No difference to 2 decimal points is seen between the Keithly and the Fluke voltmeters. 2. The RF meter may respond differently to RF input as opposed to DC input. This calibration method fundamentally assumes that at least at 7MHz, that there is no measurable difference between the DC and RF meter response. 3. Parallax error in reading the analog power meter 4. The termination load in the RF power meter is measured as 51.6 Ohms using the Keithly Ohmmeter 5. Termination load (dummy load) heating resulting in resistance variances.
15	Preparation	Start with the DC voltage on the variable power supply set to minimum.
16	Perform validation measurements	At the FSD levels the RF Power shown on the K2 Power Meter was the same as that derived from the RF Power Meter.
17	Perform the full measurement plan	Carried out as per 12 above between the levels shown below.
18	Observations	The results were consistent with the expectation.
19	Change Control	The 10VDC FSD was not possible to obtain with the variable pot and a 10K Ohm resistor in series. A ‘FSD’ level as shown in the table was used instead.
20	Computation	Calculate the RF Power for each meter reading as P = Vdc^2/2R. (where R = 50 Ohms)
21	Analysis	The plotted curve shows the transfer characteristic of the RF Power meter over the measurement range.
22	Conclusions	Using the calibration curve, the Power Meter will provide acceptable results. However this should ideally be confirmed against a calibrated RF signal generator in order to determine the accuracy with higher confidence. This procedure fundamentally assumes that a valid result can be obtained by using DC Voltage levels.
23	Documentation	Done as shown on the Blog.