10 November 2013

RF Power Meter

Today I finished building my RF Power Meter. This is a design from EMRFD section 7 by W7ZOI. The schematic is self explanatory. It is basically 2 power meters in one. The meters read Peak RF voltage that is then converted to Power using the formula P = V^2/2R.

I have completed the calibration of the bigger input using 15volts DC. I also calibrated it using 10V DC FSD. I can measure accurately a range of +34dBm - +12dBm. The measurements I took are shown in the spreadsheets.

I need to get some bigger power resistors so that I can measure at least 5 watts. Currently even measuring 2.25W generates a lot of heat in the little dummy load.

I performed a sweep using my antenna analyzer of both dummy loads. They are both flat 1:1 SWR well into VHF.

I am looking forward to calibrating the small meter which should allow me to measure down to about -8dBm. Thus I can use it to so some measurement on my receivers.

I have developed 2 calibration charts which show the non linearity of the 1N4148 as can be expected.

I built the meter into a medium quality plastic box that I had lying around.

Refer to the details of the test procedure below:

RF Power Measurements procedure.
RF Power Measurements.
1.      Author : ZS6RSH.
2.      Reference: EMRFD Section 7 paragraph 7.3.
Rev 1.
Revision History
A blog was first written on 11/10/2013. This procedure is being written ‘after the fact’ with the aim of testing the effectiveness of the measurement procedure.
Based on the recommendations in EMRFD it is valid to calibrate my homebrew RF power meter using DC power.  As confirmed in EMRFD, the calibration will be valid through the HF range and into VHF for the specified diode 1N4148. Since the resistors I have used for the dummy load are only 1/4watt rating it is only possible to calibrate between the voltage ranges of 1V-15V at this time. The lower app ~ 1 volt limit is due to the silicon diode becoming non-linear for voltages below that value. Thus the power measurement range will be approximately between +34dBm and +12dBm.
The accuracy of this DC calibration is primarily dependent on the accuracy of the DC voltage measurements (see UNCERTAINTIES below). Measurements have previously been carried out to understand the accuracy of my two DC voltmeters. The Fluke and Keithley. (Model numbers to be provided). These two meters read the same DC voltages to 2 decimal places over the range 15V – 1V. Refer to xxx For these measurements.
 The homebrew RF Power meter is connected to a variable voltage DC power supply with a variable voltage range of 1VDC – 15VDC and a maximum current capacity of 1Amp. The DC voltage was measured using the Keithly voltmeter connected across the input of the power meter.
Test equipment specifications
1.      Keithley model xxx voltmeter.
2.      Homebrew variable voltage current limited, power supply.
3.      Connection leads. Regular leads that came with the voltmeter.
4.      Jumper leads used for the power connections.
DUT specifications
Homebrew RF Power meter including dummy load. Power range of 15V FSD, Approx +34dBm - +12dBm.
Workbench process
1.      Back off the calibration pot so that the meter cannot be overdriven.
2.      Set the power supply to 15V.
3.      Quickly adjust the cal pot to achieve FSD of 1mA.
4.      Turn off the power supply.
5.      Reduce the power supply voltage so that the meter shows decrements of 1/10 of a milliamp.
6.      For each 1/10 milliamp reduction, quickly record the voltage to 2 decimal places.
7.      Take 10 readings.
8.      Change the FSD to 10Volts.
9.      Take 10 readings as above.

Expected Results
The recorded voltages against the ammeter readings should represent the transfer characteristics of a silicon diode of type 1N4148
1.      Dummy load change as a result of dissipation heating. Can be kept to a minimum if the tests are carried out quickly. This variation can be characterized in a separate set of measurements. However the plan is to build a dummy load with QRP power dissipation capabilities in the future.
2.      Specific transfer characteristics of the diode are unknown but will be discovered.
3.      Non linearities in the specific voltmeter readings. Already verified to not be an issue to 2 decimal places.
4.      Variations in ambient temperature during the test period. Not taken into account during this test but could be by recording the temperature for each measurement.
5.      Parallax errors from reading the analog ammeter. Can be read to the nearest 1/100th of a milliamp.
6.      Quick reading of the meters could result in a recording error.
7.      Calibration Graphing errors. However the data was recorded to 2 decimal places.
8.      It is uncertain that the RF Power Meter will record peak RF voltages according to the same transfer characteristics as at DC level. This is assumed to be the case based on the EMRFD reference in section 7 and nothing else at this stage. This consideration is beyond the scope of this set of DC measurements.
9.      RF coupling causing variations in load and measurement characteristics. The SWR was seen to be a flat 1:1 across the HF spectrum using an MFJ259B analyzer. As for 8 above, this consideration is beyond the scope of this set of DC measurements.

Perform validation measurements
Completed. The meter calibration pot was set to a minimum to start with to ensure that the meter would not be harmed by overdriving.
Perform the full measurement plan
No unexpected variations or observations.
Change Control
No changes were made to the original plan.
Refer to the attached tables. For each recorded voltage a power value was derived using the formula P=Vpeak^2/2R.  This formula is valid since at RF the meter records peak RF power. The diode rectifies the AC signal and the capacitor charges to the peak value. This power was then converted to dBm and graphed. Thus a major assumption is made here that the power meter will in fact correctly record peak RF values in practice. This validation is beyond the scope of this set of DC measurements.
The graph of the results shows an expected transfer characteristic in line with a 1N4148 diode over the measurement range.
The calibration curves are in line with the expected results and can reasonably be used to explore RF power measurements. Validation of the results, however, is needed in the future against a calibrated RF source.

RF Power Meter Schematic

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