Calibrating a 1N34A diode based Peak RF Power Meter

My homebrew RF power meter has 2 inputs. 1) using a 1N4148 peak detector and 2) using a 1N34A diode as the peak detector. This second port will read RF power from approximately 20dBm - 0dBm.

1	Heading	1N34A RF Power Meter Calibration
2	Label	RF Power Measurements
3	Date	11/27/2013.
4	Acknowledgements	1.      Author : ZS6RSH. 2.      Reference: EMRFD Section 7 paragraph 7.3.
5	Revision	Rev 1.
6	Revision History	RF Power calibration has been done for the 1N4148 version for power measurements up to +34dBm using a DC calibration method at 7020Khz only.
7	Scope	Calibrate a 1N34A peak power meter over the approximate range of +19dB maximum - 0dBm minimum. It is likely that the meter will not accurately perform below about 0 dBm. The calibration will be performed using calibrated attenuators and RF with the starting point being defined using a DC voltage near FSD.
8	History	This is a homebrew RF power meter built according to EMRFD Section 7 para 7.3
9	Configuration	Refer to the test schematic below.
10	Test equipment specifications	1.      Keithley model 8024B voltmeter. 20V scale. 2.      Homebrew variable voltage current limited, power supply. 1.5V – 15VDC. 3.      Connection leads. Regular leads that came with the voltmeter. 4.      Jumper leads used for the power connections. 5.      K2 QRP rig 6.      Coax connection from rig/attenuator output to power meter input 7.      Calibrated attenuators 3dB, 6dB, 10dB, 20dB. VSWR better than 1.5:1 from DC to 1GHz. Ref http://www.picotech.com/oscilloscope-accessories.html#TA050
11	DUT specifications	1.      Homebrew RF power meter using a 0.1mA FSD meter.
12	Workbench process	1.      Connect up the power supply and the Voltmeter set to read 3 volts 2.      Increase the voltage to 3volts. 3.      Quickly check that the meter is reading 3V at FSD 4.      If not then adjust the internal Pot until this reading is obtained. 5.      Set up the K2 connected into the Power meter. 6.      Adjust the power output of the K2 to read about 90mW 7.      Press the tune button and observe the Power meter reading. 8.      Make sure the reading is not greater than FSD. 9.      Record the reading on the ammeter. Being quick to do this reading. 10.   Insert the 3dB pad at the K2 RF output end of the coax connector (see diagram below) 11.   Quickly record the meter reading 12.   Repeat the above for the 6dB, 10dB and 20dB pads. 13.   Connect the 20dB pad in with the 3,6,10dB to try to get readings at 23,26,30dB levels. Not attainable.
13	Expected Results	Expect to get linear readings down to 0dBm.
14	Uncertainties	1.      Variation in K2 power output level from one reading to the next. May be necessary to repeat the readings three times to check this. 2.      Variation in actual performance of the Power meter at RF (test frequency= 7020KHz) vs at DC. 3.      The whole procedure assumes that the attenuation pads are correctly calibrated. The spec is as above in section 10.7. 4.      I used a DC voltage to obtain a reading at 0.9mA on the power meter. There could be an error between that reading and the actual RF power output from the K2.
15	Preparation	Completed
16	Perform validation measurements	Validation focused on setting the K2 to read as close to 90mW as possible (ie FSD on the meter). Once the 3VDC FSD calibration had been performed I then connected the K2. Since it is a digital power setting, the closest I was able to set the rig to FSD was a meter reading of 0.9mA. I then reconnected the DC power source and adjusted it to obtain 0.9mA meter reading. This setting was at 2.84VDC. Using the formula P =Vpk^2/2R I calculated the RF power output as 81mW. This was thus the starting point for the measurements.
17	Perform the full measurement plan	Performed as planned except for the validation/starting point measurements as above.  0dB, 3dB, 6dB, 10dB. 20dB attenuators were used.
18	Observations	The lowest practical reading was with the 20dB pad in circuit. This measurement just moved the meter to 0.04mA which is -0.92dBm (see table below)
19	Change Control	Refer above to the validation measurements change to allow for the fact that the K2 RF output could not be precisely set to 90mW FSD.
20	Computation	Refer to the table below. The Meter readings were recorded against each attenuation  pad in circuit. Thus it is assumed that the Pad is correctly calibrated.
21	Analysis	The graph shows a ‘knee’ around 0.25mA which would be as expected for this diode. More data points would be needed between the 10dB and 20dB marks in order to get a more accurate curve below 0.25mA
22	Conclusions	The calibration was completed according to the plan, however a better calibration would be possible with a calibrated RF signal generator.
23	Documentation	Completed.

Testing the ZS6RSH Measurement Procedure

The below table is developed as a teamplate for the Measurement Procedure now developed. This template is filled in using microsoft word embedded tables. The table is then simply copied onto the clipboard and pasted into the blog editor as below.

This table is also included in the RF Power measurements blog as written on 11/10/2013.

1	Heading	RF Power Measurements procedure.
2	Label	RF Power Measurements.
3	Date	11/22/2013.
4	Acknowledgements	1.      Author : ZS6RSH. 2.      Reference: EMRFD Section 7 paragraph 7.3.
5	Revision	Rev 1.
6	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.
7	Scope	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.
8	History	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.
9	Configuration	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.
10	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.
11	DUT specifications	Homebrew RF Power meter including dummy load. Power range of 15V FSD, Approx +34dBm - +12dBm.
12	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.
13	Expected Results	The recorded voltages against the ammeter readings should represent the transfer characteristics of a silicon diode of type 1N4148
14	Uncertainties	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.
15	Preparation	Completed.
16	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.
17	Perform the full measurement plan	Completed.
18	Observations	No unexpected variations or observations.
19	Change Control	No changes were made to the original plan.
20	Computation	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.
21	Analysis	The graph of the results shows an expected transfer characteristic in line with a 1N4148 diode over the measurement range.
22	Conclusions	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.
23	Documentation	Completed

A Measurement Procedure

Author: ZS6RSH Dick Hayter

Acknowledgement: ZS6BMN Jan Hattingh

Date: 11/19/2013

Revision: Draft B

Revision History: Draft B has various formatting changes and clarification additions.

1.    Document description

This document describes the measurement procedure used by ZS6RSH.

2.    Objective

     2.1. Obtain credible measurement results

     2.2.Obtain repeatable measurement results

3. Procedure

3.1. heading

Insert a heading that clearly describes the measurements covered.

3.2. label

Used for search purposes on the blog write-up.

3.3. date

Insert the date when the documentation of the procedure commenced (US format). If the procedure took > a day then insert the date in each sub section when that section is processed.

3.4. acknowledgements

Document those who contributed to this procedure including acknowledgements and references to input sources.

3.5. revision

Draft A, B… Revision 1, 2…

3.6. revision history

Summarize the latest changes made to the document.

3.7. scope

Carefully define the scope of the measurements to be carried out, including the parameters, quantities and ranges and the method of analysis and presentation of the data to be recorded.

3.8. history

Describe any related measurements that have been carried out to date.

3.9. configuration

Develop drawings, schematics, tables and supporting descriptions of the planned procedure clearly showing all test equipment, cables, and the device under test (DUT). Draw each of these schematics by hand or CAD and label each one using the nomenclature Fig 1, 2,….n. Include a heading, date and author. Refer to these figures in the text of this document. Include these figures in the blog write up.

3.10.               test equipment specifications

Describe each piece of test equipment including any model numbers and the relevant test equipment specifications that may relate to the accuracy of the test equipment during the procedure.

3.11.               device under test specifications

Describe the device under test (DUT) including any model numbers and the relevant DUT specifications that may relate to the behavior of the DUT during the procedure.

3.12.               workbench process

Document the step-by-step process that is intended to be followed during the measurement procedure. Include as much detail as possible since this will aid greatly in understanding exactly what structured actions need to be carried out on the workbench and in what sequence. Ensure that all ‘surrounding fixed parameters’ such as Vcc, Ic and Temperature measurements are recorded.

3.13.               expected results

State the expected results of the measurement in as much detail as practical. If the expected result is unknown then think again about the scope and objective of the measurement since it may need to be broken down into smaller measurement steps.

3.14.               uncertainties

Carefully identify any uncertainties and sources of inaccuracies and work hard to quantify what these uncertainties are. This can be done by studying the accuracy related specifications of the test instruments and DUT.

3.15.               preparation

Clear the work bench.

Carefully set up the measurement configuration and check that all leads, power, instruments and DUT are correctly set up. Ensure safety at all times.

Draw up the tables and documents needed to record the measurements in the lab notebook.

3.16.               carry-out the validation measurements

Make a first measurement and ensure that there are no safety issues. Check for smoke!

After ensuring that there are no safety or equipment issues, check the first measurement result to confirm that the result is as expected. Proceed with a first set of measurements (Vcc, Ic, Temp, max, mid, min…) These should ideally be tests that have a predictable result that will confirm as far as possible that the measurement configuration is indeed working correctly.

3.17.               carry-out the full measurement plan

Perform the full set of measurements, recording the results carefully in clear tabulated form according to the plan already set forth.

3.18.               observations

Carefully record any observations of possible interest as the measurements proceed.

3.19.               change control

As often happens once the measurements proceed, changes to the original configuration or tests are desired. These changes must be documented in detail against the original or amended schematics and measurement plan. Notes in this section should refer to any changes made from the original plan and the documents where those changes are recorded.

3.20.               computation

After a break (important).

Sit at the desk with the measurement results in hand.

Carefully complete any mathematical derivations on the results obtained and tabulate these results as makes sense in the form of additional columns and rows on the already-defined spreadsheet.

3.21.               analysis

Analyze the results objectively and taking into account any notes and observations made during the measurement process. Make detailed notes of this analysis.

3.22.               conclusions

Document objective conclusions that are based on established facts only!

Avoid deriving conclusions that are not credible!

3.23.               clean up

Important! Dismantle the measurement configuration and stow all equipment and cables. Switch off all powered equipment and ensure a safe environment. Clean the work bench.

3.24.               next steps

Define the next steps such as the next stage to be built in the radio project or further measurements that must be carried out.

     3.25     documentation

Edit and/or document this set of Measurements on the blog ZS6RSH.blogspot.com