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:
Refer to the details of the test procedure below:
|
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
|
 |
| RF Power Meter Schematic |
No comments:
Post a Comment