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Heading
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1N34A RF
Power Meter Calibration
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Label
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RF Power
Measurements
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3
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Date
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11/27/2013.
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4
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Acknowledgements
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1.
Author : ZS6RSH.
2.
Reference: EMRFD Section 7 paragraph 7.3.
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Revision
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Rev 1.
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Revision
History
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RF Power
calibration has been done for the 1N4148 version for power measurements up to
+34dBm using a DC calibration method at 7020Khz only.
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7
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Scope
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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.
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History
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This is a
homebrew RF power meter built according to EMRFD Section 7 para 7.3
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Configuration
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Refer
to the test schematic below.
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10
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Test
equipment specifications
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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
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DUT
specifications
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1.
Homebrew RF power meter using a 0.1mA FSD meter.
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Workbench
process
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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.
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Expected
Results
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Expect to
get linear readings down to 0dBm.
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Uncertainties
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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.
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Preparation
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Completed
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Perform
validation measurements
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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.
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Perform the
full measurement plan
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Performed as
planned except for the validation/starting point measurements as above. 0dB, 3dB, 6dB, 10dB. 20dB attenuators were
used.
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Observations
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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)
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Change
Control
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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.
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Computation
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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.
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Analysis
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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
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Conclusions
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The
calibration was completed according to the plan, however a better calibration
would be possible with a calibrated RF signal generator.
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Documentation
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Completed.
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