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Calibration Cont’d
Miscellaneous Settings
The next screen adjusts the Offset cal constant to compensate for any residual offset voltage on the output of the TLC271 op amp.
Simply adjust the Dn/Up buttons for a displayed voltage of 0.000V with no RF power applied.
The next screen compensates for any variations in the precision resistive divider which is used in the high power mode. It is adjusted by
transmitting at 100W or some other convenient value, and using the Dn/Up buttons to match the readings of the Low and High power
detectors.
Power Calibration
Before starting power calibration, it should be pointed out that calibration is not absolutely necessary. In calibrating hundreds of the
assembled versions of the LP-100A, it was found that the maximum band to band error, as read on a HP-436A power meter, was +/-
1.5% before calibration on 160-10m. The error on 6m can be more like +/- 5-10% before calibration. The Master trim setting on my
assembled meters was within 3%, and can be left at 1.000. Of course, there is more consistency in coupler construction on the
assembled units than there would be with dozens of different builders, but it would be unlikely that the error would be more than about
5% if care is taken to follow the instructions on the coupler assembly.
The first test requires no test equipment. While in the Setup > Offset screen, adjust the Dn/Up buttons for a zero reading of the
displayed voltage with no RF power applied. This nulls out the residual offset voltage of the op-amp detector.
The next screen allows for the adjustment of Master power sensitivity. This value will normally be near 1. Both the Master and Fine
power adjustments have a range of +/- 12.5% in 0.1% increments.
The Fine sensitivity adjustment is made while transmitting into the LP-100A. The adjustments affect the trim values for the band being
displayed. The frequency display follows the transmit frequency automatically when you transmit. Before adjusting either the Mstr or
Fine trims, it is necessary to provide an accurate means of measuring power that is independent of the LP-100A. The simplest
approach to this is to borrow a high quality meter like a Bird or Alpha to use as a reference, and connect it between the LP-100A and
dummy load with a UHF male-male adapter. An even better approach, which is what is used for the factory calibration, uses a
calibrated 30 dB attenuator feeding a laboratory power meter (HP436A or Boonton 4200 in my case). The power meter is calibrated
against an NIST (National Institute of Standards and Technology) traceable reference calibration signal.
The most accurate simple method for doing this with common tools is to use a high quality dummy load with a diode peak voltage
detector and DMM. H
ere is the setup…
The following formula can be used to determine power.
P (watts)=(Vpk + .25) * (Vpk + .25) / 100
The diode is a 1N5711 Schottky diode, and the cap is .01uF. A convenient power level to use is 10W, as it is within the PIV specs of the
1N5711. 10W produces a peak voltage of ~31 V across a 50 ohm load. The diode will handle up to 40W, but I have only tested the
circuit for accuracy at 10W. The voltage needs to be measured by a high impedance DMM with good accuracy. Most quality DMMs
have > 1 meg input impedance, and many have > 10 meg input impedance.
Summary of Contents for LP-100A
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