21
Setup/Calibration Cont’d
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.
The accuracy of this setup will mainly be related to the quality of the load. If the dummy load error is 5%, then the power calculation will
be roughly 5% off. You can roughly guess at the RF resistance of your dummy load by measuring it at DC with a DMM, although that
method will most likely be inaccurate at 50 MHz, and probably at 28 MHz as well. Make sure you measure the resistance with the load
at operating temperature. Also, all connecting cables / adapters need to be as short as possible. If you are unsure of the quality of your
dummy load, I recommend visiting
to look at some of their offerings. These are high quality loads, and
for a small fee they will supply you with a calibration table and chart of the return loss of the load.
The actual diode drop will very likely be within about .2V of the assumed value in the formula, for a voltage error of under 2%.
If you don’t have access to these methods, you can send your completed LP-100 back to me for calibration if you are willing to pay for
return shipping costs.
Before doing the power calibration, make sure that you have the correct style of connecting cables selected in the Connecting Cable
screen. The choices are RG-174 and RG-58. Early serial numbers used RG-174, but later models switched to RG-58. The Connecting
Cable screen allows you to select the cable type that matches your meter, which then sets the correct cable compensation table for that
cable. Note: This screen has been changed to one where you select the serial number range of your meter. This allows compensation
for other small hardware factors that may change from run to run as well.
The first step in power calibration is to set the Master Trim value. This should be done on 3.5 or 7.0 MHz. Make sure the Fine Trim
setting for this band is 1.000, then transmit at a known power level and adjust the Master Trim for the correct power reading. The Mstr
setting will not be touched after this.
To adjust the Fine power constants for each band, simply transmit on the band of interest and adjust the Dn/Up buttons for the correct
power readings. Move through all bands in sequence until they have all been adjusted. You will notice that when you transmit now, the
band indicator shows the band you are transmitting on and the Trim value changes automatically based on the band.
The Mstr Trim setting will typically be within 2% , and the variation of Fine Trim setting should be < 1% from 160-20m, <2% through
10m and <10% on 6m. This is dependent on a number of factors to do with xfmr winding, positioning and wire routing, and so will vary
from builder to builder… BUT the calibration routine will eliminate any variances. Setting the trims to the following values will typically
result in better than 5% accuracy without calibration through 10m…
Mstr – 1.000
160-80m – 1.000
60-30m – 0.998
20m – 0.995
17/12m – 0.993
10m – 0.990
6m can be left at 1.000, but it can be as much as 10% off without specific calibration.
The last screen to adjust is the Hi/Lo screen. This is used to match the power readings for the high and low power ranges. If you
transmit at a power of 100W, the two readings should roughly match. If not, adjust the Dn/Up buttons to match the readings. The Hi
reading has less resolution, so it will jump around a little and you may not get an exact match. If that’s the case, err on the high side.
Log all your constants for future reference, and you’re done. There is a page at the end of this manual to make that easy.
NOTE:
Normal use of the LP-100, including the flash programming of a new firmware version, will not disturb the saved CAL constants unless
you have the MCLoader software set to “Program Data”. Jotting the values down will allow you to return to your original settings in case
you accidentally change a value by mistake. I am planning a Windows utility to allow saving, editing and restoring of the CAL table.
Final details
If everything has checked out to this point, you can complete the assembly of the controller by adjusting the LEDs on the front panel to
line up with the holes, and snap the switch caps in place on the switches. You can also attach the rear panel to the RCA connectors
using the 4-40 x 3/8” (9.53mm) self-tapping screw provided. Don’t overtighten. You can now install the cover on the controller using the
4-40 x ¼” (6.35mm) self-tapping screws provided.
NOTE: Do not accidentally use the longer 3/8” (9.53mm) screw at the case location
near the PLED display connector. It is imperative that this screw not be longer than ¼” (6.35mm) or it will short out the connector.
If you wish to add a power switch to the LP-100, you can do so at this time. I provided a 2-pin header to wire the switch to using a plug.
In this way, the LP-100 PCB can be removed in the future by unplugging the switch.
Summary of Contents for LP-100
Page 28: ...28 Schematic Page 1 ...
Page 29: ...29 Schematic Page 2 Coupler Schematic ...
Page 36: ...36 Appendix A ...
