16
Circuit Description
Screen Saver
The screen saver works a little differently than it did in the LP-100A due to the different requirements of the GVFD display. It is a three-
step process. The first step only works in the Normal mode, while the other two work in all modes and screens. Step 1 is to dim the
display to 25% when in Normal mode. Transmitting, even just a dit, will restore full brightness as preset in the Setup screen. One
second after transmission it returns to 25%, unless you are in the Peak Hold mode. If you have the hold time set to 1 second or longer,
the screen will dim at the end of the hold time for a smoother look. Step 2 scrolls a dimmed, full screen version of your callsign across
the display after a preset amount time that you set in Setup (1-
10 minutes of inactivity). Step 3 is a “sleep” mode which turns the
filaments to the GVFD display off to reduce current draw after a preset amount of time that you set in Setup (1-10 minutes of additional
inactivity beyond the scrolling callsign). Transmitting will cancel all three steps. Tapping the Mode button will cancel the second two
steps. If you are in the sleep mode, the display will “fade” up as the filaments warm, as opposed to popping back up.
The LP-100A
is unique in it’s design in several regards. Refer to the following block diagram during this discussion.
First, instead of using a coupler that produces forward and reflected voltage signals, the LP-100A uses a pair of transformers that
sample current in the transmission line and voltage across the load. The samples are split into two paths, which provide signals to both
the gain/phase comparator and the power detector.
With a 50 ohm non-reactive load, the levels of these two signals will be virtually identical, and the phase between them will be zero
degrees. As the load varies from perfect, the relative magnitude and phase of the two samples varies, providing the meter with the
information it needs to calculate the complex reflection coefficient, rho, from which SWR and impedance are derived.
The combiner adds these two samples vectorially, providing a maximum output of 2x the input power with a perfect load, and
proportionately less with less perfect loads. The power sample is rectified in the Schottky diode detector, which uses a special dual
diode package to eliminate errors associated with temperature tracking and forward / reverse voltage drop differences. The output of
the detector is fed through precision voltage dividers to produce two power ranges, and in combination with a 12-bit A/D converter and
precision 2.5V reference chip, provides an effective resolution 12 to 13.6 bits (higher resolution on the lower range).
The power sample also feeds an AGC amp which provides a constant, clean sine-wave output signal over a 50dB+ range of input
power. This signal is sliced to create a square wave which feeds the frequency counter in the PIC to allow automatic frequency
detection at all power levels. This allows for automatic per-band calibration of all calibration parameters in the LP-100A.
The AGC amp also provides a DC “Received Signal Strength Indicator” which is used for a number of level detection tasks within the
PIC. The A/D converter also receives temperature information from the temp sensor to compensate for any residual temperature
related effects in the power detection circuitry.
Summary of Contents for LP-100A
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