PMX40 RF Power Meter – INSTRUCTION MANUAL
5-8
5.
Starting at the left edge of the screen, the processor classifies each Transition threshold crossing
according to whether it is positive-going (– +) or negative-going (+ –). Because the signal is repetitive,
only three transitions are needed to classify the waveform, as follows:
Type Sequence
Description
0
none
No crossings detected
1
Not used
2
+ –
One falling edge
3
– +
One rising edge
4
+ – +
One falling, followed by one rising edge
5
– + –
One rising, followed by one falling edge
6
+ – + –
Two falling edges
7
– + – +
Two rising edges
Figure 5-5.
Step Waveforms
6.
If the signal is Type 0, (No crossings detected) no measurements can be performed and the
routine is terminated, pending the next reload of the data buffers.
7.
The process locates the bottom amplitude (baseline) using the IEEE histogram method. A
histogram is generated for all samples in the lowest 12.8 dB range of sample values. The range is
subdivided into 64 power levels of 0.2 dB each. The histogram is scanned to locate the power level with
the maximum number of crossings. This level is designated the baseline amplitude. If two or more
power value have equal counts, the lowest is selected.
8.
The process follows a similar procedure to locate the top amplitude (top line). The power range
for the top histogram is 5 dB and the resolution is 0.02 dB, resulting in 250 levels. The level-crossing
histogram is computed for a single pulse, using the samples which exceed the transition threshold. If
only one transition exists in the buffer (Types 2 and 3), the process uses the samples that lie between
the edge of the screen and the transition threshold (see Figure 5-6). For a level to be designated the top
amplitude, the number of crossings of that level must be at least 1/16 the number of pixels in the pulse
width; otherwise, the peak value is designated the top amplitude.
