Recommended Usage of the LP-500…
For the most part, you will find the LP-500 intuitive and easy to use. Below are some recommended settings for the various modes of the LP-500, and
how to maximize your experience using it.
Power/SWR Mode…
The key thing to remember in this mode is that the Range and Alarm buttons are indexed to the selected channel. This is true whether using manual or
auto channel selection. The first thing you should do is to verify that the default setup values work for you. If not, enter Setup and set the values to your
desired settings. Then, in the Power/SWR screen, step through each channel manually, and select the bargraph range that you would like, and which
channel should be the alarm trigger for the selected channel. Normally this would be the same channel as the selected channel, but there are a number
of circumstances where you might want different channels selected for power alarm and SWR alarm, as outlined in the Power/SWR section of this guide.
The programmed values for the trigger points are displayed to the right of the SWR bargraph. If the alarm is triggered, the Alarm P or Alarm S displays
will turn red to indicate whether the alarm was triggered by high power or high SWR.
Once you have the channels configured, you can select auto channel if you like. When in auto channel, the currently selected channel is displayed to the
right of the SWR bargraph. For most operating modes, Peak power should be selected, which displays and holds the peak value in the blue area of the
display. The hold time is programmable in Setup, from ½ second to 5 seconds.
Waveform/’Scope Mode…
Again, the key thing to remember here is that the Range is indexed to channel. For instance, if you typically use 100W on channel 1, you can manually
set the range to 30V/div. If you typically use 1500W on channel 2, you can manually set the range to 150V/div. You could also set both to AutoRng if you
like, and let the meter select the correct scaling.
Here are the recommended Sweep settings for the indicated modes…
SSB… SSB F or SSB S. The slower sweep shows more cycles of speech waveforms, but is a tad slower.
Two Tone Tests… SSB F or SSB S. The fast sweep displays 2 cycles of the test pattern, while the slow sweep displays 4.
CW… CW. Displays both the leading and trailing edges of the keying waveform, independent of keying speed.
PSK 31… PSK F or PSK S. The slower sweep rate shows 4 cycles of the PSK signal, the faster rate shows a bit more than 1 cycle.
You can experiment with different combinations to get the display you prefer, and you can create your own sweep presets for custom displays where you
can choose your own trigger mode and sweep rate, from 1 msec to 20 msec in 0.1 msec steps. Just as with a normal oscilloscope, the slower the sweep
rate the slower the response.
The Wfm Style button selects the type of waveform display that’s shown. Choices are as follows…
Wfm… Traditional envelope display of modulation.
½ Trap… Top half of trapezoidal display of exciter output vs. amplifier output
Scope… Traditional scope display of modulation waveform.
Wfm/Trap… Split screen of Wfm on the left and Trapezoid on the right.
AM Mod… Split screen of Wfm on the left, and bargraphs for positive and negative modulation on the right.
Wfm/Pwr… Split screen of Wfm on the left, and bargraphs for power and SWR on the right.
There are some warnings that you should be aware of regarding this mode…
In either the ½ Trap or Wfm/Trap display, you must connect a coupler at the input and output of your amplifier, as shown in the diagram on page 1 of this
guide. The couplers must be plugged into either CH1 / CH2 or CH3 / CH4, with the amplifier output on the higher number channel. When in either trap
mode, you must select the higher number channel. If you select the lower number channel, you will see an error message and the button will briefly turn
red. If the amplifier is off, or on the wrong band, you will receive another error indicating that the amplifier gain is low. The LP-500 assumes that the amp
should have a gain of at least 6 dB.
In AM Mod display, the LP-500 looks for pauses in speech to determine the carrier level. Typically, this happens when you first key up, before speaking,
but also happens anytime there is no modulation. The meter needs this info to calculate modulation percentages.
The test tones to use in the Wfm displays are 2 Tone for SSB testing and either 400 Hz or 1 kHz for AM testing. The other choices are specially
designed for the Spectrum mode, as explained in the Spectrum Mode section.
We will be posting some video tutorials on our LP-500 web page, www.telepostinc.com/lp500.html which will walk the user through various tests like
amplifier linearity using the trapezoid display, frequency response tests using the white noise signal, two-tone IMD tests, etc.
Spectrum Mode…
Before going through the settings, we need to address some factors affecting spectrum measurements. Notice that three of the test tones have a “+”
sign added, indicating that the signals include a subcarrier. These signals are used only in the Spectrum Mode. The purpose of this is to restore the
missing carrier when testing in SSB modes. Without this, the envelope detectors in the coupler would produce a highly distorted signal in the audio
realm, which is what we are really measuring in the Spectrum Mode. So, these test tones are really designed to produce a pseudo-AM signal when fed
to a SSB balanced modulator so that the detectors in the couplers can cleanly demodulate this into two tones and their distortion products. This is not
exactly the same as a traditional two-tone test, even though it looks similar. But it allows the use of standard type couplers over very wide frequency
ranges (up to microwaves if desired) without the complicated circuitry of an RF spectrum analyzer or expensive ADC circuits which can cover UHF
frequencies… or both.
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