Adapted from Chapter 6 of The VHF/UHF DX Book
"Splatter" on SSB transmissions is high-order intermodulation distortion (IMD) caused by gross overdriving on speech peaks.
Any so-called "linear" amplifier will generate IMD if it is driven too hard. Your mission is to make sure that overdriving never happens - that's right, never!
Speech processors are usually advertised as giving your signal more "punch" or "talk power". That can be true - but a speech processor can also help to eliminate splatter. If you adjust your speech processor correctly you can have a more "punchy" signal that gets the DX, and also a cleaner signal that doesn't offend local stations. Everybody wins!
Here's how to do it. We've done it in Britain and it works. There are two stages:
Your SSB transmitter lineup is something like this:
|Step 1||Connect the transmitter to a dummy load.||Please don't do this on the air - you are going to make some foul noises before everything is adjusted correctly.|
|Step 2||Identify the MIC GAIN control.||This is before the clipper stage (see above). It controls the level of the SSB signal going into the clipper.|
Identify the processor ON/OFF button, and the PROCESSING level control.
|The PROCESSING level control is inside the clipper stage (see above). It controls the level at which clipping is applied (higher-level peaks are removed).|
Identify the RF POWER control
(often called PWR).
|This comes after the clipper stage. It controls the level of the clipped signal, and the drive to the follow-on PA if you're using one, but it does not alter the degree of clipping.|
|Step 5||Set your transceiver to full-power CW, and adjust the PA as normal.|
Set your transceiver to SSB, and the processor button to OFF.
Adjust the MIC level control according to the handbook.
|If possible, set the MIC control so that the ALC meter just below the point where the ALC meter indicates on speech peaks.|
Turn the PROCESSING control fully clockwise, and
the processor button to ON.
Give a long, loud "Haaaaaaaaalo" into the microphone, and adjust the RF POWER level for maximum output from the PA.
This will sound terrible - do not do this into the
Don't use a whistle - it's too high-pitched to give full output.
At this stage you'll probably be generating horrendous splatter!
Back off the RF POWER control until the output from the PA drops by
Note the setting of the RF POWER control, and then don't touch it again!
This is the step that will give you a clean signal!
With most transmitters, reducing the peak power by about 10% from the maximum possible (saturated) level makes a huge improvement in IMD levels.
Reduce your drive power to the level where the RF output starts to drop off quickly - about 10% usually does it.
|Step 9||Listen to your signal on another receiver. Turn the PROCESSING level control fully counter-clockwise, then advance it slowly until your voice sounds loud and crisp, but not distorted.||
If the background noise in your shack is very noticeable, you'll have to
back off the PROCESSING control.
Important: during this step, do not touch the RF POWER control!
|Step 10||Ask a local station for an SSB quality report, with and without processing. If your signal is broader with the processor on, turn down the RF POWER control until it is narrower than before.||Your signal should be narrower with the processor on than without it - because the processor should be preventing those peaks of overdrive.|
|Step 11||Check the effectiveness of the processor with DX stations. Adjust the PROCESSING control as necessary, to get the best balance between audio quality and "punch".||
Any improvement in "talk power" will be most noticeable when your
signal is weak, so ask DX stations about that.
Ask local stations about your audio quality and the width of your signal.
|Step 12||Congratulate yourself!||- and grateful thanks from all your neighbors on the air!|
Your mission... if you choose to accept it... is to build a chain of operators:
|Step 1||Find a strong local station when the band is quiet. Ask for a report on the quality and width of your SSB signal.||You aren't asking for an audio quality report, in the "hi-fi" sense. You want to know what your signal sounds like when he's not listening to your frequency.|
|Step 2||If you ask for a quality report but then continue talking normally, he won't tune around your signal for long enough - he'll politely keep listening to you.||So make it a little bit more formal: tell him that you're going to count slowly up to 20 and back down again, so he knows how much time you expect him to spend.|
|Step 3||Ask for comments on how soon he loses your signal on the HF and LF sides, and and also on carrier suppression, spurious sidebands or any other unusual features.|
|Step 1||Find a strong SSB signal that you know is of good quality. Note the center frequency, and then tune slowly either side, HF and then LF.||Choose a time when the band is quiet. You can't search right down to the noise when other stations are transmitting close by.|
|Step 2||On each side, find where the signal drops into the noise - you can't even tell if he's still transmitting. Notice how many kHz this is, HF and LF.||You will probably find that his signal (USB) seems a few kHz broader on the LF side; this is normal, and is caused by the overlap of your IF filter passbands.|
|Step 3||Try this listening test with a range of different signals, and notice how some are cleaner than others!|
|Step 4||Also try rotating your beam towards and away from a very strong local signal, to find the level at which your own receiver overloads. You will then know when the signal quality reports you give are reliable.||This opens up a range of technical areas in receiver and transmitter design - that's what The VHF/UHF DX Book is about.|
|Back to G3SEK's Amateur Radio Technical Notebook|
9 April, 2013
Page (c) 1997-2013 IFWtech.
Can't find the PROCESSOR control?
On some transceivers this variable control is called COMP. The processor ON/OFF button may be called PROC, COMP or RF FSP.
Some transceivers don't have a separate PROCESSING level control - they use
the MIC gain control instead.