A couple of years ago I made a PSK31 contact with a station in Mexico. I was running three watts. He was running eight hundred. He told me he was running that power level. It apparently never occurred to him that since he could read me perfectly with my three watts, that his eight hundred watts was out of proportion to the need.

If band conditions are such that a transmitted signal is not heard easily above the noise, then the only option is to try to increase that signal in strength so that it is heard above the noise. If band conditions are such that a signal can be easily heard above the noise and communication is successful and reliable, then the power being transmitted is enough. There is no reason to add additional power. Outside of the obvious potential for improper operation on the part of the operator using higher power (not saying every amplifier user is at fault here) the presence of the stronger signal will cause an increase in the AGC action of the receiving operator's receiver which can result in decreasing the gain and potentially causing the loss of reception of desired signals for that operator. If the signal of the operator who is using higher power is low due to band conditions, and to overcome those conditions he must resort to higher power, his resulting signal is not going to cause problems for receiving operators. In that case, it is not fair to make a blanket statement condemning all use of higher power.

Let me switch gears now and point some blame away from the high power operator. Now I am going to ask what it is about high power operating, or more specifically the presence of strong signals, that causes such a problem? Strong signals within the passband of a receiver tend to desensitize the receiver. The result of that desensitization is that the RF gain of the receiver is reduced which results in some loss of ability to copy weaker signals. If you are trying to copy a signal which is very weak, any reduction in the gain of your receiver may cause you to lose that signal completely. At best, it will increase the difficulty of copying that signal. Much of the blame for high level received signal desensitization during PSK31 operation can be blamed on improper audio level adjustments. The presence of high level received signals does not automatically mean we must suffer from receiver overload. We may be contributing to our own computer interface overload and that is what I want to focus on in this article.

It is not enough to rely solely on the PC sound card's audio level adjustments. While the Windows mixer slider will adjust the gain of the sound card so that the audio passing through it can be varied from full to nothing, this control will not affect the level of the audio signal at the INPUT of the sound card. The PC sound card audio slider can be turned down as far as possible while still maintaining enough signal to form a waterfall and overdriving can still be experienced. If the INPUT level to the sound card is too high, overdriving will occur.

When input overdriving occurs, extra signal traces may be seen within the waterfall. These extra traces are not real signals but multiples or harmonics of the real ones. Likewise, stronger signals which are real may over drive the sound card giving the impression that distortion and hiding of desired weaker signals is the fault of the transmitting station when that is not the case at all.

I use a Kenwood TS-2000 transceiver coupled to my computer through a homebrewed interface to handle digital mode operations. The points I will address in this article are specifically related to this radio but they should also relate to other radios. The software I use for detecting PSK31 and RTTY is WinWarbler which is part of the suite of software applications by DXLabs. Below is a screenshot of a part of the WinWarbler screen receiving PSK31 signals.

Figure 1

Figure 1 shows how I have set the audio level controls on both my receiver and the soundcard of my PC to display the waterfall and signal traces. Obviously how the display looks is up to the preference of the individual. I wanted it to be somewhat to the middle. No too bright so that signal traces which represent weaker signals become lost in the background noise, and not too dark so that they seem to fade out in the darkness. Two sets of level adjusting controls were used. The actual Windows sound card mixer control and the audio level output of the receiver. My ideal is for the sound card level adjust slider to reside about midway so that it is in the more linear part of the card gain curve.

Figure 2

On the next pages I show a series of screen shots of a graphical display to show what goes on with the receiver passband with PSK31 signals present at various audio level settings. There are a number of visual graphical analyzer programs available for download from the Internet. These programs function very much like an oscilloscope or even a spectrum analyzer and allow you to display either the output of your PC sound card with an on-screen display. The one I used for the following graphs is called Visual Analyzer. It is available for download FREE from the author's Web site. I won't take the time to describe the many features of this analyzer here. They are described very well on the Web site. It is more than suitable for a project such as evaluating the signal passing through your PC sound card.

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