The basic principal to follow is to keep your equipment within its linear range of operation.
I found that the linear range of operation of my sound card was essentially the middle half of its dynamic range. When signals with sample values in the upper quarter, or lowest quarter of my sound card's dynamic range were generated by my sound card, they showed evidence of non-linear operation. Since then, I have scaled signals to only use the middle half of my sound card's dynamic range.
If the line input, or microphone input, to your sound card is connected to an audio source, then that audio will be present in the audio output of your sound card, unless those unwanted sources are muted while your sound card is supplying audio input to your transmitter.
The audio level going into the radio has been the most difficult setting to make on the transmit side. A two tone test signal was developed to aid this process.
The two tone test signal consists of the sum of an 1180 Hz sinusoid and a 1520 Hz sinusoid. Note that these frequencies straddle the 1200 Hz and 1500 Hz lines on the MMSSTV spectrum display. During the first 2 seconds of the two tone signal, a constant amplitude is maintained. At the 2 second point in the signal, the amplitude steps down, such that the power drops by 3 db. This level is maintained until the 4 second point in the signal is reached. At the 4 second point, the amplitude drops again, so that the power drops by another 3 db. This level is maintained until the 6 second point in the signal. At the 6 second point, the amplitude steps back to its original level, resulting in a 6 db increase in power. The last 6 seconds of the two tone signal are a repetition of the first 6 seconds.
For the case of a SSB transmitter, if you can monitor the output power of your transmissions, then you can set the level properly by adjusting it until you see the -3db, -3db, +6db, -3db, -3db sequence in your output power while transmitting the two tone test signal. This is the most reliable way to set the level for a SSB transmitter.
If you can't monitor your output power, then a receiver can be used to monitor your transmissions. However, if the receiver is not adjusted properly, it can be the source of non-linearity, rather than the transmitter.
When the person monitoring your transmission of the two tone signal can hear the -3db, -3db, +6db, -3db, -3db pattern, then you know that both the transmitter and receiver are adjusted properly. An alternative to listening to the audio of the two tone signal is to monitor it with a spectrum display program. The MMSSTV program has been used for this purpose. When the levels are set up properly on both the transmitter and the receiver, there will be only two frequencies present in the spectrum display of the received two tone signal. When the level is too large, at either the transmitter, or the receiver, or both, then the spectrum display will show energy present at more than just two frequencies. The spurious energy is generated by non-linear operation.
Note that the non-linearity may be in the sound card at the receiver. It is possible for the two tone signal coming out of the receiver to contain energy at only two frequencies, but the spectrum display shows energy at more than two frequencies. The cure for this situation is to reduce the level going into the sound card. Unfortunately, evidence of non-linear operation on a spectrum display does not tell you where, in the chain of equipment the non-linearity is happening. It could be in:
Once you get everything into the linear range of operation, you can adjust one thing at a time to determine what it takes to begin non-linear operation of the various pieces of equipment in the chain.
One series of tests demonstrated that one transmitter operating in FM mode generated many more spurious emissions when over driven, than it did when operating in SSB mode and was over driven. Thus, you need to be much more careful in setting the audio level going into the radio when operating in FM mode than you do when operating in SSB mode.
Below is the spectrum of the part of the leader section of a transmission from Australia to the United States. Since the subcarriers are 230 Hz apart, some spurious energy usually shows up at 230 Hz, due to intermodulation distortion, when there is non-linear operation. The spectrum below shows a spread of about 15 db in magnitude among the subcarriers (which started out at equal magnitudes). It also shows there was no significant non-linear operation of any equipment in the chain.
Below is the spectrum of part of the leader section of another transmission from Australia to the United States. The receiver was the same for the spectrum below and the spectrum above. The transmitter was different for these two cases. Looking at the plot below, you can see a spike near 230 Hz, as well as many other spurious spikes, all generated by non-linear operation from the 8 unmodulated subcarriers.
This case was chosen because it is the most distorted case, so far, that was still decoded successfully (all errors corrected).
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