DSP-10 User's Manual

Chapter 2 - Weak Signal Operation

This Chapter covers user information about the specialized "Weak Signal" modes of the DSP-10. These modes, called LTI, EME-2, LHL-7 and PUA43 are all attempt to exploit differences between signals and noise by taking averages of data over long time periods. The following sections of this Chapter are external to this file, but available by htm link:

EME-2 Mode Automatically measure your EME capability
PUA43 Mode Multi-tone FSK for weak signal communication
LHL-7 Mode Seven tone code based on Morse code
LTI Mode Long Term Integration for detection and measurement of very weak signals
Sun Noise Measurements Accurate evaluation of system sensitivity

Signal Simulation - Important Note: Do not use this signal plus noise simulation on-the-air. It is intended for use as a test device using the DSP-10 transmitter as a signal generator. To use this on-the-air will not only make your signal forever noisy and hard to copy, but it can make you a poor neighbor to other operators!

What this does is to introduce wide band Gaussian noise along with any transmitted sine wave signal. The noise is generated in the DSP along with the sine wave; the S/N ratio is very carefully set. If the signal is run into another radio, the resulting noise is essentially all from the simulator. This allows for controlled copiability tests. As of now, the signal is non fading, but fading may be added for a more complete simulation. This noise addition is available in CW, LTI, EME-2, LHL-7 and PUA43 modes.

The noise addition option and level are set by the SCRL F6 dialog box. There are only two selections, an "Add Transmit Noise" check box, and an "S/N dB" entry box. The reference bandwidth for the noise is 10 Hz (the noise is much wider than the signal---the 10 Hz is a reference for calculations). This means that with S/N=0.0, the noise power in any 10 Hz bandwidth, and the signal power in the sine wave are equal. This is a convenient bandwidth for weak signal experiments, but it is narrow for copy by ear. To reference other wider bandwidths, just subtract (in dB) the 10 time the logarithm, base 10, of the new bandwidth, divided by 10. For instance for 50 Hz, subtract 7.0 dB, and for 2500 Hz, subtract 24.0 dB.

After the dialog box is closed by the Enter key, a red 'N' will show, next to "Xmit PWR." When the radio is in transmit, the 'N' will flash, as a reminder that noise is being added.

The power from the DSP-10, without external amplifier, is about 20 to 50 mW. This is a very strong signal at a receiver, and additionally, may damage front end components. It is best to attach a 20 dB, or more, attenuator to the output. Then the transmit power should be dropped, for two reasons. First, it is necessary to provide D/A converter range for the noise. The S/N in 10 Hz may be 0.0 dB, but in the total transmitted band it is much lower. This is accomplished by lowering the sine wave level with the Transmit Power controls (Scrl O and P, or o and p). To transmit a 0 dB S/N signal, the Xmit Pwr must be at 56, or lower. As the S/N is raised, the Xmit Pwr can also be raised, dB for dB. If the S/N is taken below 0 dB, the Xmit Pwr must be lowered dB for dB, or more. For reference:

 S/N    Xmit Pwr
 10 dB     66
  5        61
  0        56
 -5        51
-10        46
-15        41
-20        36
If the power rules are not followed, an error message "E50 S/N Value" will appear, meaning that the transmitted noise is being compressed in the transmitter, and that the simulation is not valid.

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