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Improving weak signal identification of FSK beacons

This is a work in progress, documentation of a current project that will change, a 'living' document.

Introduction

This article describes a simple method of improving the weak signal identification of FSK beacons by using a combination of medium speed and very low speed (QRSS) modulation with differing frequency shift.

FSK beacons are widely used on VHF as propagation beacons. FSK has the advantage of a simple low level modulator, and not requiring linear amplification after the modulator stage.

The modulation scheme proposed is, using the same MARK frequency (100Hz above the Emission Centre Frequency):

Performance analysis

FSK beacons are usually received in a SSB receiver set up for telephony, or possibly with a narrow filter for telegraphy.

Listening to the MARK frequency alone, the Morse code identification can be decoded at S/N ratios down to about -6dB, depending on the listener, speed etc. For the purpose of this analysis, the received signal level that delivers S/N=-5dB in noise with 2kHz bandwidth is taken as the reference for comparison of other configurations.

Table 1:
Speed Noise Bandwidth(Hz) Relative Noise Bandwidth (dB) Decoder Threshold S/N (dB) Relative Threshold S (dB) Comments
12WPM 2000 0 Ear -6 0  
12WPM 500 -6 Ear -5 1  
12WPM 11 -22.6 Visual / Spectran 10 -12.6  
QRSS3 0.34 -37.7 Visual / Spectran 10 -27.7  

Table 1 compares the performance of 12WPM and QRSS3 Morse reception in a range of noise bandwidths. Note that the narrow noise bandwidths achieved with Spectran do not require a narrow receive filter, for most purposes the normal SSB telephony filter is quite adequate in the receiver for decoding and preferred for finding signals in the noise.

FSK Morse beacons can be decoded to lower S/N ratios using a narrow receiver filter, or audio spectrum analysis using Spectran or the like, an improvement of up to 28dB can be achieved using QRSS3 and audio spectrum analysis. Of course, the improvement in threshold is at the expense of time to decode the message and equipment required. Many hams are already using Spectran or the like to 'see' very weak beacon signals, but they are unable to decode identification messages down to the threshold achievable using QRSS3 and narrower bandwidths.

Such a system may provide a useful early indicator of propagation.

The prototype

A prototype low power beacon on 30m was constructed using the Simple Morse beacon keyer and PllLdr application - AD9850 prototype.

The SBMK provides two outputs, one is the Morse signal and the other is high when the keyer enters the periodic high speed message cycle. The first of these signals (TX) is connected to the PllLdr sel[0] input, and the second (AUX) to the sel[1] input. The latter effectively selects the MARK and SPACE frequency pairs for high speed and low speed modes.

{
"ver":"01",
"rbo":"R",
"options":"0300",
"regs":[
["0014C9B0A2"],
["0014C995CA"],
["0014C9B0A2"],
["0014C9AFF6"]
]
}

Above is the configuration data for the PllLdr to load an AD9850 DDS chip.

Fig 1:
 

Fig 1 shows a Spectran display adjusted for the high speed message received in 11Hz noise bandwidth. The low speed mode would look and sound much like a stead carrier (though it does have 5Hz FSK carrying a low speed signal). The S/N ratio measured with longer integration period is about 10dB in 11Hz bandwidth or -13dB in 2000Hz bandwidth, well below the level than can be decode by ear even with a 500Hz filter.

Fig 2:

Fig 2 shows a Spectran display adjusted to view the low speed message received with S/N of about 10dB in 0.34Hz bandwidth, signal level 28dB lower than the threshold that can be decoded by ear in 2000Hz bandwidth. If the signal above was from a 10W beacon, it would take more than 6kW to decode the signal by ear using a SSB telephony receiver.

Higher frequencies

The AD9851 DDS chip has a step size of 0.05Hz using a 180MHz reference clock, so it is quite capable of producing 5Hz shift on signal multiplied into the low GHz range. The output of an AD9851 at say 50-75MHz could be used as the IF into a transverter to mix to the target band.

Image output at 144MHz of the AD9851 referenced to a 10MHz high stability oscillator (using x6 multiplier) may be usable with a transverter, depending on the spurious content and filtering. There exists a wide range of possibilities just with this one DDS chip.

An alternative method of modulation is amplitude modulation (ie A1A), and it may suit higher frequency applications where the step size of the PLL or DDS is an issue, but at the expense of losing the near constant carrier between high speed messages. Many PLL and DDS chips support controls to turn the RF output on and off, and these could be used for low bit rate modulation. They should be followed by linear amplifier stages.

Fig 3:
 

Fig 3 shows a Spectran display of the prototype adjusted to amplitude modulate the QRSS keying. The fuzzy part at the left is the high speed message seen through a narrow filter.

Licencing

Many VHF beacons are licenced for FSK with a Necessary Bandwidth of 850Hz. Though operating practice is different, that allocation permits frequency shift of up to 690Hz with 20WPM keying without exceeding the licenced Necessary Bandwidth. (It is naive to think that 850Hz would accommodate a frequency shift of 850Hz with keying at the speeds used.)

Since these allocations fix upper and lower allocation limits, it would be wise to use a lower frequency shift to accommodate the variation in frequency that is characteristic of most beacon transmitters. 200Hz shift is more than adequate to carry 20WPM Morse code and be easily decoded by ear with an SSB receiver.

The FSK scheme proposed here is compatible with existing licence allocations of 300Hz or more Necessary Bandwidth.

Conclusions

FSK beacons are widely used as propagation indicators, and they have the benefit of being decodable by ear using just a SSB telephony or Morse code receiver.

An improvement in minimum decodable signal of 28dB could be achieved by using a combination of high and low speed messages with appropriate frequency shifts.

The combination allows identification of the signal by ear, and using audio spectrum analysis, decoding to much lower signal levels.

Links

Simple Morse beacon keyer

PllLdr application - AD9850

Changes

Version Date Description
1.01 02/02/2012 Initial.
1.02    
1.03    
1.04    
1.05    

 


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