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ALE Automatic Link Establishment (MIL-STD-188-141; FED-STD-1045)
Laboratory measurements of ALE Signal and Waveform
ALE SPECTRUM ANALYSIS - RF - 23dB Bandwidth=2.157kHz
Spectrum Analyzer Image
Conditions of Transmitter
  • Icom 756pro
  • IF bandwidth 2.6 kHz (No speech compressor)
  • RF Power Output 70Watts
  • signal center freq 29.000MHz.
  • Audio source: PCALE
  • Audio path: Soundcard to Rigblaster to mic input. 
  • Test transmission time 240 seconds
Spectrum Analyzer
  • Horizontal graticule 0.5kHz/div
  • Vertical graticule 10dB/div
  • IF bandwidth 30Hz
  • Video bandwidth 30Hz
  • 14 and 30 seconds sweep time
  • Peak hold
  • Markers set delta mode at -23dBc +/- 0.6dB 
  • Measured -23dBc bandwidth = 2157Hz
  • It is easy to see the sharp spikes at the centers of a few of the tone carriers (above). Upon close inspection of the audio waveform (below) using 16bit sampled audio, and looking at the excerpt of the specification (below) for the ALE system, notice that the 8-ary FSK tone frequencies are shifted at the peaks of the sinewaves. Perhaps this is the cause of the spectrum spikes. In any case, this spectrum analysis shows that a properly adjusted amateur transceiver running soundcard- generated ALE signals can transmit a clean signal and easily operate within an occupied bandwidth of less than 2.5kHz, similar to a voice transmission.
Photo below: 
Audio waveform showing the shift in frequency of the 8-ary FSK signal transmitted by ALE

Excerpt of Specification for MIL-STD-188-141; FED-STD-1045 signal:
A.5.1  ALE modem waveform.
18 A.5.1.1  Introduction. 
 The ALE waveform is designed to pass through the audio passband of standard SSB radio equipment.  This waveform shall provide for a robust, low-speed, digital modem capability used for multiple purposes to include selective calling and data transmission.  This section defines the waveform including the tones, their meanings, the timing and rates, and their accuracy.
20 A.5.1.2  Tones.
 The waveform shall be an 8-ary frequency shift-keying (FSK) modulation with eight orthogonal tones, one tone (or symbol) at a time.  Each tone shall represent three bits of data as follows (least significant bit (LSB) to the right):
· 750 Hz  000
· 1000 Hz 001
· 1250 Hz 011
· 1500 Hz 010
· 1750 Hz 110
· 2000 Hz 111
· 2250 Hz 101
· 2500 Hz 100

 The transmitted bits shall be encoded and interleaved data bits constituting a word, as described in paragraphs A.5.2.2 and A.5.2.3.  The transitions between tones shall be phase continuous and shall be at waveform maxima or minima (slope zero).

21 A.5.1.3  Timing.
 The tones shall be transmitted at a rate of 125 tones (symbols) per second, with a resultant period of 8 ms per tone.  Figure A-5 shows the frequency and time relationships.  The transmitted bit rate shall be 375 bits per second (b/s).  The transitions between adjacent redundant (tripled) transmitted words shall coincide with the transitions between tones, resulting in an integral
49 symbols (or tones) per redundant (tripled) word.  The resultant single word period (Tw) shall be 130.66... ms (or 16.33... symbols), and the triple word (basic redundant format) period (3 Tw) shall be 392 ms.

22 A.5.1.4  Accuracy. 
 At baseband audio, the generated tones shall be within +1.0 Hz.  At rf, all transmitted tones shall be within the range of 2.0 dB in amplitude.  Transmitted symbol timing, and therefore, the bit and word rates shall be within ten parts per million.

Figure A-5 ALE timing and coding
figure A-5 - ALE Timing and Coding.

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