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IC-756Pro & IC-756Pro II Occupied Bandwidth Measurements

Contributed by George T. Baker, W5YR

Conditions:

  1. Transmit frequency: 14175 kHz USB, with a reference power of 100 watts at 1000 Hz into a dummy load.
  2. Readings for baseband (audio) input via ACC1, Pin 4, except where otherwise stated.
  3. COMP OFF.
  4. Baseband source: NCH Tones computer program output through Behringer 802A mixer/EQ to ACC1 or MIC connector input.
  5. All EQ in the system (internal & external) set flat.
  6. Output (Po) normalized for all runs at 100 watts (= 0 dBr) with 1000 Hz sine-wave input signal (f o).
  7. Output (Po) measured at low-end (f 1) and high-end (f 2) frequencies.
  8. Power output measured by Bird 43 and PRO/PRO2 panel meters, which have previously been cross-checked with the HP 436A Power Meter at the specific power levels shown.
  9. The 5W level represents essentially zero output power.
  10. Occupied bandwidth (OBW) = f 2 - f 1. Note: ITU-R Recommendations specify -6dB OBW.
Run #1: PRO, ACC1, WIDEPo =100W (0 dBr) at fo = 1000 Hz.
f1 (Hz) f2 (Hz) Po (W) dBr OBW (Hz)
63 2934 50 -3 2871
51 2948 25 -6 2897
40 2968 10 -10 2928
33 2980 5 -13 2947

Note: Further testing has revealed no substantive difference between the PRO and PRO2 for ACC1 transmit audio input.

 

Run #2: PRO2, ACC1, WIDEPo =100W (0 dBr) at fo = 1000 Hz.
f1 (Hz) f2 (Hz) Po (W) dBr OBW (Hz)
64 2933 50 -3 2869
52 2948 25 -6 2896
40 2970 10 -10 2930
33 2981 5 -13 2948

 

Run #3: PRO2, MIC, WIDEPo =100W (0 dBr) at fo = 1000 Hz.
f1 (Hz) f2 (Hz) Po (W) dBr OBW (Hz)
96 2935 50 -3 2839
85 2953 25 -6 2868
72 2977 10 -10 2905
65 2986 5 -13 2921

 

Run #4: PRO2, ACC1, MIDPo =100W (0 dBr) at fo = 1000 Hz.
f1 (Hz) f2 (Hz) Po (W) dBr OBW (Hz)
260 2733 50 -3 2473
243 2747 25 -6 2504
220 2770 10 -10 2550
205 2783 5 -13 2578
Note: These results are fairly close to ITU-R Recommendation M.1173 (350 ~ 2700 Hz @ -6 dB).

 

Run #5: PRO2, ACC1, NARPo =100W (0 dBr) at fo = 1000 Hz.
f1 (Hz) f2 (Hz) Po (W) dBr OBW (Hz)
462 2730 50 -3 2268
446 2744 25 -6 2298
422 2766 10 -10 2344
406 2777 5 -13 2371

Low-end response for ACC1 Pin 4 vs. MIC input.

With reference to Runs #2 and #3, it will be seen that in the WIDE setting, ACC1 Pin 4 yields -6dB points at 64 ~ 2868 Hz, as compared to 85 ~ 2868 Hz for MIC input. This difference is a function of the high-pass filter in the speech-amplifier circuit. Read Adam, VA7OJ's notes on the transmit audio inputs.

Effect of compression on occupied bandwidth:

Previously-conducted tests have shown that the COMP function increases the occupied bandwidth by about 100 Hz. This increase is due to the signal-level-following action of the COMP function. As the signal is reduced near the edges of the passband, the COMP amplifies it back up, thereby slightly raising the skirts on both ends of the spectrum. I monitored the output signal on the scope using the MONITOR at all times, and could never see any obvious distortion of the sine-wave output signal, even with very high input audio levels. The output power held steady at 107W over a wide range of input levels.

Compression and PSK31:

Using a PSK31 two-tone input signal, the COMP produced serious flat-topping distortion for any power output over about 25W average on the Bird 43. I have previously found that best IMD is obtained for PSK31 with the PRO operating at 40W PEP or 20W average with the PSK31 idle tone excitation.

Copyright 2002/2003, George T. Baker, W5YR

Page created by A. Farson. Last updated: 09/30/2018

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