IC-756Pro Occupied Bandwidth Measurements

Contributed by George T. Baker, W5YR

Conditions:

1. Transmit frequency: 14175 kHz, into a dummy load.
2. Mode: USB.
3. Readings for baseband (audio) input via ACC1, Pin 4: black text on white background.
4. Readings for baseband (audio) input via MIC socket: white text on navy background.
5. COMP OFF for first runs, and then ON for the last run.
6. Output (Po) normalized for all runs at 100 watts (= 0 dBr) with 1000 Hz sinewave input signal (f o).
7. Output (Po) measured at low-end (f 1) and high-end (f 2) frequencies.
8. Power output measured with Bird 43 wattmeter calibrated against the PRO and Kachina output meters.
9. The 5 watt level represents essentially zero output power.
10. Occupied bandwidth (OBW) = f 2 - f 1.

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

The comparatively small difference in the frequency response functions between the ACC1 Pin 4 input and the MIC input is quite surprising. If we consider  the -6 dB bandwidth in the MID setting, the ACC1 Pin 4 input provides 290 Hz to 2710 Hz, while the MIC input provides 300 Hz to 2710 Hz. This amounts to no difference at all within experimental error.

In the WIDE setting, ACC1 Pin 4 yields 90 Hz to 2910 Hz, while the mic input provides 150 Hz to 2910 Hz. In the NAR setting, the limits are 490 Hz to 2710 Hz for the ACC1 and 500 Hz to 2710 Hz for the MIC input.

This leads me to believe that any advantage from using the ACC1 Pin 4 input must come from reduced distortion and noise and not from a restricted frequency response, other than at the very low end. Read Adam, VA7OJ's notes on the transmit audio inputs.

Effect of compression on occupied bandwidth:

It is interesting, though not unexpected, 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 sinewave 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.

IC-756Pro Monitor Bandwidth Measurements

Contributed by George T. Baker, W5YR

Conditions:

1. Transmit frequency: 14175 kHz, into a dummy load.
2. Mode: USB.
3. Baseband (MOD) input: ACC1, Pin 4. 1000Hz test tone; input level set for 100W RF output.
4. Monitor output: PHONES jack. Monitor level set for 316 mV audio output ( 0 dBr).
5. Monitor output measured at low-end (f 1) and high-end (f 2) frequencies.
6. Bandwidth (BW) = f 2 - f 1.

Fig. 1: Monitor amplitude-frequency plot (linear)

Remarks:

1.These bandwidths agree closely by eyeball with the spectral plots. I was surprised at the small -3 dB bandwidth, but it is verified by the third-octave plot. It does, however, explain the lack of highs in the Monitor output from my reference audio recording, when I can readily hear the highs in the audio being fed to the PRO. These bandwidths are about 400 to 500 Hz smaller than the WIDE transmit bandwidths. I suspect that the highs are being rolled off in the receive analog audio section of the IC-756PRO, in an effort to reduce hiss and high-frequency noise. Read Adam VA7OJ/AB4OJ's notes on the Monitor.

2. The downward slope of the linear plot is presumably a function of some low-end emphasis residing within the PRO, probably in the output audio circuits.

3. I am almost completely sold on using the Monitor to assess the transmitted signal audio on SSB. For most operators, lacking a second receiver or being unable to use it properly as a signal monitor (not as simple as just tuning in your own signal!), the Monitor output is the only valid example they  have of what they are transmitting. And as we have seen from the above measurements, the frequency range and lack of distortion speaks well for its capabilities.

4. By listening to the Monitor signal and adjusting the bass and treble level controls and the bandwidth setting I can quickly and easily arrive at a setting that I want for a particular purpose, be it "hi-fi" for rag-chewing, or down and dirty for pile-ups. This capability is rather like the mirror image of the ease with which a receiving passband can be constructed and placed as required.

Third-Order Intermodulation Test:

This test was performed using a PSK31 two-tone idle signal of sufficient amplitude to drive the IC-756PRO to 40W PEP output.  At the signal-source output, measured IP3 was -64 dB relative to PEP.  At the Monitor jack output, measured IP3 was approx. -58 dB relative to PEP.

Copyright © 2002, George T. Baker, W5YR

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