Adam, va7OJ/AB4OJ's ic-756Pro/Pro II user Review




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Adam, VA7OJ/AB4OJ's IC-756Pro/Pro II User Review


In July 2000, I sold my IC-756, and bought a 756Pro from a local dealer. I am delighted with the Pro. I find the Pro a big improvement over its predecessor, the 756. The Pro receiver seems much quieter than that of the 756 - probably due to a cleaner DDS LO implementation.. The DSP IF filters have much steeper skirts than the analogue crystal filters in the older rig, and are much more effective against adjacent-channel QRM than analogue filters. George, W5YR's IF Filter Page dramatically illustrates this point. Also, read George's "Notes on roofing filters" (below).

The manual pre-AGC IF notch filter (70 dB deep) is dynamite. It makes an S9+20 undesired tone disappear off the S-meter. The DSP-IF filtering, including a tuneable notch filter, is all inside the AGC loop (unlike the IC-756). The combination of the DSP-NR and noise blanker renders night-time 40m listening much more pleasant and less fatiguing. I observe significant artifacts under strong-signal conditions only when the noise blanker is enabled. These are clearly due to the NB gating on signal peaks, and are eliminated by switching the NB out.

Overall, the Pro pulls the "weak ones" out of the noise noticeably better than the 756 (or any of its other predecessors in my shack) did. The measured sensitivity on 20m with Pre-amp 1 on, and 500 Hz bandwidth, is 0.1 μV for 10 dB S+N/N (using an HP 8640B generator). I am able to copy easily SSB signals which do not move the S-meter. Those signals would have been barely intelligible on the 756. The ability to tailor the filter passband to the received signal (using the Twin PBT or the filter tables) also provides a superb tool for pulling out the "weak ones". The manual notch is also helpful in improving the SNR of the received signal.

With the Pro, you can optimise the IF bandwidth by tailoring it to the occupied bandwidth of the received signal, thus yielding optimum S/N ratio. Also, the DSP IF filters are inside the AGC loop, so strong signals outside the DSP filter bandwidth will not swamp the receiver. The fact that all the DSP IF filters, including the Manual Notch (but excluding the Auto-Notch) are inside the AGC loop sets the Pro (and the late, lamented Kachina 505) apart from all other amateur HF transceivers on the market.

The vertical sensitivity of the 756Pro Spectrum Scope is significantly higher than that of the 756. A signal of less than 1 μV is visible, whilst the 756 requires at least 20 μV to produce a spike. The only alignment procedure for the IC-756Pro spectrum scope is vertical (amplitude) alignment and calibration. The horizontal (frequency) display is in the digital domain, and thus never goes out of cal. The CAL control on the lower right side of the chassis will center the marker correctly. Incidentally, you can observe the spectral content and occupied bandwidth of your transmitted signal by setting "Scope during Tx = ON" in the "Scope Set" menu.

The transmit speech amplifier of the Pro has a little less gain than that of the 756, requiring a slightly higher MIC GAIN setting when using a Heil microphone with a dynamic insert (HC-4 or HC-5). The new Heil HM-i electret will fully drive the Pro with Mic Gain at around 9 o'clock.

The infinitely-variable DSP IF filters have far better shape factors than classical analogue filters. The tuneable IF notch filter is 70 dB deep. And once you have got used to the spectrum scope, you will never wish to be without one again. The AGC voltage is also derived from the DSP.

The IC-756Pro and an Icom amplifier - PW-1, IC-4KL or IC-2KL/AT-500 - make an excellent combination; the amplifier tracks the radio. The Pro can also be interfaced to a Yaesu Quadra. My first impressions of the Quadra are documented here.

The IC-756Pro/Pro II Monitor is excellent. A sample of the 36 kHz analogue transmit IF at the output of the main DAC is down-converted to baseband in a mixer whose LO is the 36 kHz  ADC/DAC clock. The resulting audio is fed to the speaker/headphone output and the accessory audio output (ACC1 Pin 5).  At the sampling point, the IF parameters are those of the transmitted signal; the next step in the main signal path is the analogue up-conversion and power-amplification chain.

Using the Monitor and a good pair of headphones, you can set up the Pro for the desired transmit audio quality with very little trouble. Try switching between NAR, MID and WIDE TX occupied bandwidth. View George, W5YR's IC-756Pro Monitor page.

The recommended microphone for the 756Pro is the Heil HM-i. The HM-i is plugged directly into the front-panel [MIC] socket. 756Pro Settings: [MIC GAIN] at 9 o'clock, Treble +5 dB, Bass -2dB, compression OFF and Tx occupied bandwidth = MID (COMP OFF MID). No auxiliary equipment is interposed between the microphone and the radio. If compression is used, set COMP ON MID, and adjust [COMP] for 5 to 10 dB compression, no more. This will avoid overdrive.

Notes on the IC-756Pro II:


In May 2002, I purchased an IC-756Pro II at the Dayton Hamvention.  Upon returning home, I installed the Pro II in my station and began evaluating it.

One week later, I can report that the verdict is very favourable. I have been using the Pro II, and have observed quite an improvement in the receiver performance compared to the Pro. So far, I have noticed superior strong-signal handling, DSP IF filtering and DSP noise reduction (NR). 

The adjustable noise-blanker (NB) threshold is also a big advantage over the fixed NB level in the old Pro. I find that by increasing the NB threshold from 50% (default) to 75%,  I can almost completely eliminate local HV power-line noise.

The improvement in strong-signal handling on the IC-756Pro II is dramatic. My nearest ham neighbour is 1 km down the street from me, and has a 3-element quad at a height of 18m. When he transmits SSB on 20m with approximately 1.2 kW PEP (S9 + 60 dB at my QTH), the peak values of received artifacts are as follows:



Offset kHz

 Strength

Scope

10

S5

+40 dB

15

S3

+20 dB

20

S2

+10 dB

35

S1

+10 dB
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