Notes on Icom IC-756Pro II and IC-746Pro

Contributed by Lidio Gentili, IØGEJ

ICOM has taken the DSP technology introduced in the 756PROII, and created a whole new radio category with the 746PRO. It is interesting to compare the two rigs, which present very similar performance and were launched on the market almost at the same time.

A) IC-756PRO II


The following improvements/added features have been incorporated into the IC-756PROII:

Sharp and soft IF filter shape factors are independently selectable for SSB and CW.

In the front end of the RX, the first mixer and RF bandpass filters (BPF) have been redesigned. 
Inductors and capacitors used in the 756PRO BPF generated distortion; thus, the 756PROII has much bigger inductors than the 756PRO.
In addition, the 756PROII uses new Motorola PIN diodes for signal switching in the RX front end to reduce the distortion.

The third mixer was completely redesigned, and now converts 455kHz signals to 36 kHz to be passed into the 24-bit A/D converter. This change allows enhanced filtering after the third mixer, where the input bandwidth is limited only to 15kHz. – as distinct from conventional receivers, which have a mechanical or crystal filter before the third mixer. With this design, ICOM achieves the lowest distortion possible.

In this project, the new DSP filter was heavily redesigned in software (with little modification in the circuits), but retained the same DSP chipset: Analog Devices ADSP 21061L Sharc

ADSP 21061L Specifications:

The IC-756Pro II DSP Board. Click for ADI data sheet.
Fig. 1  IC-756Pro II DSP Board.

B) IC-746PRO (IC-7400 in Japanese & European market)


ICOM also presents the new IC-746PRO transceiver, with "Supercharged Performance”, all the features of 756PROII and a bonus: 144-148 MHz! The IC-746Pro offers the following new features:

Sharp and soft IF filter shape factors are independently selectable for SSB and CW (as in the IC-756Pro II and IC-756Pro III)

And much more.

*In this project, ICOM changed the DSP chipset. The IC-746PRO uses the Texas Instruments TMS320VC33. 

TMS320VC33 Specifications:

The IC-746Pro DSP Board. Click for TI data sheet.
Fig. 2  IC-746Pro DSP Board

The unique difference between the IC-756Pro and the IC-746Pro is in the 746Pro's "simplified monochrome display", the lack of Dual Watch and the lower price – very attractive!

The IC-746Pro. Click for Icom America IC-746Pro page.
Fig. 3 IC-746Pro HF/6m/2m Transceiver

In March 2000, I bought an IC-756PRO. I think it's a good rig; I am tempted by the new 756PROII. Still thinking about it - the comparison raises some doubt.

Which ICOM rig is the real top technological solution? Which has better price/performance ratio? To quote Icom’s advertising:

IC-756PROII. The best just got better!! 
IC-746PRO. Supercharged Performance!!

Which is the real motivation for the change in the DSP chipset? Which platform does ICOM intend to carry on in future projects?

Perhaps the change of DSP chipset confirms that the756PROII is a transitional product - essentially an enhancement of an “old” project - whilst the 746PRO is the first of a new family of products based on next-generation Texas Instruments DSP devices.

We now know....Icom unveiled the IC-7800 at the 2003 Dayton Hamvention. 

The IC-7800 uses four TMX320C6713GDP DSP devices. Please read my IC-7800 DSP Page.

I would be very glad to have your thoughts on the evolution of the IC-756PROII and 746PRO.

Best 73, 
____________________
Lidio Gentili, IØGEJ, Perugia
e-mail
28.XII.2001

Copyright © L. Gentili  2001 All rights reserved. 
Last revised: November 15, 2012.

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