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PCB Design

Unlike the GeForce 7900 GTX, the GeForce 7900 GT uses a unique PCB design, developed almost from scratch.

The new card is shorter than the GeForce 7800 (for details please see our article called NVIDIA GeForce 7800 GT: Full-Throttle Graphics for $449) because the GPU and memory chips have moved along with their accompanying components closer to the DVI-I connectors; the previously empty part of the card is now densely populated. The graphics memory wiring has been redesigned for GDDR3 chips in new 136-pin FBGA packages (Nvidia used 144-pin chips earlier). The video-capture chip is missing here, although there’s place reserved for it on the PCB. It may probably be installed on serially produced cards.

The power circuitry has been greatly simplified. It now includes fewer power elements because the G71 needs less power than the G70; the number of electrolytic capacitors is almost the same, though. The cooler’s fan is still connected via a 2-pin plug while the senior members of the GeForce 7800 and 7900 families use an intelligent 4-pin connection. So, you shouldn’t expect the fan speed control system to be very clever, but there shouldn’t be too much noise, either, considering the low heat dissipation of the G71 chip clocked at a reduced frequency.

This sample of the graphics processor is dated the 3rd week of 2006, i.e. it is older than the chips installed on our samples of the GeForce 7900 GTX. Perhaps early batches of the G71 didn’t have a high frequency potential, but anyway the GeForce 7900 GT is going to be manufactured out of chips that can’t pass a frequency check for installation on GeForce 7900 GTX cards. As mentioned above, the GeForce 7900 GT has the same number of active pixel and vertex processors as the GeForce 7900 GTX: 24 and 8, respectively. According to the official specification, the GeForce 7900 GT’s pixel processors and raster operators are clocked at 450MHz and vertex processors at 470MHz. The chip lacks a protective frame, but it’s not a big problem since the GeForce 7900 GT employs a smaller and lighter cooler than the one they install on the GeForce 7900 GTX.

The card has Samsung’s K4J55323QG-BC14 chips on board. These chips have an 8Mb x 32 structure, 256Mb capacity, 1.8V voltage, and are rated to work at 700 (1400) MHz. The total of graphics memory is 256 megabytes; its clock rate is 660 (1320) MHz or a little lower than the recommended 700 (1400) MHz. The peak memory bandwidth of the GeForce 7900 GT is much higher than that of the GeForce 7800 GT: 42.2GB/s against 32GB/s.

The technical characteristics of the GeForce 7900 GT and GeForce 7800 GTX are very similar, so we can expect them to be in the same level of performance in games.

 
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