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What are the differences between the SRa and SRb?


The biggest difference between the SRa and SRb receivers is the use of a six layer printed circuit board rather than the original four layer board. This single change reduced internal interference (birdies) to the point that gain could be reduced in multiple amplifiers in the RF and IF stages. In fact, one IF stage was simply removed since it was now unnecessary to the design. In spite of the reduced gain, the sensitivity of the receiver improved overall. As you would expect, intermodulation and overload performance improved greatly with the reduced gain. This improved board was first used by exchanging customers' boards on Block 606 in England as they were having the most problems.

With what we learned on the Block 606 changes, we added the same general changes and the six layer board to all the SRa units without saying much about it for several months. These improved units seemed to work very well and tested much better in production. While shipping the improved units, we went back to the SRa RF board and spent several months re-matching every amplifier and filter in the RF and IF sections. This was necessary because of all the other changes that had already been made. Many small improvements were made at each stage leading to fairly impressive additional improvements overall. One of which was an improvement in sensitivity of more than 6 dB. The last major change was to relocate the two IF filters. We found an optimum location by making up 5 different prototype boards, with the IF filters located in various trial positions with different shields and ground plane configurations. The final result had one IF filter on the top of the PC board and one underneath with a solid ground plane and our usual shielding. This simple change improved out of band selectivity by a huge 30 decibels.

With the improved sensitivity, better out of band rejection, improved intermodulation performance, and the elimination of internal spurious responses we decided to advance the model designator from "a" to "b", thus the SRb. As a service to our customers with SRa units, we offered an upgrade for the difference in retail price between the SRa and the newer SRb. The change in model designator also indicates that we think this is a major improvement in the SR receiver and it encourages our customers to get the upgrade.

Posted 1 year agoby LectroAdmin