SETI League Technical Manual -- Amplifiers
Seen here with its cover removed (click on thumbnail to view full-sized image) is a commercial low-noise amplifier (LNA) for the 1.3 to 1.7 GHz Water-Hole, based upon a SETI League design. In 1995, we completed a prototype preamp incorporating the Hewlett-Packard MGA-86576 GaAs MMIC. The unit was constructed in microstrip on 1/16" thick G-10 (fiberglas-epoxy) circuit board, double-clad with 1 oz per sq ft copper. The schematic is available here. [CAUTION: this is a large file, the downloading of which will consume considerable Web bandwidth] . Etched, drilled and plated printed circuit boards are available for those who wish to build their own preamps (see below).
Computer modeled circuit performance plots are seen here. Gain is relatively flat, and exceeds 23 dB from 1296 to 1691 MHz, suggesting the unit will prove useful in the amateur 23 cm band, for 18 cm weather satellites, and at all frequencies in between (including, of course, those favored for SETI and radioastronomy). Reverse isolation exceeds 40 dB (indicating excellent stability), and projected noise temperature is at or below 150 K. Lower-noise devices are certainly available at spot frequencies, but some compromise in noise temperature appears necessary in order to economically achieve the wide bandwidths desired for SETI. Input VSWR is projected to approach 3.0 at the higher frequencies, a necessary result of matching for optimum noise performance. According to the computer model, output VSWR is expected to be better, at well under 2.0.
Although preliminary test results in the SETI League lab upheld the above projections, it is always instructional to measure any circuit's performance using state-of-the-art equipment. We had that opportunity in late October of 1995. The amplifier prototype was demonstrated on Hewlett-Packard network and noise figure analyzers at that year's Microwave Update conference, with encouraging results. Measured gain varied from 24.0 dB at 1296 MHz to 26.4 dB at 1660 MHz. Input VSWR was best at the lower frequencies, ranging from 1.4 at 1296 MHz and 1.8 at 1420 MHz, to a more disappointing 2.6 at 1660 MHz. Output VSWR fared somewhat better, showing 1.2 at 1296 MHz, 1.15 at 1420 MHz, and a still acceptable 1.7 at 1660 MHz. At all frequencies noise figure was under 2 dB, with lowest noise at the design-optimized 1420 MHz Hydrogen line. Measured noise temperatures exceeded theoretical by 10 K across the band, typical of real-world component losses.
All in all, this design should prove suitable as a SETI front end in all but the most demanding applications, or as a second stage or general purpose gain block over the 1 to 2 GHz range. If cost is a factor, the GaAs MMIC approach is quite appealing. This particular amplifier is being supplied commercially, in both kit and assembled form, by Down East Microwave. Their catalog number is SETI-LNA, and the preamp is available with a choice of connectors and DC feed options. A complete preamp kit costs $75; assembled and tested LNAs in a weather-proof enclosure sell for $105. As our gift to you, all SETI League members in good standing will receive upon request a $10 cash discount coupon for this preamp. You can also purchase just the PC board and documentation package, although our members' discount coupons are not applicable to such purchases.
Please Note: Successful completion of the above preamplifier kit demands precision microwave assembly technique and, in some cases, the use of complex electronic test equipment. This kit is considered appropriate to advanced microwave experimenters. SETI League members lacking extensive experience building and troubleshooting microstrip assemblies should purchase an assembled and tested unit, since a nonfunctional or improperly assembled kit probably can't be economically repaired, even if the vendor is willing to try.
For those applications which demand the lowest possible noise temperature, a two-stage amplifier can be implemented by cascading a discrete PHEMT preamplifier with the above GaAs MMIC (used as a second stage). The concept was tested at Microwave Update 1995, using a PHEMT preamp supplied by Radio Astronomy Supplies ahead of the prototype of this GaAs MMIC amplifier. The first stage, designed by Tommy Henderson, WD5AGO, exhibited no instability when driving the MMIC. This combination delivered well under 1 dB noise figure across the entire Water Hole. 1 dB improvement in noise figure translates to about a 3 dB sensitivity improvement, but (appropriately) carries with it roughly a 3 dB increase in cost!
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