SETI League Technical Manual -- Block Diagram
Copyright © 1999 by H. Paul Shuch, Ph.D.
Executive Director, The SETI League, Inc.
PO Box 555, Little Ferry NJ 07643
email n6tx @ setileague.org
The microwave receiver takes a small, selected portion of the radio spectrum, and converts it to audio for signal analysis. Selection of the appropriate receiver leaves more to the discretion of the experimenter than any other portion of the amateur SETI system. Four distinct options present themsleves. In descending order of cost, they are:
- High-end microwave scanning receivers. These units (typified by the Icom models R-7000, R-7100, and R-8500, as well as the AOR 3000 and 5000) are multi-mode receivers which can receive AM, FM, CW, SSB, and sometimes video and digital modes. Various IF bandwidths are usually available, and these receivers are normally programmable to scan a selected range of frequencies. They typically tune from a few hundred kHz all the way up to about 2 GHz, which actually exceeds our SETI needs. Prices are likely to start around $2000 US, making these receivers as expensive as all other portions of an amateur SETI station combined.
- Modified radio-telescope receivers. One of the very few vendors of commercial radio astronomy receivers for the amateur market is Radio Astronomy Supplies. Their microwave receivers, which are designed specifically for continuum radio astronomy (that is, searching for natural astrophysical phenomena), can sometimes be modified for SETI use. Such modifications generally require considerable electronics expertise, but offer the ultimate in performance.
- Computer-controlled receivers. The first generation were built on ISA cards, and plugged directly into one of the vacant slots on the motherboard of a personal computer. These units were prone to radio frequency interference generated by the computer itself. Later units, like the Icom PRC1000 and WinRadio 1500e, are separate boxes which plug into a computer via a serial, parallel, or USB port. They have many of the features of the high-end microwave scanning receivers, but since they rely on a companion computer for digital control, typically cost half as much.
- Downconverter/receiver combinations. Several converters are available to shift a selected portion of the microwave spectrum down in frequency, for reception in a shortwave or VHF ham radio receiver. Popular units are available from Down East Microwave in the US, and VHF Communications in Europe. Downconverters are appealing for those who already own a high-performance communications receiver, which unfortunately doesn't tune to the SETI frequency of interest. Downconverters cost about half as much as the computer-controlled receivers, but require the user to couple them to an existing receiver.
Whichever receiver scheme is selected, present practice suggests operating it in single sideband mode (either USB or LSB), and leaving it fixed-tuned, rather than scanning it across the spectrum. The reason for avoiding frequency scanning is that the Earth is turning the antenna continually, so that the spatial dimension of the observation is always changing. Only by holding frequency constant for at least one rotational period of the Earth (that is, one day) can we avoid the problem of "too many variables."
The bandwidth of the receiver's audio stages will typically be the limiting factor, as far as instantaneous frequency span is concerned. Many SSB receivers cover as little as 3 kHz of spectrum at a time, which is an inefficient way to search for ETI. Advanced SETI experimenters sometimes modify their receivers for up to 22 kHz of instantaneous IF and audio bandwidth, while custom-built receivers can cover several hundred kHz all the way up to a few MHz of spectrum at a time.
Information on various commercial and kit receivers, along with vendor links, may be found in the Receivers and Converters chapter of The SETI League Technical Manual.