The SETI League, Inc. Hydrogen Line Receiver Development Photos

SETI League Photo Gallery

Hydrogen Line Receiver Development Photos

(Click on thumbnail to download full picture)
Caution: be advised that these tend to be large files. Importing them will consume considerable Web bandwidth and connect time.

Attn. Journalists and Photo Editors: please check our Fair Use Policy before reproducing any of these images. Thank you.

The photos below document the development by SETI League engineers of a complete SETI receiver, incorporating the SETI League's 1420 MHz to 144 MHz microwave downconverter, along with a suitable direct-conversion IF. More details on this and other equipment may be found in the SETI League Technical Manual.

Many SETI League members have been asking for a complete receiver package, much like the commercial Icom communications receivers used in our first Project Argus stations, but more nearly optimized for SETI, and at a more affordable cost. Here is the prototype of just such a receiver, optimized for hydrogen-line SETI and made up of the modules seen below. We are trying to interest manufacturers in producing this receiver as a commercial product.

SETI League photo

ARRL editor Steve Ford found executive director H. Paul Shuch demonstrating the SETI League Hydrogen Line Receiver prototype at the AMSAT Space Symposium in Toronto, in November, 1997. This photo appeared in QST for December, 1997, and is used here by permission.

WB8IMY photo

The SETI League hydrogen line downconverter is designed to drive a 144 MHz SSB receiver. One of the best is seen here, the R2 receiver designed by Rick Campbell, KK7B, and published in QST for January 1993. The unit is available in kit form for $109 US from Kanga US. SETI League engineers have successfully modified the R2 to optimize it for wider-bandwidth SETI.

Kanga US photo

The prececessor of the R2 is the R1 receiver seen here. This unit is also a KK7B design. Many amateur SETI stations employ computer sound cards for digital signal processing (DSP). Since the maximum sampling rate of popular sound cards is 44 ksps (kilo-samples per second), they can do a Fast Fourier Transform on up to 22 kHz of spectrum at a time. However, the R1, R2, and many other HF and VHF communications receivers exhibit only about a 3 kHz audio bandpass. The modifications shown here increase their bandwidth to 22 kHz for SETI use.

Kanga US photo

The R2 receiver was originally intended for the reception of single sideband (SSB) voice communication. Its input diplexer had a 300 Hz - 3 kHz bandpass, appropriate to that application. For SETI use a wider bandwidth is desirable. The input diplexer can be modified for the 250 Hz - 22 kHz bandpass seen here, merely by changing a few resistor, inductor and capacitor values. The new component values are seen at the top of the diagram (component designations are per the schematic in the original QST article). The IF ports of the two mixers see a VSWR less than 1.05 from DC to beyond 10 MHz.

SETI League drawing

After direct conversion to audio, the signals from the R2's two mixers must be phased for image cancellation. The original all-pass phase shift networks are retained, but six precision resistor values are changed to produce a 90 degree phase difference over the entire 250 Hz - 22 kHz band. The new component values are seen at the top of the diagram (component designations are per the schematic in the original QST article).

SETI League drawing

Once the I and Q channels have been recombined, the spectrum is applied to a passive low-pass filter. The original design used a 7th-order elliptical filter with a cutoff frequency of 3 kHz. For SETI use, we subsititute a 22 kHz cutoff frequency. The new component values are seen at the top of the diagram (component designations are per the schematic in the original QST article). Skirt selectivity and impedance match are seen here.

SETI League drawing

After the low-pass filter, the audio is applied to a low-noise preamplifier, which drives an audio power amplifier through a simple third-order Butterworth high-pass filter. We double the value of the two series capacitors, to slightly lower cutoff frequency while retaining reasonable 60 Hz hum rejection. In all, adapting the R2 to SETI use has required the substitution of 14 capacitors, 7 fixed inductors, and 6 resistors. The new component values are seen at the top of the diagram (component designations are per the schematic in the original QST article). Kanga US will be offering a commercial version of the R2 receiver kit, incorporating these modifications, for SETI use.

SETI League drawing

Along with the R2 or mini-R2 kits, you may want to use the companion LM2 "Little Mother Board," sold for $80 US in kit form. The LM2 provides you with input filtering and gain, a necessary LO phase shift network and power splitter, and a variable crystal oscillator LO circuit, as seen in this block diagram.

Kanga US drawing

A view of the prototype hydrogen-line SETI receiver with the cover removed. The SETI League 1420 to 144 MHz downconverter is seen at the right rear. A modified R2 receiver is to the left of the converter, and an LM2 VCXO circuit appears in front of the converter.

SETI League photo

Looking down on the SETI receiver with the cover removed, the interconnections between the various modules can be seen. Room exists to stack a second R2 board above the first, permitting two independent channels, one USB and one LSB, each 22 kHz wide, to be analyzed by the two stereo inputs of a computer sound card. If a 2048 point FFT is run on each channel, we can analyze a total of 44 kHz of spectrum at a time, in 4096 bins to around 10 Hz resolution.

SETI League photo

This latest packaged version of the SETI League Hydrogen Line Receiver is a pre-production prototype. The enclosure measures 8 by 10 by 2 inches, exclusive of the connectors. On the front panel (left to right) can be seen the +12 VDC power connector, monitor speaker, baseband audio output connector, coaxial input connector, fine-tuning knob and volume control.

SETI League photo

A look inside Model RX-1420 Prototype #2 reveals the familiar Down East Microwave downconverter boards and Kanga-US local oscillator and direct conversion receiver. The advanced experimenter can purchase in kit form everything found inside the enclosure, for under $350 US.

SETI League photo

Click here for lots more pictures.

email the
Webmaster

Top of Page