Antenna, Downconverter and TV/VCR
The antenna is a 426.250MHz turnstile reflector (it looks like 2 dipoles at 90 degrees) antenna tuned to 50ohms. The 426.250MHz signal is then downconvertered to Channel 3 (~62-64MHz) by a PC Electronics Downconverter. The downconverter is in that small grey box on the antenna. Note that it needs a 12V powersupply which we actual sent in the signal coax via a 12V supply and a feed through capacitor for the signal. Less wires = good.
The "bird feeder" ground plane antenna and ATV downconverter
?ATV Downconverter Schematics
From here, our trustee old VCR picks up channel 3 and records the video image and the data coming down the audio channel.That way, we can always go back and recover the data again in case of the ground computer crashing or whatnot. From the VCR, the "Video Out" plug goes to our B&W mointor (a small B&W TV set) so we see what the rocket is seeing real time.
Modem (Demodulator) and Ground Station Computer
?Demod Thumbnail.gif
AFSK Demodulator
?AFSK Demodulator Schematic
Finally, the "Audio Out" RCA jack on the VCR leads to our home brew modem. Actually, it's only a demodulator - it takes the two audio tones from the modulator in the rocket and converts it to RS-232 signals. A 2225Hz tone is translated into a 1 ("mark") and a 2025Hz tone is translated into a 0 ("space"). This stream of 1s and 0s then gets sucked up by our trusy old 386 laptop from the mid-eighties. Running a really killer OS (Dos 3.3), the laptop blazes away at a whopping 15MHz to record our 300bps data stream using a free terminal logging emulation package called. Ok so maybe it's not that high tech but it works like a charm.
Ok, it would have worked great except for the short between the grounding and data stream from the PIC board to the modulator in the rocket which reduced the modulator to all "0"s. Arrrrrrrgh. Next launch, we'll have pre-flight testing in place before we launch!
For more information on all of this junk, feel free to contact us at info@psas.pdx.edu!