Choosing a receiver If you have a high performance digital communications receiver you can spare for 24 hours a day, 365 days a year, then by all means tune it to the transmitter of your choice and leave it there. The rest of us need something a little less capital-intensive, which is where a talent for sniffing out recycled treasure can be handy. The setup described below is very basic. It's just enough to get you started, but it has limitations as a statistically reliable source of meteor data because of the wide bandwidth of this type of receiver. Build it, for sure, and prove that your setup is working, from antenna to data analysis software, but then add a narrow-band IF section so that you are detecting statistically useful data. Details about how to do this are available on Ilkka Yrjola's site (look under the section MS observing with VHF Radio & Computer. Incl. software, then scroll down to "An economic narrow band FM receiver for the FM broadcasting band"). Most car radios have excellent sensitivity and, since you're using a preamp at the antenna, the signal-to-noise ratio isn't a big issue. Garage sales, school fairs and the like inevitably offer a selection of junked car radios. Choose a digital one: nothing else will do. Make sure the FM section is working and don't pay too much for it. Old hi-fi FM receivers may be OK, but they must be digital (for stability purposes) and sensitive. I was lucky enough to buy two matching Pioneer units for $10, so I have an alternative receiver of comparable performance ready to take over from the main one if anything goes wrong with it. If I ever discover a second suitable transmitter to monitor, this receiver can be interfaced via another bit on the games port, thus providing twice as much data. (See note on games port and lightning.) If you have access to a frequency counter or digital VHF communications receiver, it's worth checking the frequency accuracy of the car radio before doing anything else. The local oscillator frequency should be 10.7 MHz away from whatever frequency the car radio shows on its display, usually higher. (For example, if the display shows 100.0 MHz, the local oscillator should be running at 110.700000 MHz.) If the frequency is more than a couple of KHz off, try a ceramic capacitor in the 2.2 to 47 pF range across the crystal to pull it back into line. I built my receiver into an old desktop XT computer case rescued from our suburb's annual roadside junk collection. It has a good power supply, the case provides plenty of ventilation and there's heaps of room to mount everything. It also looks slightly less ugly than a nest of cables and boxes side by side, and makes the whole setup easy to transport. The antenna lead attaches to a BNC socket in one of the card slots in the back panel, and the 2-wire data cable to the computer leaves from the slot containing the interface card (see photo). All 5 and 12 volt power wires connect to a chunky great terminal block mounted down the middle of the computer case. It's inelegant, but that way it's easy to change things around without a soldering iron, and it's all very accessible. Power for the masthead preamp also came from the XT supply and is fed up the coax in the usual way.   Setting up the receiver Connect the computer's speaker to one of the receiver's audio channels so you can monitor the channel, when necessary. Most of the time you'll want the audio turned right down. Set the mode switch to "mono", and if there's "Local/DX" control set it to "DX". If the receiver defaults to a particular frequency on power-up, make sure you configure this to be the one you're observing so that it will be able to keep observing after power failures. You'll be using some kind of data logger to record the time when our system picks up a signal. That means we need a digital output, a feature I've yet to see on any car radio, so it's off with the covers and out with the multimeter or CRO. We're looking for a mute signal or, failing that, an AGC line. This may be easy to find if you have a circuit diagram, but it's unlikely you'll be that lucky. Take some time to look at the circuit board layout. First try to identify the RF section (the antenna lead is a giveaway), then the frequency synthesiser (probably near a crystal) and audio sections. The signal you want will probably not be in these sections, so now you know where not to start. It's more likely to be near a large IC containing the IF and demodulation components. Just measure the voltage on each pin methodically. Tune in a local signal and try to find a pin where the voltage level changes when you switch to an unused frequency. Don't be fooled by voltages that change gradually as you shift frequency: these will probably control the local oscillator. The Pioneer receiver I used didn't appear to have a mute line, but the AGC wasn't hard to find. It varied from +1.40 volts with no signal down to +0.05 volts on a very strong station.
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