Radios

The hardware requirements for a Simple interferometer are not tied to any particular radio kit or manufacturer. We can offer suggestions on how to some kits, the RadioJove kits and the Ten-Tec 1056 kits, but you should keep in mind that the same ideas apply to many other kinds of radio and you're encouraged to experiment - that's what makes it all fun, after all. We remind you that this site is about sharing ideas and we take zero responsibility for any damage you cause!

You should note that this information only really applies to AM, SSB or radios without a detector. An FM radio destroys the information we require (we want the noise!) and cannot be used without modification.

Most modern radios work by amplifying a signal from the antenna and mixing it with another signal generated locally by a Local Oscillator (LO), to bring the radio signal down to audio frequencies we can listen to or sample with a sound card. Radios that have just one LO and mixer are called direct conversion receivers. Direct conversion receivers are well suited to backyard radio interferometry because we need to lock the LO's of the two receivers together - and with a direct conversion receiver we only have one to worry about. <some links, like radiojove direct conversion intro>.

Locking the LO's of the two receivers makes them run at precisely the same frequency - they are "phase locked". It essentially allows us to measure the time difference between when a radio wave hits one antenna and the other. See the Dark Art page for a background on why this is important in radio interferometry.

The kit is incredibly easy to build with maybe 6 components. Only one semiconductor to get around the right way.

Note that the kit comes with a small amount of 50ohm coax. Although it depends on how you choose to mount your kit, you're likely to need a bit more 50ohm coax. Note the 50ohm coax - not the same stuff we use for the antennas!

You'll also want to make sure that this preamp is installed as close as possible to the incoming power supply for your unit. Long power lead runs are likely to oscillate = bad.

You might also want to consider some power filtering. There are some cute in-line modules available that run to A$20 a pop. Failing that a low value ceramic capacitor could do the same job.

In order to link the LO's you'll need to "disable" the LO of one receiver and inject a copy of the other receiver's LO into that receivers mixer. If you are working with pre-assembled receivers this probably involves desoldering some capacitors from around the mixer.

We then use an amplifier to "make a copy" of the LO from the first receiver. Many kinds of RF amplifier would be up to the task. We've had success with the Ten-Tec T1001 <link> kit. The photo shows an amplifier board wired up to boost the LO signal out of a Ten-Tec receiver.

Note, you'll probably need 15-30dB of attenuators to bring the LO signal back down to ~1.6V peak-to-peak <check this number> required by the NE602 in the second receiver. Failure to attenuate will toast the mixer of your second receiver!! <need link or photo of attenuators to show what we're on about. you can make 'em from 3 resistors>. If you don't have a CRO to measure the signal voltage you should try about 25dB of attenuators if your amp is running off a 12V supply rail.

Once you've got the LO amp in place, you can test if your system works. If you connect antennas and speakers up to each of the receivers and turn the power on you should hear noise from both receivers. As you tune past radio stations you should hear roughly the same output from both receivers, ie, they should both be running at the same frequency. Technically speaking we say they are coherent.

If the LO injection is working and the radios tune together then you need to calibrate the phases by inserting a suitable length of coax between the amplifier output and the second receiver. This step basically makes sure that the beam pattern of your interferometer is pointing at the right part of the sky.

It's worth mentioning that a different approach to phase locking the LOs is to use some kind of signal generator as your LO and inject the signal from it into both receivers. With this technique, both receivers must be configured to have their internal oscillators disabled and accept an injected LO. We're working on that so stay tuned <boom boom!>