The Ten-Tec 1056 <link to ten-tec> receivers require a fair amount of tweaking before they are optimally configured for use as an interferometer - but the kits are relatively inexpensive. The main disadvantages to compared the Jove kits are that these kits don't have front-end preamplifiers like the Jove kits, and also the audio bandpass filter is setup for narrowband signals like Morse code and must be augmented to improve the astronomical performance of the kit. On the plus side, Ten-Tec provide all the components to operate the kits over the entire HF band so lots of experimentation is possible.
We cannot provide full instructions for assembling the kits - but you get good instructions when you purchase them. Here we discuss the steps that are necessary to make an interferometer out of the kits - read the following before assembling your two kits!
The first receiver is basically constructed as per the Ten-Tec instructions. If you want your system to run near 20MHz then use the components suggested for the 21MHz band. You can replace C1 and C3 (47pF suggested) with 56pF capacitors to bring the frequency down nearer to 20MHz (these extra 56pF capacitors don't come with the kit, you'll need to find some elsewhere). At 20MHz your data will be more comparable with Simple and with participants in the RadioJove project.
Having assembled the first kit you can now wire in the amplifier for the Local Oscillator (LO). The amp sniffs a small amount of energy out of the LO section of your first kits and boosts it up so that we can force the other receiver to run at precisely the same frequency.
Note that the amp 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.
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.
You'll probably need 15-30dB of attenuators to bring the LO signal back down to the level required by the NE602 in the second receiver. Failure to attenuate the LO amplifier output could toast the mixer of your second receiver.
|The 1056 kits convieniently have a "Frequency Counter" output which is perfect for sniffing the LO signal in order to amplify it. You can solder some coax to the Frequency Counter output and connect it to the input of your LO amplifier. This photo shows a Ten-Tec amplifier wired up to the Frequency Output of a 1056 receiver (the coax is soldered to the underside of the board).|
Now that you've completed receiver 1 and wired in the LO amp, you need to assemble the second receiver. There are a few deviations from the 1056 kit design so take care when assembling the second receiver. You need to set this receiver up for the same frequency band as the first receiver. If you changed the capacitor C1 to 56pF you'll need to make the same change to this receiver, but don't change C3 because we don't install it into the second kit!
The main deviation from the kit is that we leave the LO section of the receiver unassembled. This includes L2, L3, D1, R2, R3, R4, R5, R23, C3, C4, C5, C7, C25 - leave all of those pieces in the bag, we don't need them - but we do need the board real-estate!
Instead of inserting those pieces, we use a few components to inject the LO from the LO amp into pin 6 of the NE602 mixer. In particular the coax from the LO amp (probably via a connector) can terminate into the unused pads for L2. The shield of the coax should go to ground and the core should be wired to the track which is supposed to connect L2, C5 and C7. You might wire a 47 or 51 ohm resistor between the C5 pad of that track and ground. This will help to transfer the energy from the coax more efficiently by matching the impedence of the circuit to that of the coax. Finally install a capacitor (say 10nF, the value isn't important) across where C7 is supposed to go, in order to ensure no DC voltages make it to pin 6 of the mixer. This photo illustrates this complicated description.
The 1056 receivers use a resistor to drop the 12v supply down to about 6 volts for the NE602. However because we haven't installed the LO section, the circuit doesn't draw enough current to step the voltage down sufficiently. You need to increase the resistance (of R?, I think it's 220r and it's near the bandspread control) to around 4k7 and/or ensure you insert the zener diode to prevent cooking the mixer.
You should now be ready for the smoke test. Connect some antennas and speakers to the two receivers and turn the power on. You should be able to hear noise from both of the receivers. As you tune past radio signals the output of both of the receivers should sound roughly the same.
At this point you should really consider modifying the bandpass filter of both receivers to make them better suited to astronomy. The bandpass filter that comes with the kit is very narrow - it only lets a small frequency band through. For astronomy we like to have a bit more bandwidth so that we are able to capture more of the astronomical noise at once.
Finally, you now have a functional radio interferometer which is ready to capture it's "first fringes"! At some stage you will want to calibrate your phases so that you know where the interferometer beam is pointing on the sky.