2008-02-04
Autodyne Receiver Now In a Box
I've whacked the 30 Metre Autodyne Receiver into the an Altoids tin along with a front-end low-pass filter and impedance matching network.
The input impedance magnitude was measured to be approximately 820 Ohms, dropping slowly with increasing frequency suggesting a somewhat capacitive reactance as well. A low-pass Pi matching network was computed to transform this to a standard 50 Ohms. The filter was built on a small piece of circuit board, 2.5 uH implemented as 29 turns on a T37-6 core, the shunt capacitances as a mixture of NP0 ceramic and Polystyrene capacitors. The input capacitance was absorbed into the high-side shunt capacitance, although its magnitude is quite small and the moderate design Q of only 6 means it wasn't especially critical to do this.
The filter was tested after construction with a hacked together return-loss bridge. Matching my signal generator into an 820 Ohm resistor it showed better than 0.5 Γ over about 2 MHz centred around 10.2 MHz. This is quite consistent with the design, and the precision of the measurements which wasn't very high. Similar results were seen into the receiver front-end once installed - actually I accidentally installed the filter backwards at first, additional testing of the input return loss showed my error quite quickly. (The receiver also performed extremely poorly with the filter reversed which was a big hint - it was quite deaf and had troubles locking even strong signals.)
The Antenna input and RF output are made via BNC connectors. The DC power and AF output pass through feed-through capacitors. The shielding with the lid closed is quite good, completely eliminating any tunable-hum and the VHF interference problems. The low-pass input, while not very aggressive is quite sufficient for my purposes. You can try a two or three resonator band-pass if you experience persistent problems.
There is sufficient room inside for an AF amplifier and even a small mylar speaker in the lid of the tin. This would make the receiver self-contained (except for the PSU of course).
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Parent article: 30 Metre Autodyne Receiver.