Sunday, July 30, 2023

Built VK3XU's "Simplex sidebander"

I'm on a bit of a retro transmitter trip at the moment. Armed with some low profile 3.6864 crystals I built the "Simplex Sidebander" described in the first volume of Drew Diamond, VK3XU's wonderful "Radio projects for the radio amateur".

The transmitter is simplicity itself. An NE602 with a crystal pulled up in frequency produces double sideband that goes through a 4 crystal bandpass filter. Three RF gain stages produce about 4W out. The final is a low cost IRF510. Mic amp is a 741. Here's how mine looks:

Despite trimming the oscillator crystal I couldn't get decent sounding sideband out of it. The reason is that the little crystal wouldn't pull high enough. Acting on the advice of SmartFriends™I replaced the single crystal, first with two and then three of the little ones and in the end with three larger crystals which seem to pull more. In the end I have a full size polyvaricon for tuning:

Drew warns that the whole thing needs to be in a shielded box and I can attest to the fact that when attached to my antenna it takes off magnificently. 

The exciter itself seems very narrow to me - here it is fed into a local SDR.

Off to the shops to buy a tin of biscuits... (to get a tin to put it in).

Boxed up and it no longer takes off when connected to the antenna.

I have a bit of instability as you can see in the dips in the AM test signal here:

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