Well, like a lot of things I do the 40m DC receiver I have been discussing never quite made it into a case. Despite ticking all the boxes on performance, I just have this uneasy feeling I am missing something. I listened to the audio in a you-tube video demonstrating an alternative design, the juma-RX1. The differences are minimal:
- 2 pole BPF v 3 pole
- different implementation of AGC
- 1W v 5W audio stage
But something is nagging me. Perhaps the reason there are so few signals around when I get a chance to fire up the receiver is that Perth, where I live, is the most remote capital city in the world. It means just about every signal is "DX" distant for me. I looked really hard to the Juma schematic and they way audio is being recovered from the differential outputs of the product detector has me intrigued. I plan to hack my board around to allow a similar differential audio pre-amp stage to be used.
One of the nagging doubts is a carry-over from when I looked at RF transmitting transistors. Just because a part is labelled as a BC549 doesn't mean it is a "low noise" version. Since I don't know the origins of the NPN transistor I used in the audio pre-amp it is possible I am not getting the best performance possible.
In addition, tying the two mixer outputs together as I have done does not have the same intuitive appeal as the differential approach used by the Juma.
So my intent is to modify my receiver to use a similar differential audio pre-amp stage. I'll keep you informed of what happens.
73's
Richard VK6TT
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