Rick N3FJZ has completed his the direct conversion receiver that we will soon be building with students at a local high school. See video above.
We are hoping that a number of people will build the receiver as we designed it. Some folks have sent us versions of the receiver that they have built, but these versions often include significant deviations from our design, rendering them less-than-useful in checking our work. Rick built it just as we prescribed. His build is very useful in confirming the validity of our design. So if you are working on one of these receivers, I would encourage you to -- for the moment -- dispense with innovations and build it the way Rick did: as per the design we have been using.
We know that our design is not perfect. But we have decided to stick with it because it is very simple and very easy to explain. Examples: We know there is an impedance mismatch between the mixer and the AF amp. But fixing this would introduce complexity that we want to avoid. And the receiver works fine with the imperfection. We know that a push-pull AF amp would probably work better than the one we have. But we do not want to have to explain push-pull amps, biasing schemes, and PNP transistors in this short introductory course. So we stuck with three common-emitter AF amp circuits and an 1K-8ohm transformer.
Rick did a really excellent job not only in building this receiver, but also in documenting it. His diagrams and drawings are really superb. We will probably use these in our presentations to the students:
We will keep all of you informed on the progress of this project. We will begin this week. But if you are still working on the receiver, please send us your work, even if it comes in after we begin the course.
Thanks Rick!
Rick's sound really good. In my build--still ongoing--I'm following your schematic to the letter, but I'm building the modules on perfboard rather than Manhattan on pine board. I hope I'm not being bumptious or uppity.
ReplyDeleteA friend of mine is planning to build one of your receivers and told me about the oscillation problems some people are having with the audio amp. A possible reason for that is the lack of smaller capacitors in parallel with the 47uF electrolytics. Electrolytic capacitors do not have a low impedance at higher frequencies and good designs almost always put a smaller ceramic capacitor in parallel with them ... maybe a couple thousand pF depending upon the frequency of oscillation. Emitter bypass caps provide feedback and phase shift if they aren't really bypassing.
ReplyDeleteRick - Thank you for the very fine schematics. I noticed that:
ReplyDelete-- The Attenuator / BPF diagram shows the inner parallel capacitors at 120 pF.
-- The final Full Schematic diagram shows 150 pF.
-- Bill's original LTSpice diagram shows 164 pF.
Is this an area of the circuit where someone building can use what they have if it is in range of the 164 pF LTSpice model?