40m Direct Conversion Receiver - Yes, I'm doing another one!

OK, so I saw a blog post by someone who suggested they were getting a batch of boards done at a great price. I won't name them because their bargain was way too expensive once freight was included. But by the time I discovered that it was too late, I'd made the mental commitment to one final, best I could build version.

Since then I've sat on my hands for almost a year. But this weekend I needed a break from the maths of loop filters and modifying 10GHz equipment. I wanted something I could do without much thought.  And since all the planning for this was done around a year ago it seemed like it was time to start.

I had wanted to see if toroids would offer a significant advantage over the tuned inductors in the RF bandpass filter.  While the existing filter works really well,  I considered moving from adjustable inductors, with a Q of perhaps 70, to toroids with a Q of say 200.

The tighter filter passband became less attractive when I worked out the maximum inductance I could wind on the toroids was around 1uH. A larger inductor has wire too fine to allow for compression and expansion of the coil turns to tune the filter. The advantages of the higher Q are offset by the lower impedances within the filter and the net result is not much difference. Had I had a larger toroid then things would have been different. So I will stick with the existing filter and this iteration will really be about incorporating all the modifications I made on the fly.

Here is a piece of advice I agonised over putting to print. I'm not comfortable denigrating other people's designs. But I was gob-smacked to see someone recommend an AM broadcast filter for a 40m receiver as a way of fixing receiver overload from short-wave transmitters. I'd like to know how that works. Because I can see no way that it can work. Remember, AM broadcast signals are around 1MHz. Those pesky short-wave signals run from 40m upwards to perhaps 16MHz.

And another matter that galls me is people still promoting simplistic designs as being worthwhile. If the project doesn't have at least a double tuned band pass filter between the aerial and first mixer then it's a glorified crystal set. I do wonder if those that propose the "easy" way know what they are doing.  For all the effort that goes into building a receiver reward yourself many times over with another 1% of effort and put in two or three extra components in one of the most critical parts of your receiver.

Here's a picture of progress to date. I hope to have this finished by the end of the month. Then I'll think about what to do with all the spare boards. Any takers??

Regards
Richard VK6TT