Building a basic QRP homebrew BITX - Part3

[@acestudioscouk-Ace-G0ACE]

Enjoying this series as a returning amateur. You de-mystify the process and I feel I might give it a go.

[@boxingday11]

Thanks Mark.

[@nevmarr]

Hi Mark. I'm enjoying your posts as I'm working on a similar project. I built up 4 balanced modulators before I got something to null nicely! I mucked about matching diodes (1N4148's) across a range of currents, trying to find a pair with the same IV curve (I think that's the right term, but ignore if I'm wrong). On reflection, a bit of an overkill. After more mucking about I found diodes with the same voltage drop driven with 5mA worked in my setup (you tested at 1mA?). The other thing that buggered me up was trying to put other elements on the same board as the balanced modulator (mic amp, rf amp after the pot). Things seemed to be getting in to things - I never figured what was upsetting what, but a small amount of physical separation made a big difference. And a curiosity … what's on the output of your balanced modulator to complete the DC path? I have a RFC (as many windings as I could on a small ferrite bead).

[@MarkG0MGX]

I''ll try matching the diodes at 5mA, but I would think that the actual current in circuit is very small. Will report back! Thanks for the feedback

[@MarkG0MGX]

I've added some capacitance either side of the balanced mod and the results are excelent. 20pf on one side and a 5-40pf trimmer on the other. Please see the bottom of this blog post: g0mgx.blogspot.com/2020/02/back-to-bitx.html