Doing ot the hard way, because you can. - Page 1
Forum section: electronics / projects

author: Howardlong
author_rank: Super Contributor
timestamp: May 05, 2015, 02:56:36 am
content: When I was about eight years old, back in the early '70s, my old man and I set to making the Ladybird Book radiohttp://www.mds975.co.uk/Content/george_dobbs_trf_radio.htmlIn its simplest form, a basic crystal set will do. But in contrast to keeping it simple, I present to you how to make an AM radio the hard way, in software on a microcontroller, with RF direct into the MCU's ADC, and AF out.http://youtu.be/lD3WSCN-5u0I have to say this was one of the hardest leisure projects I've ever done.There are some comments about the optimisation difficulties encountered herehttps://www.eevblog.com/forum/microcontrollers/how-are-micros-programmed-in-real-world-situations/msg660776/#msg660776And yes, the amp does go up to eleven.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 05, 2015, 03:30:43 am
content: Excellent job there! Would like to see how sensitive and selective it is if you make another video.

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 05, 2015, 09:12:44 am
content: HowardMy God that brings back memories! I remember getting that book fromthe school library (primary school) in the early seventies. I must have been about 8y old. I never managed to build it as it was a bit too advanced for me then but I did build some crystal sets from toilet roll formers. Thanks for the fond memory!

author: smjcuk
author_rank: Frequent Contributor
timestamp: May 05, 2015, 10:08:31 am
content: That is indeed the hard way! Nice work.This is a rather large contrast to the 40m QRP receiver I'm working on which is basically the lowest part count direct conversion that doesn't suck completely and doesn't use any ICs.My first radio was a crystal set - it's what started it all for me.

author: tautech
author_rank: Super Contributor
timestamp: May 05, 2015, 10:37:30 am
content: Quote from: smjcuk on May 05, 2015, 10:08:31 am+1Mk 1Coil wound on a toilet roll center and tapped every 10 turns for tuning.REAL Germanium crystal and a "cats whisker" for a diode.Mk 2Updated the crystal to a 6H6 double diode cause it withstood the knocks.Mk 3Philips transistor beginners set.That was 45 years ago.

author: Howardlong
author_rank: Super Contributor
timestamp: May 05, 2015, 12:12:28 pm
content: Quote from: joeqsmith on May 05, 2015, 03:30:43 amThis was more of an academic exercise than for a final result, to see if it's possible, with a single MCU, to do such a thing. It is so close to using up the entire Cortex M4 core's capacity that to do much more is going to require offloading bits to the other M0 cores on chip.Still it's been an educational few days, I learned an awful lot, and on several occasions after a long stint of no progress, walking away and coming back an hour or so later with a fresh mind has frequently proved a very effective way of fixing those inevitable bugs that creep in. Debugging and testing DSP code, due to the enormous amount of data that's processed, can be quite intensive.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 05, 2015, 01:24:37 pm
content: Quote from: Howardlong on May 05, 2015, 02:56:36 amThat seems like a strange way to mount the ferrite, 2 closed loops around it would kill the Q of the coil I think.

author: smjcuk
author_rank: Frequent Contributor
timestamp: May 05, 2015, 02:08:50 pm
content: Probably fine at MW frequencies. I've seen all sorts of cruddy hacks like that in old MW sets I used to take to bits and they all worked fine. Well they did until I'd finished with them

author: Howardlong
author_rank: Super Contributor
timestamp: May 05, 2015, 02:11:24 pm
content: Quote from: smjcuk on May 05, 2015, 10:08:31 amWhy use three transistors when you can use three million?My original intention was to see if I could get it to work on 40m. It could be done but it could be considered cheating, you'd have to subsample and have a reasonably sharp bandpass antialias filter which might need tweaking. The current one is just fixed value lumped LC. The ADC bandwidth is fine though, it's designed to run up to 80MSa/s, but there's no point running it much higher than the current rate, the poor old CPU wouldn't be able to cope with the downconversion filtering, that is highly optimised assembly and already uses up 70% of the M4 core at 2.04MSa/s.Technically speaking, I guess I am subsampling already on frequencies over 1.02MHz as the sample rate is only 2.04MHz. The current bandpass filter cutoffs are 500 and 1600kHz, so there will inevitably be images present.It might make a good video on the more detailed design aspects, from antenna through band pass filter, amplifier and onto the software. There are plenty of concepts to cover, including the ADC, DMA, PWM as well as the SDR fundamentals.