The Neophyte is bread-boarded, with a plethora of tiny parts and pieces as well as a tangle of jumpers and clip wires. Let the building and testing begin!
What I have not managed to created is a functioning direct conversion receiver...yet. I can hear....stuff...in the headphones, but I'm not able to copy any phone or CW signals on 40 meters. Let me tell you what I've done to make it work (full disclosure: as of this post, it still ain't working).
In some respects this project is fully functioning as a theremin, albeit without the wide audio range normally enjoyed in this type of musical instrument. If I wave my hand around the breadboard, particularly in the vicinity of the oscillator components, the noise characteristics change in the headphones. So it would seem to be that I've at least managed to get the oscillator to percolate.
When I adjust the main tuning capacitor (C12), the headphone noise changes as well. In fact, there are two distinct spots in the rotation of the capacitor at which the audio is distinctively "radio-like." As I'm tuning through those spots, the audio changes from background noise to a high pitch signal, descending down in frequency until the signal disappears, and the rising back up through the audio range. A better description would be the swooping sound of trying to zero beat a carrier. This occurs twice across the range of the capacitor, both in the upper and lower thirds of the capacitor. From this I can surmise that the oscillator is both percolating and variable.
Since I cannot even copy a single intelligible signal, I am completely unable to estimate if the Neophyte is oscillating and variable in the correct band. So, out comes the frequency counter....oh, wait, that's right, I don't have one. What I do have is my first unknown!
The conclusions regarding the oscillator (functioning and tuning) allow me to infer one other key piece of information: Since the headphones produce sound, the audio amplifier section is functioning correctly. To share a secret learned from one of N6QW's videos, I already knew that the audio amplifier was functioning by touching the input components to the LM386 chip. If you hear "hum" on the headphones when you do this, you can confirm that the amplifier is working.
Having found no obvious problems so far, at least with the tools available here, I started to look at the front end. The tunable front end works, since I can peak the signal level using C4, the polyethylene variable capacitor and the 42IF123 transformer can. Also, when I remove the "antenna" leads, the noise floor drops away to nothing in the headphones. More on the antenna in another post as this might really be part of my problem.
So, no working receiver, but some useful findings.
What I know:
- The oscillator is percolating.
- The oscillator is tuning.
- The mixer is functioning.
- The front end is working and tuning.
- The audio section is functioning.
What I don't know:
- The oscillator operating frequency and range.
- The antenna input level sufficiency.
- What needs to be hammered on to make the Neophyte work.
From here I'll have to look at the antenna (currently ten clip wires set end to end) as a potential problem, but I'm also going to have to wire up my Arduino and Si5351 clock generator to see if I can squirt some RF into the Neophyte and make a little coherent sound this time.
No comments:
Post a Comment