My first HF receiver project is complete.
It’s not fancy. It’s rock-bound with no ability to QSY. It only has a single RF gain control up front. It’s direct conversion, so it hears all signals on both sides of its frequency. It’s bodged together and probably not super durable.But it’s my own, made from scratch, and I couldn’t be more happy. So without further ado, here’s the finished product:
I received this (scandalous!) container in a White Elephant gift exchange at a holiday party this Christmas, and I immediately thought it would make a good home for a radio. It seems a fitting case for the receiver that’s taking my homebrewing virginity.
The circuit is nothing particularly new: it’s based, as I’ve said before, on the work of Dave Richards AA7EE, some fine projects from GQRP, and a very useful document from Bill KV2AWC. The unit is a direct-conversion receiver based around the ubiquitous NE602/NE612 mixer/oscillator IC and an LM386 audio amplifier circuit. Gain is controlled solely by the RF gain pot in the front-end. I find this provides more than enough gain control. There’s also a position for a jumper to boost the audio output from the LM386, but with the amount of RF noise in my apartment, this proves more detrimental than helpful. I may turn that jumper into a switch if experimentation shows its use.
The only filtering present is the handful of passives that sit between the NE602 and the LM386, filtering the audio a bit between converting to baseband and the audio amplification stage. A little experimentation showed that the present passives only seem to round off higher audio frequencies, say about 6 khz. They’re not really meant to enhance selectivity, but just to reduce noise introduced by signals further away from zero beat.
I found some nice panel-mount BNC connectors at our local Fry’s electronics, and fitted some Anderson Powerpole pigtails as a power input. (Pro-tip: if you use Powerpoles regularly, as I do in my theatre job, get yourself a ratcheting crimper. Less than $40, will change your life.) There’s a 1/8″ jack attached to one side and the RF gain control is mounted up front. The whole thing weighs maybe half a pound.
Signals come in clear and hot on this thing! With minimal audio filtering and just a basic front end filter, it’s pretty wide open, but that’s kind of the point – at this point, I’ll take sensitivity over selectivity. Particularly up in the QRS part of the spectrum, I’d rather be able to hear the one guy who’s within 3kHz of my crystal than lose him. Remember, this thing is rockbound, so changing frequencies means swapping in new crystals.
While I’ll probably never really be ‘done’ with this receiver – with tinkering projects, are we ever really done? – it feels great to have it packaged up, in a case, and usable. It’s freeing to have something complete enough to show off.
Hear you on the air!