Tag Archives: radio

DC40 Receiver

June 15, 2019 Michael LeBlanc 5 Comments

In the late winter of 2018-19, I decided to build a receiver that would provide a performance improvement to the regen I built a couple of years ago. I wanted the radio to receive AM and SSB signals between 160m and 20m (1.8-14.5 MHz).

Another regen seemed to be out of the question; although regens are very sensitive, they have other drawbacks: they’re difficult to tune, and once tuned, the configuration needs to be changed often to prevent howls of oscillation and other audio problems.

It made sense to me that the next step in my radio-building self-instruction should be a ‘direct conversion’ receiver. DC radios have an oscillator running at the same frequency as the signal you’re trying to receive. This oscillating wave gets combined with the signal in a double-balanced mixer. The mixer converts the high-frequency signal down to audio frequency, where it can be amplified using several audio amp stages. It’s a very simple configuration, and the result is a very clean sound.

The DC radio can also be fitted with a frequency display, which is in this case is an Arduino that controls an SI5351 oscillator at the frequency I want to listen to, and a digital display that shows the frequency I’m tuned to.

Design

The basic design comes from Farhan VU2ESE, with modifications by Ryan Flowers. I used a different audio amplifier than the ones specified in the original plans. I tried an LM381 preamp and an LM386 but was unhappy with the results. The amplified audio coming out of the LM386 was barely enough to power a speaker and could only be used with headphones. I tried several different audio amplifiers and finally settled on the TDA2003A.

I faced difficulty with hum and there was a tendency for the audio amp to self-oscillate. I added a power line filter, which helped a little, but the best results seem to be to use battery power. Self-oscillation is still somewhat of an issue with the TDA2003 at max volume. You can see from the schematic that I tried to gather 0v (ground) for both the digital and audio sections to a common point, which should help in reducing hum and other problems.

This was the first opportunity I had to experiment with homebrewed double-balanced mixers. I used four closely-matched germanium diodes for the diode ring.

Another first for me: a reverse polarity protection circuit—using an IRF9540 P-channel MOSFET—is placed between the +13v power socket and the rest of the radio. It delivers full power with no voltage drop.

The DC40 is tuned using a digital encoder. I initially tried a cheap $3.00 encoder but wasn’t at all happy with the feel. Instead I ordered a Bourns EM14 optical encoder from Arrow that provides 64 pulses per revolution. By the way, my experience with Arrow has been very positive, despite the fact that the order wasn’t shipped until I contacted them two weeks after placing it, and then the shipping container for the encoder arrived slightly opened and empty. Support was excellent however and a replacement arrived two days later. Other stuff ordered from Arrow has been on time and in perfect shape.

Construction

The radio is enclosed in two boxes, one for analog and the other for digital circuits; to avoid interference between the digital signals and the analog radio signals, the components are placed on separate boards. Digital circuits are on the top box and the radio/audio circuits are in the lower box.

Enclosure

The enclosure was designed using Autodesk Fusion360 and 3D printed using PETG filament on my Prusa i3 Mk2.

In contrast to my previous and previous 3D modeled enclosures, this one is designed so that each facet of each box is printed separately and later bolted together. Because each facet is printed face-down, the surface is very smooth and flat. The disadvantage to this approach is the “bolting-together” bit.

One might use the term ‘nightmare’ to describe it. Although Fusion360 makes it easy to ensure that parts mate well, it was up to me and my tiny brain to try to visualize how I would be able to bolt the pieces together. To my credit, for the most part my design ensures that I could torque each screw without other parts of the enclosure interfering—if I assembled things in the correct order. However, multiple times during assembly I had to disassemble parts because the arrangement obscured or interfered with other connections that had to be made.

However, over the course of a few days, I was able to fitfully achieve complete assembly. I incorporated holds on the insides of some of the facets to facilitate using nylon ties to organize the inter-board wiring.

Here’s video of the radio in operation:

Radio

When the Lights Go Out

April 5, 2019 Michael LeBlanc Leave a comment

…and when they come back on

Here in Nova Scotia we get a lot of wind. So much so, that sometimes the wind gets the better of our local power utility. Trees fall over and that takes down the power lines.

For most people, when the lights go out, there’s not much to do other than to curl up with a good book and try to keep warm. For radio enthusiasts, a power failure is a pristine new playground sans QRM! I’ll explain.

Most of the time, legitimate high frequency signals have to compete with less-than-intentional electronic noise from FCC-unapproved wall-wart transformers, LED lighting, plasma TVs, and power line hash, to name a few. It’s a mess for the ears. But when the lights go out, my neighbours no longer trespass on my high-frequency peace of mind.

So when the power service dropped off in my Halifax neighbourhood, I grumbled—but I then realized I had a tremendous opportunity: I pulled out my McHF QRP transceiver to listen and record the high frequency bands. I could quickly tell that my reception had improved. It was as if I had moved way out into the country, far from “noisy neighbours.” I turned on an audio recorder because I was most interested in that instant when the power returns. Although it was plain that my reception was better, I wanted to hear how much of a difference there was in background noise levels.

The radio was tuned to CHU, the Ottawa-based time signal station, broadcasting at 7.850 mHz. At 22:03, almost three hours into the power interruption, the power returned, as did the rf noise:

Compare the peace and clarity with the overwhelming din of civilization.

This unwanted noise is so strong that I’ll be soon starting work on a noise cancelling unit that does for RF what Bose noise cancelling headphones do for audio noise. Stay tuned.

Amateur Radio, design, Radio

BITX40 Ergonomic Enclosure

August 30, 2017 Michael LeBlanc 2 Comments

Back in April, as a birthday present to myself, I received a BITX40, an inexpensive 40-metre band SSB transceiver, designed by the brilliant radio amateur from India VU2ESE Ashhar Farhan.

Farhan is well-known among amateur radio homebrewers, and the BITX40 board has been purchased by thousands by enthusiasts around the world. It’s been designed specifically to facilitate experimentation, and you can find hacks and mods on the BITX20 discussion group, the Bitx Hacks site and on Github.

The BITX40 comes in a plastic dollar-store box: the bare board and some sockets, potentiometers, a tiny electret microphone, connectors and other hardware, and once the buyer solders these things on, adds 12v battery power– ‘bob’s yer uncle’– you have a working SSB rig. We call it al fresco operating.

But it’s really just a board, and it needs its own home. In keeping with the homebrew ethos of the BITX, users are designing cases and posting them. Bill Meara’s Soldersmoke blog/podcast regularly features BITX40 radios using powder-coated steel mesh, a metal popcorn box, a peppermint bark tin, and even a cakepan. Talk about retro, how homebrew can you get?

However, having a couple of months earlier bought a Prusa i3 Mk2, I thought that I’d like to design and build a 3D printed box for my new BITX.

The BITX40, being a very economical design, lacks many of the extras that grace more commercial rigs, like a signal-strength meter, CAT (remote) control by computer, or even AGC (automatic gain control). Without AGC, you can tune across the dial at weak stations and then all of a sudden be blown out of your chair by a strong station. AGC prevents that, by automatically adjusting the signal stronger or weaker, as required. Not too hot, not too cold. Just right.

So the absence of AGC requires the operator to use both hands: one for the volume control, and the other for tuning. In that sense, it’s a lot like a regenerative radio. And if you’ve read my previous post, I discovered that improper positioning of these controls can eventually get uncomfortable. Because we’re used to the conventional “dials-and-speaker-facing-front” radio case, it’s not something we’re aware of. But once you use a radio where the controls are ergonomically-placed, the experience becomes much more pleasurable.

My concept is different from other box designs because it’s ergonomically more efficient–the volume and tuning dials are positioned for ease of use and the speaker is tilted up towards the operator’s head. It’s small and has a much larger heat sink to accommodate 100% duty cycle transmissions that digital modes require. The design for the BITX40 evolved over several weeks; I started just playing around with Fusion 360 without making much headway. I experimented with the Sculpt environment in 360. I found it unwieldy and went back to Modelling. I made a total of 9 false starts before settling on a final design.

About halfway through this process, I realized that I was designing the shell without taking full account of what needed to go inside. So I modelled facsimiles of the BITX40 board and all the other components, so I created a “starting point” model from which all later models would derive: