Building a Direct Conversion Receiver: Part 3

KI7MNW’s Homebrew QRP Transceiver

Building a Direct Conversion Receiver: Part 4

We last left off with having built the Diode Ring Mixer portion of the DC40 direct conversion receiver that we started on in Part 1. As was discussed, the Diode Ring Mixer accepts input from two sources: The antenna (through a Band Pass Filter) and the VFO. Now it’s time to examine these two critical but very different stages of the radio, and build them. Follow along!

None Shall Pass!

An antenna picks up signals from just about any RF source and sends it on to the radio. That’s an oversimplification of sorts, but the point is that it presents a lot of signals to the receiver that we do not want. This causes the radio receiver to be less sensitive and puts a lot of noise even into the audio. To solve this, we need the Band Pass Filter. The Band Pass Filter is a tuned circuit that only allows the signals we do want. It rejects everything above and below the band we’re aiming for.

For this project I decided to use a filter that I already had, which is a QRP Labs Band Pass Filter kit for 40 meters. So, this part will be blessedly short. I’m going to refer you to QRP Labs to purchase and build the 40m Band Pass Filter kit. Or the 20m. Or the 80m. Pick the band(s) you want to receive, because these things can be swapped out very easily!

Yes, you could buy the discrete components and build a band pass filter yourself, but you won’t come out ahead. The QRP Labs kit build instructions are top notch so be sure to go there and follow them carefully. Also check out our page “Building QRP Labs Band Pass Filters: 3 Tips you should know!

To install the Band Pass Filter you could solder it into your circuit, but I prefer to socket it using 0.1″ pitch header sockets. I bought These Here (Click Link) and use them for all sorts of connectors. You can cut them to whatever length you need. Very handy!

A simple way to use a single filter for your radio. The opposite side uses a manhattan pad for the RF in from the Antenna

Cut two 4 pin sockets and plug them into your completed filter. The “out” on the filter attaches to L2-A winding of T1. The “GND” goes to ground on both sides of the filter, and IN goes directly to your antenna connector. That’s it! Solder it all up, being careful not to get the socket too hot. You don’t need to solder it directly like I did, and it’s fine to run a wire between a pad for the OUT on the filter to the pad for T1’s L2-A.

Another benefit of using a socket is that you can build other filters and switch them out. Want to listen to 160m or 20m instead? No problem, just turn off the radio and swap out the filter. Or you could even automate it so that it automatically switched to the correct filter based on the VFO frequency. Speaking of the VFO…

What’s the Frequency, Kenneth?

In Farhan’s DC40 he builds a 40m VFO, which is a classic style oscillator tuned with a variable capacitor. It has a lot of its own caveats and adds a lot of complexity to the build. Plus, I like being able to see exactly which frequency the radio is tuned to, which adds even more complexity.

We’re not doing any of that.

Here to save the day and make our lives easier is the QRP Labs VFO/Signal Generator. It takes all the guesswork out of the build and produces a stable frequency that’ll be injected into the Diode Ring Mixer. It also displays the frequency clearly on its LCD display

For our build, we’re not going to use any of the fancy features that the QRP Labs VFO has. We’re going to use it as-is to tune the radio, so you don’t need to configure anything. So I’ll leave it to you to build the QRP Labs VFO and get it working. The instructions are excellent, so be sure to read them thoroughly.

The VFO is shown on the schematic as connecting directly to T2’s L7-A, and if you’re going to use the analog VFO that Farhan designed, you can do exactly that. But we’re using the QRP Labs VFO, which puts out a signal that’s actually too strong for the Diode Ring Mixer. We need to attenuate (weaken) the signal some, and for that we’re going to use an attenuator pad.

The name “attenuator pad” sounds really complicated, but it’s not. It’s just three resistors in a T configuration or a Pi configuration. I built a 3dB T pad using 3 resistors. I chose this configuration based on the chart at the following site: RF Attenuator Resistor Values Table

You can see the different configurations, but we don’t need anything complicated. I chose the T pad because based on their chart, a 3dB pad used resistors that were close to ones I had in stock. You can use whatever works for you. Here is how my T pad worked out, and a Pi would work similarly.

Three resistors are soldered together at the top. R2 goes to ground, and one R1 goes to the VFO and the other goes to C3, which feeds T2 L7-A. If you’re not sure what I’m talking about with R1 and R2, check the RF Attenuator Resistor Values Table link.

Final Assembly

We’re very close to firing this thing up! It’s time to put it all together. You’ll need a speaker or headphones that match whatever audio amplifier you’re using, and connect it. Install a 5v regulator at the edge of the board and power it from the 12v buss. Refer to the article “A Simple 5v Power Supply for Arduino, U3S and more” for details on how to setup the voltage regulator.

Use the header pins and sockets to make connectors for the VFO and get it wired up. We’re almost there!

First power up

Do a quick power test and make sure it all fires up without any smoke. If it all looks good, then power it off, connect your antenna. Power it back up, tune to the 40m band and see what you can hear! If it works, well then… Your receiver is done. You did it! Read Farhan’s DC40 page for troubleshooting steps if you run into any trouble, they’ll apply even though we’ve changed things up a bit.

Here is one of my first power-ups after completing my build. I did something a little different than what I’ve documented- I built a relay controlled Band Pass Filter switch in order to be able to automatically switch between bands. The QRP Labs VFO has the ability to control relays based on the current frequency, and my board uses that function. I have filters installed for 40m and 20m, and if one were to use the LPF Switch Kit (also usable for Band Pass Filters) that QRP Labs sells, you can do up to 5 different bands if you desire to.

From here, the next thing to do is to install the radio in some sort of enclosure. Metal is preferred for the sake of keeping stray RF out of the radio, but it’s not strictly necessary.

Up Close and Personal

I want to share with you some detailed pictures of my build, and encourage you to try it. Building a receiver from nothing more than a schematic is extremely rewarding. And there’s the added bonus that there won’t be another one like it anywhere- it’s just yours!

Retrospective

As I worked to complete this installment and wrap up the series, I ran into a problem. It was kind of anti-climatic. I poured over this very article, looking to figure out what I was missing. I looked through the whole series and I thought to myself “What, that’s it? I recommend a couple of parts and we’re done?” But the answer to that question is yes. It’s really that simple. Add a VFO and the Band Pass Filter and you have a working radio!

I purposefully glossed over a few details like a power switch, power supply, and connector. I didn’t talk about what kind of RF connector to use for the antenna. I didn’t talk about an enclosure except to say that you need one. The reason for this is that these things are trivial and don’t matter. Use what you have. If you don’t have, make! Get creative. The worst that can happen is that it doesn’t work and you have to try something else. Experiment. Don’t be afraid of failure, and don’t get discouraged if it doesn’t work out the first time.

The hardest part really is starting. Once you decide to take the plunge, the rest will fall in line. And if your first version doesn’t pan out, then try again.

I hope this series inspires you to be adventurous, to try something you’ve never done before. To learn things you might not have even considered, and to ask “Why?” Allow me to illustrate.

In the first article I mentioned Robin, AC7LX. She really helped me wrap my head around some of the concepts I was unfamiliar with such as the Diode Ring Mixer in Part 3. She really helped me to grasp what the mixer does, but one question remained. “Why are there input and output transformers?” Neither of us knew the answer, and she did some research and found the answer that I presented in that article. To understand it deeper requires a deep grasp of higher level math than I care to even bother with- but maybe you’d like to explore it.

This series also leans heavily on things that are already written such as Farhan’s DC40 page, the QRP Labs documentation, and other posts in this article. In fact, the post “Building QRP Labs Band Pass Filters: 3 Tips you should know!” started with a tip from Randell, W5ZJ. It was originally intended to be part of this series but became its own article fairly quickly.

Errors Along The Way

I told you in Part 1 that I’d share my errors, and I won’t let you down!

  1. TDA2822M. I should have built an audio amplifier out of discrete components, or else ordered an LM386. The TDA2822M is the very same chip that cause Farhan so many headaches in the uBITX fiasco. He got bad chips, but even the good ones are sensitive. I think I ruined mine by fiddling with the radio while it was on. I am going to replace it with an LM386 that I ordered when I bought the 1n4148 diodes.
  2. Trying to “save time”. I alluded to this in Part 2. I thought I’d be smart and save time and board space by installing not another pad. I shouldn’t have done that. That part of the circuit is messier, and I wanted a 100% clean build. That’s what I get for trying to be clever!
  3. I blew up the QRP Labs VFO. Again, I was handling the radio while it was on. I’m not sure how, but I killed the microcontroller on the VFO. I had a spare with an older version of the firmware, so I put that in so I could keep going. My automatic band switching won’t work again until I get the newer chip in the mail.
  4. I bought the wrong glue. A minor thing, but I got thicker CA glue than I intended. It takes longer to set up and has to have some pressure on it. On the up side, it gives you working time.
  5. Cold solders/Jumping to conclusions. In item 1 above I concluded that the self oscillations were happening because the audio stage had died. I was wrong. I disconnected the audio stage from the radio and it was fine. I looked around and found three different electrolytic capacitors with cracked solder joints. I warmed them up and added some solder, and the radio is now fine.

References

As I’ve mentioned, there are references that I used throughout this and I want to share them with you so that you can use them too. I’ve peppered a few others throughout the series, but these are the ones that laid the foundation for these articles.

Direct Conversion Receivers—Some Amateur Radio History – Wes Hayward, W7ZOI
A Simple Receiver – the DC40 – Ashhar Farhan, VU2ESE
Taking the mystery out of diode double-balanced mixers

And the Emmy goes to…

This project was possible because of (and inspired by) three QRP ARCI Hall of Fame alumni: Wes Hayward W7ZOI, Ashhar Farhan VU2ESE, and QRP-Labs founder Hans Summers G0UPL. On the shoulder of giants, indeed!

I also owe a special thanks to Robin Marks, AC7LX for her contributions. I’d still be doing a lot of head scratching without her generous help.

Thank you for reading this series. I hope you’ll comment and ask questions or tell me what you think. Be sure to subscribe to the blog (top right corner ^) to get notification of new posts. We’re only getting started 😉

12 comments

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  1. Nice project Ryan. Love that you added the retrospective and the error log. Gives good insides of things to think of.

  2. Very good Ryan! what is that pumping kinda noise on the reception?

    1. Hello, Thanks for taking the time to comment! That pumping sound must have been some local RFI or perhaps some band oddity- 40 meters was all over the place that day. It doesn’t persist.

  3. Great series of posts Ryan.

    • michaelleblanc on March 19, 2019 at 5:03 AM
    • Reply

    I’ve built a radio from the same design, but I’m finding that the audio is very low level. On 40m, going to a quiet part of the band, the audio is so quiet I can’t hear the underlying background hiss without putting my ear right up against the speaker. It sounds like you’re tuned into quite a strong signal in the video.

    1. Thanks for the comment! Yes it was a strong signal, but not the terribly so. Are you sure the diodes in the ring mixer are well matched? Also what are you using for a filter? Have you considered bypassing it?

    • michaelleblanc on March 20, 2019 at 4:51 AM
    • Reply

    The ring diodes are matched to within 1.0mV voltage drop of each other. In place of the bandpass filter on the front end I have a wide low pass filter with a -3dB cutoff at 10.5 mHz. I’ll see what happens when I bypass it–thanks for the suggestion. Yesterday I added an LM381 low noise preamp between the 10k LOG pot and my audio amp, and it really improved the audio level. I’m happy with the result so far. I’ll probably work on the VFO next–it’s currently a function generator but I plan to use an AD9850.

    1. Hey that’s a pretty good idea, I might have to try that 🙂 Also, another question: did you build from Farhan’s original schematic, or the corrected one from Part 1 of this series? The original had some serious errors in the diode ring mixer layout which could make it very quiet.

    • michaelleblanc on March 20, 2019 at 4:27 PM
    • Reply

    I used the schematic from part 1… I have a high respect for anything that works well in LTSpice. 🙂

    1. Well in that case, thank you so much for following along with the project! I seriously appreciate your readership and comments. I have an update to this project coming up once I can get some quality time with my soldering iron and hot glue 😛

    • vaishakh em on July 9, 2019 at 10:24 AM
    • Reply

    I am hav a doubt..The radio impedance is 50ohm..i didnt get it

    1. Can you elaborate on your question?

  1. […] month I built the DC40 receiver (and wrote a giant series on it Here, Here, Here and Here. And kinda sorta Here!) and at the same time, Hackaday.com posted their Circuit Sculpture contest. […]

  2. […] Building a Direct Conversion Receiver: Part 4 […]

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