Building a Direct Conversion Receiver: Part 3

In the first installment, we talked about choosing a Direct Conversion receiver design to build and getting started with the most basic parts. Part 2 dove into Manhattan construction and building the amplification stages. For this third installment, we’re going to discuss the functions of the Diode Ring Mixer, Band Pass Filter, and VFO stages. Then we’ll build the Diode Ring Mixer and be that much closer to a working radio. Let’s get started!

The Things that Make a Radio a Radio

Since we’re talking about the Diode Ring Mixer, lets see where it fits into the radio. We’ll start at the antenna and work our way through the radio.

The antenna will connect to a Band Pass Filter, which strips out all the signals we don’t want. For a 40m receiver, we don’t want anything that isn’t in the 40 meter (7.0-7.3mhz) band. So a filter that passes those frequencies but filters the rest is called a Band Pass Filter. The Band Pass Filter connects to the Diode Ring Mixer.

The VFO (Variable Frequency Oscillator) is a device that produces a small RF signal. For a Direct Conversion receiver, it produces a signal that matches the frequency we want to receive. So, if we want to listen to a 40 meter net on 7283.50khz then we use the VFO to produce a signal at 7283.50khz. The VFO also connects to the Diode Ring Mixer.

The VFO and the Band Pass Filter both shove their signals into a mixer, and out comes audio. We amplify the audio a bunch with the amplifier stages (which we built in Part 2), then the audio amp (built in Part 1), plug in a speaker, and we have a radio receiver.

All Mixed Up

The diode ring mixer is a core component of the radio. As its name implies, it uses a ring of diodes to mix signals together. Signals coming from the VFO and the Band Pass Filter go in, and what comes out is both the sum and the difference of the two signals. The output contains both RF and audio. The RF is shunted to ground via a .1uf capacitor (C5) and what makes it past that is just audio, which is then amplified.

Now that we know what it does, lets talk about building the diode ring mixer. We’ll start by examining the schematic. Below is the entire Double Balanced Diode Ring Mixer (isn’t that a mouth full!) in all its glory:

Outlined in yellow is the entire mixer. The red highlights the input and output transformers, and the green highlights the four diodes that make the mixer itself. To the left is the band pass filter (we’re going to use a different one) and to the right are the VFO and the output the the audio stage.

The input and output transformers are important because the radio operates at 50 ohms impedance, but the diodes need an impedance of at least 200 ohms to work in their operating range. For that, two 4:1 tranformers (T1 and T2) are needed. Take a look at the toroidal transformer on the left, and keep it in mind while we talk about how to build such a beast!

More Than Meets The Eye

The transformers before and after the diode ring mixer are 4:1 transformers, which means (because Maths) that there are twice as many windings on one side of the transformer as the other, and the side with double windings is also center tapped. T1 transforms 50 ohms into 200, and T2 does the opposite.

What makes the transformers complicated looking is that they are wound on toroidal inductors. So lets break it down into simpler chunks. Four leads represent the secondary windings, and two leads represent the primary. On the schematic to the left, I’ve labeled each winding as having an A and B side which correlates to the left and right side of the finished transformer, respectively.

The Primary winding (L2) has 2A going to another part of the circuit (which we’ll examine later) and 2B goes to ground. 3A and 4B go to the diode ring mixer. 3B and 4A are shown as being tied together, but they individually go to ground.

The Transformation Begins

To build the transformers you’ll need 6-8 feet of 32-34ga enameled wire and two FT37-43 toroidal inductors. The designation means Ferrite Toroid, .37″, type 43 metal. You can purchase both at KitsandParts.com, and their “15 Feet of 34 AWG Red (solder heat stripable)” is what you’ll want. Enameled wire (aka magnet wire) has a very fine enamel coating on it that insulates each wire from the wires next to it, but burns off with heat from solder.

You need 3-4 feet of wire per transformer. That will leave plenty left over which is fine. You can cut it into three even sections, but I prefer to fold the ends across each other and pull, like you’re tying a shoe lace. You end up with three equal lengths without making a cut. Then I like to fold the ends over by about half an inch and twist it to give it some heft and make it stay together.

Chock one side of the wire into a drill, and hold the other end with a clamp or even a pair of pliers. Twist the wires with the drill until you’ve got about 10 twists per inch. If you don’t have a drill you can use a pencil or pen as a handle and turn it manually. While it isn’t a critical number, you don’t want less than 6 turns per inch to be safe- it’s just how these transformers work.

Now that you have the trifilar (three wires twisted together) wire, wind 10 turns each on two T37-43 inductors. Each time the wire passes through the inside of the toroid, that is one turn. See This Article for information about winding toroids. Leave a couple of inches on either side, and cut off the excess. Unwind the ends so that you have three leads on each side. Tin them with a solder blob to burn off the enamel, and get out your continuity checker. I use my Innova Multimeter for everything in I do. Check continuity between the left side windings and right. Arrange them so that each lead has continuity to the same lead on the opposite side.

Look at the transformer from the top view. On the left side of T1 you have 2A, 3A, 4A. On the right side you have 2B, 3B, 4B. For T1, 2B, 3B and 4B go to ground. 2A goes to the band pass filter (and subsequently to the antenna) while 3A and 4B go to the diode ring mixer.

T2 is wired a bit differently in that the secondaries go to the diode ring mixer, the center tap goes to the next stage (the amplifier) and the primary goes to the VFO. Use the same principals you did for installing T1 and follow the schematic. You’ll be good to go. For the center tap on T2 I joined the windings with a longer piece of copper strip, and used that to connect its next stage as seen below:

T2 and its pads for the non-grounded leads. The others are soldered directly to the groundplane.
The opposite side of T2. You can see the 50 ohm resistor (R1) tied to the pad that ties the center tap windings together.

After T2’s in place, you need to build the diode ring mixer itself, then install T1. The diode ring mixer consists of 4 pads with four 1n4148’s in a ring, all biased the same direction per the schematic. Let’s get to it!

Matching your Diodes

In the previous installment, I said you’d need to buy more than four 1n4148’s, and encouraged you to buy several dozen. I ended up buying 100 of them at Arrow Electronics for just a couple of dollars, and they came overnight- free shipping too! I highly recommend you do the same.

Why so many? We have to test the diodes and find 4 of them that are as close to electrically identical as possible. We do that by passing voltage through them (limiting its current with a resistor) and then measuring the voltage difference between anode and cathode.

A physical diode as it relates to the schematic

If you physically examine the diode, you’ll see a marking on one side. This marking denotes the cathode (negative) side, and the side with no marking is the anode (positive) side. Connect the anode to a 13.8v power supply through a 10K resistor. Connect the cathode to ground. Measure the voltage directly on the anode and ground. This is the forward voltage of the diode. We want to measure it to the third decimal.

I used a breadboard and an extension jumper to do the testing as shown below, and I simply wrapped some wire around my test leads and put the wire into the breadboard to make a connection. Some of the diodes settled down after 10 or more seconds, but I waited each one out and then sorted them by value until I had four of one value. Any value is fine as long as all four match.

A breadboard makes it easy. I’m measuring the voltage between the resistor and the anode. This one goes in the .634 pile

At this point I’m going to refer you to a YouTube video by Charlie Morris,
ZL2CTM. He goes through all of these things on video and it’s how I learned to do it.

It’s also quite interesting to see how he goes about construction of his. Skip to the 2m30 mark for the information on the diodes themselves.

Above is the completed double balanced diode ring mixer. The diodes in this picture are some test diodes I used while I waited for my 1n4148’s to show up, but you can see how each transformer is connected.

Sneak Preview

In Part 4 we’re going to cover using a QRP Labs Band Pass Filter and also the excellent QRP Labs VFO. Check out my other articles on those two devices to get familiar, and coming up we’ll explore them within the context of the Direct Conversion receiver.

Stay tuned!

I hope this installment has been educational for you and has given you the confidence you need to build the diode ring mixer circuit and connect it to the previous stage. Once you’ve reached this point, you’re very close to having a working radio! Make sure you subscribe to the blog (top right corner of the site) to get notifications of new posts. And don’t forget to follow us on Instagram for exclusive progress pictures! And remember: If I can do it so can you!

CLICK HERE to continue to Part 4

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  1. […] Click Here to continue to Part 3 […]

  2. […] Building a Direct Conversion Receiver: Part 3 […]

  3. […] 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 […]

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