Thoughts on Direct Conversion Receivers

Let's look at the pros and cons

Before we get into the meat of the post, please allow me a few personal background paragraphs. I’m not on social media any longer and I don’t blog about personal stuff very much these days, so I realize that a lot of readers, even those who may be familiar with me from my earlier work, don’t know my current status.

For quite a few years, our family lived in Beaverton, Oregon. I had a one-man business named Etherkit that manufactured some small open source amateur radio projects. Everything changed because of the pandemic lockdowns. We decided to make a significant move, by leaving the city and pursuing a homestead in rural Willamette Valley. I could literally write a whole book about the travails of selling our old house, finding a suitable bit of land, and getting the raw land developed with all of the requisite utilities and a house, but I’ll spare you. The main thing is that we saw our goal to the end after two long years, even if it didn’t work out 100% as we were hoping (does anything that significant?)

The main thing is that while I’m now happily on our new country estate, things are quite a bit different from our old living situation a few years ago. Etherkit is in hiatus (although I’m working hard towards a relaunch), we had to downsize our new house a fair bit from what we originally planned, and I’m having to basically start over on my work after a couple of years pause. Because of the house downsizing that we were forced to do, I no longer have a dedicated office/ham shack with plenty of room, like I did at our old house. My wife Jennifer graciously agreed to let me take a corner of our bedroom to use as a bare-bones workbench until we can get a proper workshop on our land, hopefully in a few years. Until now, I’ve been developing Project Yamhill almost exclusively “on paper”, or more accurately “on PC”. Now that the first batch of boards is ready to be fabricated, I have to have a way to build and test.

NT7S Workbench late 2023

The Next Phase

Now that I have a place to actually melt solder, make measurements, and use a real mechanical keyboard again (holy crap, so nice after using a laptop for a couple of years!), it’s time to start integrating all of these theoretical building blocks into real radios, as well as make measurements of blocks and radios, and experiment with new and old circuit designs.

The first integrated radio experiment for Project Yamhill will be the direct conversion receiver. The classic DC receiver is often a homebuilder’s first “real” radio, as it can be executed very simply, which allows for good chance of a successful build. In the photo above, you can see that I’m building some modules into a DC receiver on my bench. The largest circuit board is actually a different audio amplifier than the one that will be included with Project Yamhill, and there’s a good chance that this design will also see the light of day soon…stay tuned.

In preparation for this unit, I’ve re-read chapter 8 of Experimental Methods in RF Design (EMRFD), which does a nice deep-dive into the topic. If you are already familiar with homebuilding, that’s a great place to learn the nuances of this receiver architecture.

So what exactly is a direct conversion receiver? At its most basic, it’s a radio that uses a mixer fed with a local oscillator (LO) at essentially the same RF frequency as the desired signal, which gets converted directly to an audio signal. This audio signal is then amplified to a level where it can be heard, and usually is filtered to eliminate annoying and unnecessary high-frequency audio.

Let’s do a brief look into the unique features and challenges of building and using a direct conversion receiver.

The Good

The aforementioned simplicity is definitely one of the biggest draws of the direct conversion receiver, but perhaps the most striking thing is the audio fidelity of a good, or even decent, design. Since the incoming RF is downconverted directly to audio, there is very little distortion in the received signal. One can do plenty of audio filtering and processing, but there’s something to be said about the crystal clarity of DC audio with perhaps only a bit of audio low-pass filtering. You simply have to hear it to understand it. The best is to hear it directly from your own receiver, but video recording can give you a taste of the audio if you’ve never heard one before.

Here is a video from QRP legend Dave Richards AA7EE showing a relatively simple DC receiver that has high-quality audio filtering tuning through the CW portion of the 40 meter band.

Here is another video from famous SolderSmoke podcast founder Bill Meara N2CQR that is a great example of tuning through SSB signals. I’ve cued the video to the portion where he actually listens to the bands, but feel free to watch from the beginning if you want more information.

Hopefully you can see the appeal from these videos.

The Bad

Of course, there was a reason why direct conversion receivers were not widely used in amateur radio for decades (although they have made a comeback in the form of software defined radios). Superheterodyne receivers were the standard for a long time, until the most recent batch of ham radio manufacturers started using some form of direct sampling or downconverting software defined radio. As we experiment with DC receivers, we’ll encounter these shortcomings and find ways to mitigate some (but not all) of them.

No Sideband Rejection

The most obvious, and unavoidable, problem with a direct conversion receiver is the complete lack of sideband rejection. In a superheterodyne receiver, one of the two sidebands of a detected CW or SSB signal can be selected by the IF filter, while the unwanted sideband is rejected. When you tune a superhet across the band, you hear just one signal.

A direct conversion receiver has no such ability to filter out a sideband, since it’s mixing an oscillator signal with the desired signal at the same carrier frequency. When tuning on a DC receiver, you will always hear first one sideband, then a null as the oscillator frequency exactly matches the desired signal’s carrier frequency, then the opposite sideband.

Audio filter in a DC receiver helps to cut out unwanted portions of the instantaneously-received audio signal, but can never eliminate the unwanted sideband. This isn’t normally a huge problem when the band is lightly or moderately busy, but can be very difficult to deal with when the band is heavily used.

Microphonics And Transients

A common problem with simple DC receivers that you’ll hear about is microphonics. That is, when there is a sharp mechanical jolt to the receiver, or even the bench it is resting on, you’ll get a loud impulse thump in the audio. Without getting into too many details here, this is a result of amplitude and phase transients in the local oscillator signal that get reflected in the mixer.

These transients can be tamed by using good isolation techniques to minimize local oscillator signal going out through the RF port of the mixer. A buffer amplifier with good reverse isolation placed at the RF port is a common way of mitigating this problem. Also, good mechanical construction techniques will help as well.

Tunable Hum

The phenomena known as tunable hum is probably the most insidious out of this entire list, especially in the current times. Tunable hum is a low-frequency interfering signal in the audio that peaks and ebbs as you tune the local oscillator through the band. It is the result of the the LO signal mixing with RF leaked from mains power or from other strong, low-frequency systems such as the plethora of switching power supplies in the modern home. This composite signal re-enters the receiver through the mixer RF port, which then leaves a nasty low-frequency audio hash on top of your desired signal.

Other than trying to eliminate the low-frequency offenders like noise battery charges, LED lighting circuits, etc., this problem can be mitigated with shielding around the local oscillator. This can be a bit of a challenge to get right, so expect to see work in the area of tunable hum during this phase of Project Yamhill.

AF Oscillation and Instability

Due to the high level of AF gain (~100 dB) and poor construction techniques (poor grounding, layout), sometimes the audio system can break into oscillation, causing howling in the headphones. This isn’t that much of a problem these days with well-design amplifier chains, but you may still run into the issue when trying to do a bare-bones design with minimal or no negative feedback in the audio amplification.

What’s Next

Now that we’ve taken a high-level look at direct conversion receivers, I’m going to be starting on the formal documentation outline for this learning unit. I think I may also design an audio filter module to go along with this tranche of boards, so that a homebuilder can experience the difference between lightly-processed and more heavily-filtered DC receiver audio.

I’m still breadboarding and working with Manhattan-style prototype modules here, so that I can hit the ground running when I get real PCBs in hand. Speaking of PCBs, I do want to order them soon, but I need to save a bit more funds so that I can order all of the many PCBs in one batch from the fab. Hopefully I can gather that together sooner rather than later.

I’ve also got that side-project that leverages this work, plus some previous old circuits that I found in my notebook that could be a fun little cheap and cheerful “popcorn” project. Further details on that to follow.