Direct Conversion Receivers - Simplistic designs do not do them justice

I was pulling apart some goodies that I recently acquired to see what was inside and found myself looking at switched capacitor filters, NE570 compander chips and low noise op amps. It took me back to when I was actively building HF transceivers consisting of Double Sideband Transmitters and Direct Conversion (DC) receivers and the long path to success. I found the final result well worthwhile so I thought it was worth updating my receiver onto one PCB and posting about what I learnt.

Firstly, I'm going to be controversial and point out that the majority of DC receiver projects usually found on the web are not serious receivers. I don't mean they aren't enjoyable, rewarding or capable of hearing signals. They can do all of that. But they are just cheap. Thrown together with a minimum of parts as if saving a few pennies becomes a virtue allowing you to overlook the shortcomings.

Nope. I've been at the cheap end and it is very unsatisfying. My design criteria are:

• lots of volume from a speaker since headphones don't work if you're moving about the workshop while you listen to a net,
• no audio instability, 
• no hum, 
• good AGC so you are not jockeying the volume control, 
• good audio filtering, and 
• a squelch. 

In short, I want a DC receiver that can be used without having to apologise for it's failings.

I was able to achieve these goals and build a 40m DSB transmitter / DC receiver combination that I actually used in a contest. This is not suggesting it was competition grade. Rather, it was a radio which worked well enough that it had to be punished to uncover shortcomings. And while it struggled at times with a band of strong contest signals I considered the results acceptable.There were many, many, iterations over more than a decade before I deemed the results acceptable.

Over three or four posts I will take you through the final configuration that achieved this stage be stage. I will discuss what I tried, the shortfalls and the improvements (or corrections!) I made before I was satisfied.

Today we will start with the product detector, or mixer. This is perhaps the single most important design decision that shapes everything else. In chronological order I've tried the following product detectors:

1. Discrete transistor product detector
2. LM1496
3. SBL-1 diode mixers and equivalents
4. FST3125 mixers

What I found was they all worked, but some are better than others. The discrete transistor mixer, used as a product detector, actually worked surprisingly well. I couldn't find a LM1496 back then, or a CA3028 or similar. So I lashed one together from discrete transistors (BC109's from memory) as if it was a CA3028. It was so long ago I couldn't find my notes but it was similar to this one: Discrete transistor Mixer

Eventually I found a LM1496 and that  looked more conventional, see below. The output of the product detector was feeding a low pass filter built around a LM324. As I discovered later the LM324 was too noisy. But since I didn't know any better I was running the LM1496 at maximum gain by connecting a capacitor between pins 2 and 3. I could hear stations and I made several contacts with the transceiver this was used in, but I still suffered from cross modulation and strong signals overloading it.

Lesson Learnt: With no RF preamp you need a low noise audio stage after the product detector even when using an active mixer.

LM1496 Product Detector

So I moved on to a diode double balanced mixer, the SBL1. The holy grail I thought. A triple band pass filter to reduce the chance of cross modulation and overloading from transmitters outside the 40m band.

SBL Product Detector (Note: Filter values shown proved to be wrong!)

But I still had a deaf receiver. About this time my collection of test equipment had grown to include a HP signal generator and an oscilloscope. Now I could make repeatable measurements i.e. a measurement made today would be the same as a measurement made 2 weeks prior. Poking and prodding revealed the RF bandpass filter had too much loss and the audio strip was still too noisy.


The temporary fix was to install a RF preamp between the band pass filter and the mixer. This transformed the radio. Ultimately I sorted out the loss in the RF bandpass filter, fitted a low noise audio amp directly after the mixer and was able to remove the RF preamp.

And while this served as the product detector for a good year or more I still wasn't happy. I live in a low noise location so connecting the aerial did not always produce a large increase in noise in the loudspeaker. So I built a product detector around a FST3125. With my now further improved collection of test gear I could see that this approach had promise. Importantly, this IC is far cheaper and more readily available than the SBL1. It can often be found on early server motherboards for free.

While I was satisfied with the SBL1 style mixer, I consider the FST3125 product detector to be at least as good and far cheaper. Next time I will discuss the RF bandpass filter..

More on that later.
73's
Richard