Direct Conversion Receiver:
making friends with the Signetics SA602 IC
How simple can it be? If you are a minimalist and like to mess with small electronics projects then you should meet the SA602 chip. The Signetics SA602 IC is a double balanced mixer and tunable oscillator in one shot. Sounds to me like the front end of a receiver.
By adding a handful of other components to the SA602 you can build a direct conversion reciever. Direct conversion means that you process the incoming RF signal at its frequency without down converting it to an IF (Intermediate Frequency) and then processing the IF frequency.
The Ramsey HR QRP Receiver Kits
The least frustrating way to mess with the SA602 is to buy the Ramsey HR Amateur Radio Kit. This cost is about $39. These are advertised by Ramsey as QRP receivers and they are available for 20 meters, 30 meters, and 40 meters. These receivers can receive CW, SSB, and AM. The kits differ only in the external components that determine the frequency of the internal oscillator of the SA602. The oscillator in all these kits is a Colpitts varactor tuned tank circuit. The audio section is the very popular LM386 chip – the same in all the kits.
We have already written about the the non-linear mixing process, varactor tuning, and the LM386 audio amp IC in our article on Superheterodyne. Read that article to get some background on receiver design that uses IF frequencies. This posting is on direct conversion and is an alternative, and historically earlier, radio receiver design.
Building the kit
It takes about 1 hr to build this kit. There is nothing complex about it. It has 84 solder points and would be appropriate for a younger person to build. Kids buiding this kit should have no problem – they will learn how to follow instructions, solder, identify components, and then after 1-2 hrs have something that actually does something. So if you are in to “instant gratification” then this kit may be for you (or your kid)
Don’t expect the world from this kit. This kit is used to demonstrate the principle of direct conversion receiver design – this is a design from the 1920’s. In the history of radio, the obvious short comings of this design was overcome by Superheterodyne.
We built this kit, performed the alignment, and hooked it up to a 20 ft piece of wire. It actually does work and we heard CW, SSB, and broadcast AM on 20 meters.
We dragged out the scope to take a look at the oscillator signal. If you look at the picture above you will see a wire-grabber attached to the circuit board. This grabber is attached to the external components that determine the oscillator frequency of the internal SA602 oscillator. As you can see the internal oscillator in the SA602 is cranking. This is what is being mixed with the incoming RF signal. The oscillator has a tunable range of about 12MHz to 15MHz using he L2 coil (see schematic below)
If you don’t have a scope and you want to check to see if the oscillator in the SA602 is working then use a shortwave radio or ham radio. Place the SW radio or Ham radio bear the HR radio and tune around 14MHz. You should be able to hear the SA602 oscillator as a hissing or dead silence at some point 12MHz-15MHz on the SW radio or Ham Radio.
Direct Conversion – How to do it
There are 4 basic sections in the schematic
- The rough tunable input tank circuit
- The varactor tuning portion of the colpitts oscillator inside the SA602
- The SA602 double-balanced mixer – mix incoming RF with LO
- The LM386 audio amp
Pins 6 and 7 of the SA602 want the tuning portion of SA602 internal local oscillator. The oscillator is a Colpitts oscillator varactor tuned by a 10K pot which reverse biases the varactor diode to tweak the capacitance which in turn affects the resonant of the tank circuit (L2 allows rough tuning). The effective capacitance and L2 control the oscillator frequency. For the 20 meter kit, the frequency is tunable around 14 MHz +- about 1 MHz.
The SA602 does all the work. It mixes the incoming signal with the local oscillator, suppresses the carrier, and produces the difference frequencies. Now here is the big deal. For some incoming RF frequency the difference of this mixing process is in the audio range. This is direct conversion. The output of the SA602 is on pin 4. Pin 4 drives the input of the audio section.
The audio section is a slam-dunk. Its a black box LM386 audio amplifier. The LM386 has enough gain to drive a small speaker.
So, that’s it. At a conceptual level, how much simpler can it be?
The HR20 won’t win any prizes for receivers. But, it is a simple kit that you can build in about 1 hr and demonstrate a basic principle of receiver design – Direct Conversion.
It is suitable for kids to build and “instant gratification” can be had in 1-2 hrs. Perhaps this kit can trigger the interest in a young kid who may have a yet-to-be-discovered interest in electronic or radio. Or, if you are a older ham with a lifetime of appliance operating then this kit may open the black box of a very basic reviver design that is easy to understand and actually works.
Note: The NE602 is mixing two frequencies and the result is an audio frequency according to the multiplication of the local oscillator and the detected signal.