March 31, 2009

Build Yourself This Cheap Tuned Radio Frequency Receiver

Building a simple tuned radio frequency (TRF) receiver is an easy job. TRF receivers work by tuning to the transmitting carrier frequency (duly amplitude modulated, of course) and amplifying them before feeding this RF to the detector stage. Three major types of AM receivers are:

1. The Crystal Radio Receiver: The crystal was actually an impure form of lead sulphide, called 'galena'. A spring was made to contact this crystal through a fine tip; thus as if working as a point contact diode. This was known as 'cat's whisker' in the past era. Acharya Jagadish Chandra Bose (and G. Marconi) devised this contraption. You will be surprised to learn that this radio consumed no external power supply. The induced current was sufficient to drive a high impedance crystal headphone. You can learn more here.

2. Superheterodyne set: Here we have two oscillators; one tunes with the incoming carrier signal while the other, 'the local oscillator' generates a frequency so that there is always a difference of 455 kHz (470 kilo Hertz in some countries). This constant difference is achieved by 'ganged' variable capacitors, i.e. rotating the shaft will cause movements of both (tuner and local osc) vanes together. For example, if the tuner freq was 500 kHz, the local osc freq would be 500+455 kHz. So, you see that the intermediate frequency (IF) is always 455 kHz no matter what the tuner freq is. In other words, those 2 freqs would produce 'beat formation' in much the same way two tuning forks of different frequency does. (Should you try to simulate 'beats' in your very own computer, want to use your computer as an oscilloscope and study various waveforms, then this software by Christian Zeitnitz might be of interest to you.) Now, since this beat is not occurring in the audible range, the phenomenon is referred to as 'superheterodyning' (for supersonic heterodyning; ultraheterodyning would have been a better term as supersonic relates to the speed of sound) and the receivers are called superhet sets for short. Anyway, the IF is amplified and demodulated before being fed to the loudspeaker.

3. Tuned Radio Frequency (TRF) receiver: This is the one I'm going to talk about. Tuned radio frequency radio receiver using MK 484 or ZN 414Here in the adjoining figure you can spot our good old LC tank circuit. This is our tuner that detects the amplitude modulated carrier frequency. The RF signal is then fed to the input pin (pin 2) of IC ZN 414 (or MK484; YS414 in India). The IC originally produced by Ferranti, has 10 transistors that has RF amplifier stages, a detector circuitry and an AGC (automatic gain control) built in. The current consumption is extremely low and the IC itself operates in 1.2 to 1.6 volt range. Now follow the procedure of cleaning and soldering described in the radio transmitter article and populate the components on the stripboard. Solder them as shown in the figure keeping these points in mind:

1. the output decoupling capacitor (0.1 MFD) should be soldered as close to the IC as possible.
2. keep all leads short.
3. the tuner assembly should be kept as far away from the battery, headphone and their leads as possible.

The amplified RF signal is first 'half wave rectified' by diodes inside the IC and the bypass capacitor smooths out the carrier waveform. The preset (5 k ohm pot) is adjusted for better response. You may want to have a look at the ZN 414 datasheet and different connection diagrams here.

Nowadays, FM transmission is in vogue across the globe for its unmatched audio quality which includes stereo reception. The concept is quite complex which I don't understand well. Briefly, just as we consider force or velocity a vector quantity; the waves are also quite like them. They have magnitude and direction, and they are called 'phasor's. FM receivers work out the signal by a mechanism called the 'phase discriminator' or a 'ratio detector' circuit. The way FM sets do this analysis has a lot in common the way doctors analyze your ECG/EKG (vector analysis).

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