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Fig.2: The three main sections — tuning Circuit,detector and audio amplifier — are separated by C13 and C15 respectively.

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T^NNG DETECTOR AUDiO C«CL«T IZN4K) AMPLIFIER

Fig.1: Simplified diagram of the K7 radio.

ing Circuit does in effect “amplify” the tuned signal and weakens the other sig-nals.    *

Back to our simplified Circuit. Having picked the radio signal we want out of the ether, it needs to be deciphered or', to stay within the jargon, to be demod-ulated. As you may know_t AM stands for Amplitudę Modulation. It mcans that the amplitudę or strength of the radio frequency carrier has been modu-iated with the audio frequency musie or voicc signal.

When you physically look at the modulated signal (on a CRO) you will notice that the envelope of this signal corresponds exactly to the original AF signal. Now, to demodulate the radio signal, the most obvious thing to do would be to tracę an “electric” linę across the envelope. Basically this is what happens in our detector The diodę cuts off half of the symmetrical RF signal and the capacitor smooths out the remaining half cycle pulses into a continuous signal. The RF signal is now demodulated and the audio signal is re-trieved

To make the AF signal audible in the

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headphones it needs to be amplified, and this is done in the audio amplifier.

Ali this might secm very basie and simplified, but you should realize that the first AM receivers did not even have an amplifier. They literally had only four componcnts. Yet at the time, they were regardcd as hi-tech!

The Circuit

As described in the previous section, our K7 cassette box recciver consists basically of threc parts: tuning, detec-tion and amplification. If you haye a closer look at the Circuit diagram (see Fig.2) you wijl notice that the three sections are separated by the capacitors C13 and C15.

The tuning section is quite straightfor-ward. The inductor LI (the coil winding on the ferrite rod aerial) is connccted in parallel with any of the six tuning capacitor combinations through switch SI. Each capacitor combination consists of a fixed capacitor and a trimmer capacitor. The value of the fixed capacitor determines the frequency rangę of the tuning Circuit and the trimmer capacitor is used for the actual preset tuning. Each of the capacitor combinations should be tuned to different stations, depending on your personal tastes and preferences.

Capacitor 03 couples the RF signals into the next stage, the detector. The AM detector consists of one single IC chip which physically resembles an ordi-nary transistor. Don’t be fooled, the ZN414 does the work of several transis-tors. To start with it has a high imped-ance input and buffers the input signal (to prevent loading down the tuned Circuit). It then amplifies the weak radio ^SWITCH-TUNED RADIO

signals, with a typical gain of 72dB — about 4(XX) times. In the next interna! stage, it detects the AM signal; and last but not least, it provides an. output buffer for the audio signal.

The output level of the ZN414 would be enough to drive a set of light hcad-. phones, although the sound would be quite weak. Morę of this soon. We have not mentioned the last and most impor-tant feature of the ZN414 yet: the Automatic Gain Control (AGC). The strength of radio signals depends en-tirely on the power of the transmitter and distance of the station, and the dif-ference in the signal level of detected signals can be very big. To get a consis-tent output level, somewherc along the linę the signals should be attenuated or amplified. This is exactly what AGC does, by using the output level to control the gain of an amplifier.

The AGC of the ZN414 radio chip is very effective. The output signal of the ZN414 is fed back to its input via the potentiometer VR1 and the resistor Rl. The potentiometer VR1 is used as a RF gain control.

Although the AGC of the ZN414 is very good, it has its limits. If a signal still gets through too strongly, then the signal level can be adjusted with VR1. You basically reduce the feedback DC voltage, which will fool the AGC which then reduces its gain. If a signal is too weak, then VR1 should be adjusted in a way that the DC feedback is inereased.

When you are receiving very strong signals then it is possible for the ZN414 “to go crazy” and start protesting in your ears. The obvious remedy for the problem is to decrcase the gain.

This may all sound very confusing and complicated, but rcally we are only talking about one single potentiometer which has to be adjusted; in practice ifs quite easy to use. Resistor R2 determines the RF gain of the ZN414 and capacitor 04 filters out the remaining RF and determines the time constant of the AGC Capacitor 05 blocks DC, and leads both us and the AF signal to the audio amplifier stage.

The l.5V supply voltage does not allow a great “working space” for audio amplification. The output level is quite reasonable espccially when you use the “in your ear“ typc headphones. The biasing method used in the first transis-tor stage protects us against problems due to transistors with different current gains.

This first transistor stage amplifies the AF signal to an acceptable lesel. The signal is then fed into the buffer stage

ELECTRONICS Australia, Seplember 1987

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