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Superheterodyne AM Receivers

On Pic.4.1 you can see the block diagram of a radio-broadcast superheterodyne receiver The input circuit (UK) refines the signal of the tuned station from all the voltages created in the antenna (A) by various radio transmitters and sources of disturbances. In our example, it's an AM signal that has the carrier frequency fs, and is modulated by a single tone, as seen in the rectangle above its label. This signal is being led into the stage called the mixer. Another voltage is also led into it, the voltage from the local oscillator that has the frequency of f0, and a constant amplitude. Under the effect of these two signals, the phenomenon called the outbreak takes place in the mixer, and an AM signal appears on its output, its frequency being fm=455kHz. This signal is called the inter-frequency (IF) signal, and its frequency fm the interfrequency. The IF signal has the same envelope as the station signal entering the mixer. That means, that the information from the transmitter to the mixer is carried by the signal frequency fs, and in the mixer it is being assumed by a new carrier, that has the frequency fm. When transferring to another station, the user changes the capacitance of the variable capacitor C by turning the knob, setting up the resonance frequency of the input circuit to be equal to that station's one. Another variable capacitor, Co, is located on the same shaft as C, so its capacitance changes simultaneously to that of C. This capacitor is located in the local oscillator and that is how it gets the new oscillating frequency, having such value that the difference of the oscillator and station frequencies is again equal to the inter-frequency value.
Here's one numerical example. The interfrequency is being adopted by the constructor of the device, and it is mostly fm=455 kHz. When the receiver is set to the station that has the frequency of fm=684 kHz, the frequency of the local oscillator shall be fO=1139 kHz, therefore making there difference be
1139 kHz-684 kHz=455 kHz=fm.
When tuning to a station that operates on the frequency of fS=1008 kHz, the listener will change the capacitances of the two capacitors until the resonant frequency of the input circuit becomes fS=1008 kHz, and the oscillator frequency fO=1463 kHz, therefore yielding
1463 kHz-1008 kHz=455 kHz=fm.
If the receiver has more wavebands (LW, MW, SW1, SW2…) it is being constructed to have the same inter-frequency value for all of them.
What do we gain with this change of the carrier frequency? So far we haven't mentioned one very important thing, that is that the input circuit can never be selective enough, to extrapolate only the signal of the tuned station, from all the signals that exist in the antenna. On the output of this circuit, besides the station signal, also signals of strong and local transmitters are obtained, especially the signals from the neighbouring channels (their frequency being very close to the one of the tuned station). All these signals are receiving new signal carriers in the mixing stage, with their frequencies deviating fm as much as their carrying frequencies differ from fS. E.g., if the input circuit is set on the station whose frequency is 1008 kHz, another two signals from the neighbouring channels can also emerge on its exit.

Their frequencies would be 999 kHz and 1017 kHz. The ordinary TRF receiver would in this case be totally incapable of suppressing those signals, which is not the case with the superheterodyne receiver. These 3 signals are entering the mixer, which gets the 1463 kHz voltage from the oscillator. The outbreak occurs, and 3 AM signals are exiting the stage, their frequencies being 455 kHz, 464 kHz and 446 kHz. All 3 signals go to the IF amplifier (MFP), which has several amplifying stages with oscillatory circuits set to 455 kHz, making it very selective, so it amplifies only the 455 kHz signal and suppresses the others enough not to disturb the reception.
the signal exiting the IF amplifier is led onto the detector (Det.), the LF voltage amplifier (NFP) and the output stage (IS), the circuits we spoke about in the previous projects.
The ARP signifies the circuit that turns back the DC component of the detected signal into the IF amplifier, to obtain the automatic amplification regulation.
Above every block on the picture you can see the signal shape exiting that block, as seen on the oscilloscope, in case the modulation in the transmitter is done by the single, sinusoidally-shaped tone. The upper part of the picture contains the average voltage amplifications for each block, for the mass-production devices. Total voltage amplification, which is the ratio of the voltage on the loudspeaker to the voltage in the antenna is A=750000. The amplification in decibels is therefore: A(dB)=20logA=117.5


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