WhitakerAudio AM/FM Stereo Tuner 2012, Assembly & User Manual, Page: 13 / 166

13
AM/FM Stereo Tuner
remaining the same as with no modulation. The additional 50 percent sideband power is furnished
by the transmitter modulator. Since sideband power and amplitude varies with modulation
percentage, the term amplitude modulation is applied.
Although AM could conceivably be a high-fidelity service, it normally is not for three primary
reasons.
• First, broadcast frequency allocations in the U.S. (and elsewhere) are 10 kHz apart. As
such, sidebands from each station must be limited to 5 kHz on each side of the carrier in
order to prevent interference with the next channel, unless the station has relatively clear
channels on each side, or employs a protective field pattern to prevent interference.
• Second, the dynamic range of the program material is limited. Modulation by the station
over 100% will result in spurious emissions and must be prevented. A limiter is used to
compress louder peaks, with a resultant loss of dynamic range.
• Third, the medium-frequency band in which AM broadcast stations operate exhibits a high
level of man-made noise, which limits the minimum noise floor.
Detection of AM signals can be accomplished by removing one half of the modulated carrier
envelope by rectifying the carrier in some manner, usually by employing a non-linear
unidirectional current device such as a vacuum tube or solid state diode. RF energy remaining
after detection is removed in a low pass filter that attenuates high frequency energy but passes
audio signals with little loss.
An unconventional type of detector is used in this AM tuner, although the basic principle of
operation is much the same as the more conventional single diode type described above. As
shown in Figure 1.1, two diodes (D1 and D2) are employed in a voltage-doubler circuit connected
to the secondary of second IF transformer T4. One diode is connected between the IF transformer
and ground and serves the normal function of rectifying or detecting one peak of the IF signal,
storing the developed negative dc voltage across a 47 pF capacitor (C18) also connected between
the IF transformer and ground. This diode conducts during the positive portion of the IF signal.
Because the positive portion of the signal is shorted to ground through the diode, only the
negative portion will be available to charge the capacitor—the diode appearing as an open circuit
to a negative potential. When a negative swing takes place, diode D1 connected between IF
transformer T4 and the filter will conduct, allowing the negative voltage to appear at the filter
input. Both diodes develop a negative voltage and these voltages add to yield twice the dc