Electronics Demystified, 2nd edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 10

1. The circuit shown in Fig. 10-10 employs a P-channel junction field-effect transistor (JFET), which requires a source of DC power in order to do anything. However, there's no evidence of any power supply here at all! We can provide the JFET with a suitable source of power by connecting the non-JFET end of the drain resistor to a source of negative voltage, say -12 V DC, instead of to ground. The correct choice is C.

2. The circuit of Fig. 10-10 will behave as a weak-signal amplifier for radio frequencies (RF), provided that we choose suitable values for the inductor, capacitors, and resistors. The best choice is B. Although you might think that choice A has some validity here, the existence of the tuned input circuit strongly suggests that the system is intended for RF use, not for audio-frequency (AF) use.

3. We can design a voice communications receiver to "ignore" all incoming signals except those modulated by a subaudible tone at a specific frequency, or an initial sequence of audible tones having specific durations and frequencies. Engineers call this scheme selective squelching. The correct choice is B.

4. A direct-conversion receiver can demodulate a single-sideband (SSB) signal if we tune the local oscillator (LO) so that its frequency coincides with the suppressed-carrier frequency of the incoming SSB signal. In that case, the sidebands "beat" or "mix" with the unmodulated LO signal, resulting in an exact reproduction of the original modulating audio at the transmitter. The LO, in effect, "puts the carrier back in," giving us an AM signal with only one sideband at the receiver's mixer output. The correct choice is B.

5. Figure 10-11 is a schematic diagram of a product detector. This type of circuit can demodulate SSB signals, on/off-keyed signals, or frequency-shift-keyed (FSK) signals. The correct choice is D, "All of the above."

6. In order to provide the necessary mixing effect, the active device in the circuit of Fig. 10-11 should exhibit nonlinearity. The correct choice is D. We don't need much signal output power, so choice A isn't right. The shape factor and bandwidth have nothing to do with the performance of this system, so choices B and C lack relevance here.

7. Digital signal processing (DSP) can improve the performance of a communications receiver when the overall reception conditions are marginal or poor. The correct choice is A. Although spread-spectrum technology can reduce the likelihood of catastrophic interference from a single unwanted signal, that technology offers no particular advantage when the desired signal is weak, or when the background noise level is high; therefore, choice B isn't nearly as good as A. Selective squelching and passive mixers offer no advantage in weak-signal or high-noise situations, so choices C and D are inappropriate.

8. As incoming signals come into a superheterodyne receiver (or, for that matter, practically any wireless receiver), the front end is the first electronic circuit that the signals encounter. The front end comprises a preamplifier and/or a tuned circuit, providing some measure of sensitivity and/or selectivity. The correct choice is D. Although we could, in theory, apply the RF signal from an antenna straight to a detector or first mixer (choices A or C), we wouldn't do either of those things in a well-designed superhet, and the question says that the system must have good design! We'd never let the antenna's signal serve as the first intermediate frequency (IF), so choice B is completely wrong.

9. We can employ either of two technologies to combine multiple signals on a single carrier. We can continuously modulate a single main carrier with multiple signals, each with its own carrier (called a subcarrier). That scheme is called frequency-division multiplexing. Alternatively, we can break the signal up into segments by time, and modulate the main carrier with those segments in a rotating sequence. That scheme is called time-division multiplexing. The question describes time-division multiplexing, so C is the right answer here. The terms intermodulation (choice A), mixing (choice B), and product detection (choice D) all sound a little bit tempting, but none of these technologies are used to accomplish the task described in the question.

10. In the RF stages of a receiver, a preselector helps to reject incoming signals whose frequencies lie far above or below the desired signal's frequency. The circuit provides selectivity in the initial RF amplification stages, often in the front end. The correct answer is A. Even the best preselector circuit can't do anything about birdies (which arise from oscillator signals originating entirely within the receiver), so choice B is wrong. The preselector has nothing to do with the IF stages or the detector, so choices C and D are irrelevant.