# Modern Communications Receiver Design and TechnologyAnswers to RF Cafe Quiz #32

All RF Cafe quizzes make great fodder for employment interviews for technicians or engineers - particularly those who are fresh out of school or are relatively new to the work world. Come to think of it, they would make equally excellent study material for the same persons who are going to be interviewed for a job.

Note: Some material based on books have quoted passages.

This quiz is based on the information presented in Modern Communications Receiver Design and Technology, by Cornell Drentea. Published by Artech House.
Note: Some of these books are available as prizes in the monthly RF Cafe Giveaway.

1.  What is the modern version of a "coherer?"

Edouard Branly, of France, discovered that a glass tube with two silver electrodes, filled with loose iron particles, will conduct DC electricity better in the presence of so-called Hertzian waves generated with conventional spark generators. (see page 5)

2.  What is the difference between a heterodyne (1) and a superheterodyne (2) receiver?

b)  (1) is direct conversion to 0 Hz while (2) uses an IF frequency prior to conversion to 0 Hz

The superheterodyne method converts the incoming radio frequency (RF) to an intermediate frequency (IF) prior to conversion to 0 Hz, to allow use of lower cost and complexity components prior to converting to baseband (BB). It also can provide superior mixer intermodulation spur mitigation. Superheterodyne can be single conversion, but the signal is processed at the new frequency - as with an envelope detector. (see page 13)

3.  What is an image frequency?

d)  The signal on the other side of the LO that will also convert to the same IF as the intended signal

Along with intermodulation products, a mixer generates fundamental output frequencies equal to LO - RF and RF - LO. So, two signals equal distances above and below the LO frequency will result in the same IF. Without adequate filtering, the two separate signals (or signal plus noise) will combine at the mixer output. Some mixers use phasing techniques to help cancel the image without a filter. (see page 20)

4.  What is a High Probability of Intercept receiver?

a)  One that surveys broad frequency ranges in narrow bandwidth samples at high sensitivity

These receivers are like high sensitivity, fast-scanning spectrum analyzers designed to reliably detect every relevant signal in the band. (see page 105)

5.  Why would a double conversion be used instead of a single conversion?

c)  To avoid high level intermodulation spurs from the mixer

Using an intermediate frequency (IF) prior to converting to baseband (BB) usually results in being able to avoid having the higher power intermodulation (IM) products falling inband at the output. Once those products are generated, there is no way to filter them out. The same applies to an upconversion. (see page 77)

6.  What is the 1 dB compression point of an electronic device?

a)  The power level at which the device gain is reduced by 1 dB from the linear value

No device can output an infinite amount of power. The power is limited ultimately either by thermal failure (mainly passive devices) or by the limit imposed by the properties of a semiconductor device. As such, the gain of an amplifier or mixer is linear only within a specific dynamic range. Beyond that range, the input-output gain is reduced until the output power reaches a saturated level. The point at which the input-output gain is reduced by 1 dB from the linear value is often used as the point at which an intolerable amount of distortion will be introduced into the signal. (see page 185)

7.  What is one of the main reasons for using a fractional-N phase-locked loop (PLL) synthesizer?

b)  Small frequency steps can be obtained without using multiple PLL loops
Dividing down the reference frequency achieves small tuning increments without suffering the spurious product impairments generated by multiple mixer steps for getting small frequency steps. (see page 230)

8.  What type of diode usually makes the best RF signal switch?

d)  PIN

A PIN diode, because of its construction, exhibits a variable RF resistance within it region of operation without operating in a highly nonlinear mode. This prevents the generation of high level harmonics and/or intermodulation products. (see page 304)

9.  Why would you use a logarithmic amplifier (log amp)

d)  If you must accommodate a large dynamic range at the input while producing a smaller dynamic range at the output

Because the input-out power gain follows a logarithmic relationship, a log amp allows a very large range of input powers to be translated into a smaller range of output signals. Using a log amp can potential avoid the use of selectable gain schemes where stages of gain are switched in or out of the path. However, there are situations where switching cannot be used, such as when continuous monitoring for the presence of signals is required for threat assessment. (see page 331)

10.  What is(are) the main difference(s) between an infinite impulse response (IIR) filter and a finite response response (FIR ) filter?

d)  All the above

An IIR filter uses feedback to retain part of the response "in the loop" indefinitely whereas an FIR filter has no feedback; i.e., poles and zeroes in the transfer function vs. only zeroes, respectively. (see page 392)