Modern Communications Receiver Design and Technology
Answers to RF Cafe Quiz #32
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Cafe Quiz #32
quiz is based on the information presented in Modern Communications Receiver Design and Technology, by Cornell
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?"
b) Radio receiver/detector
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)
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
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?
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
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)