Frequency Synthesizers: Concept to ProductAnswers to RF Cafe Quiz #35

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. Bonne chance, Viel Glück, がんばろう, buena suerte, удачи, in bocca al lupo, 행운을 빕니다, ádh mór, בהצלחה, lykke til, 祝你好運. Well, you know what I mean: Good luck!

Note: Some material based on books have quoted passages.

This quiz is based on the information presented in Frequency Synthesizers: From Concept to Product, by Alexander Chenakin (Phase Matrix). Artech House is the publisher. Note: Some of these books are available as prizes in the monthly RF Cafe Giveaway.

1.  What would the spectrum analyzer display for an ideal sinewave output look like?

d)  A single vertical line at the center frequency that has no width.

See page 3.

2.  For a fundamental frequency of 100 MHz, what is the first harmonic frequency?

b)  100 MHz

By definition, the first harmonic is the fundamental (1f), the second harmonic is 2f, the third

is 3f, etc. The first overtone frequency is the same as the second harmonic (2f). See page 5.

3.  Increasing the oscillator signal-to-thermal noise ratio has what effect on the noise curve?

b)  The entire curve shifts down.

Maintaining a higher power level in front of the resonator and/or reducing the active device

noise factor while setting the active device gain to its optimum value will increase the oscillator

signal-to-thermal noise ratio. See page 44.

4.  When is harmonic mixing typically used?

a)  When the LO signal needs to be multiplied.

A lower fundamental frequency oscillator can be used for cost and simplicity reasons, and then

a harmonic of the fundamental is used for mixing. For example the 3LO x 1 RF product might be

the preferred mixer output rather than the more common 1LO x 1RF product. See page 71.

5.  What distinguishes a lumped element from a distributed element?

b)  The physical length of the component compared to signal wavelength.

A loose rule-of-thumb is that if an element is greater in length than 1/10 of a wavelength of

the highest frequency, analyze it as a distributed element. See page 96.

6.  Which circuit is most likely to be found in a

feed-through EMI filter?

d)

An EMI filter is designed to block high frequency signals will passing DC bias and relatively low

frequency control signals. That dictates a lowpass filter topology. The series inductor chokes off

high frequencies while the parallel capacitor shunts high frequencies to ground. See page 106.

7.  Why might resistors be added in series to

control ports on a synthesizer's control port?

a)  Current limiting to protect the input circuitry

against a short or overvoltage.

See Figure 4.13, on page 121.

8.  What PCB layout strategy is required for synthesizer design?

c)  Each component must be located and oriented based on how other components are placed.

Many components are sensitive to cross-coupling with other components and much be situated

to minimize the interference. For example, inductors should not me placed parallel to each other

when in close proximity in order to avoid effectively creating a transformer. See page 133.

9.  What is the phase noise degradation of a 1 GHz synthesizer that uses a 10 MHz reference

oscillator?

c)  40 dB

Phase noise is degraded according to the equation 20 * log (fc/fRef).

So 20 * log (1000/10) = 40 dB. See page 149.

10.  What advantage does a Fractional-N synthesizer have over Integer-N?

d)  Fractional-N permits output frequency steps that are small that the input reference source.

See page 151.