Frequency Synthesizers: Concept to ProductAnswers to RF Cafe Quiz #35
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Note: Many answers contain passages quoted in whole or in part from the text.
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?
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
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.
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