# Microwave Mixer Technology and ApplicationsAnswers to RF Cafe Quiz #54

All RF Cafe quizzes would make perfect fodder in 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 Microwave Mixer Technology and Applications, by Bert Henderson and Edmar Camargo.

Note: Some of these books are available as prizes in the monthly RF Cafe Giveaway.

1.  What is the name given to a receiver circuit whose oscillator circuit is biased just shy of the oscillation point in order to increase signal sensitivity?

d)  Regenerative

Positive feedback in the oscillator circuit is adjusted to be just below the point of excitation so that a very small received signal sends it into oscillation for use in the downconversion and detection circuitry.  (see page 9)

2.  What is the name given to a mixer configuration that cancels out received signal frequencies that are equidistant from the oscillator frequency as the intended signal, but on the opposite side of the oscillator; i.e., on the opposite equivalent sideband?

a)  Image reject

If the intended received frequency is f1 and the oscillator frequency if fLO, then the multiplication (mixing) process (not including harmonics) produces, in the example of a high-side LO, fLO + f1 and fLO - f1 . If f2 = fLO + (fLO + f1) = 2fLO + f1, then it is the same distance above fLO as f1 is below fLO. Now, substituting f2 into the f1 mixing products yields fLO + f2 and fLO - f2 = fLO + (2fLO + f1) and fLO - (2fLO + f1) = 3fLO - f1 and fLO + f1. Note that the fLO + f1 term is that exact same as the original f1 products. Hence, it produces the same output frequency. That is why it is called the image frequency.  (see page 37)

3.  What is Error Vector Magnitude (EVM)?

c)  The difference between the expected vector magnitude and the measured vector magnitude.

As the drawing to the right shows, the Error Vector is the difference between the ideal vector end point (line V) and the measured vector end point (line Vm).  (see page 67)

4.  What is a "cold FET?"

d)  A FET circuit without DC bias

An example of a cold FET is one used in a frequency mixer circuit where all ports are AC coupled.  (see page 140)

5.  Why might a Lange coupler be preferred over a 3 dB hybrid coupler?

c)  Wider bandwidth for impedance matching

The physical construction and phasing of its rectangular construction limits the wavelength over which a 3 dB hybrid coupler provides a good impedance match.  (see page 222)

6.  What are mixer spurious products comprised of?

c)  Large signal LO and small signal RF inputs

The LO (local oscillator) signal is at high enough power to quickly drive the mixer diodes into saturation during the "on" portion of the cycle while the RF input signal are kept within the more linear conduction region of the diodes.  (see page 299)

7.  Why does the mechanism of noise generation in a mixer differ than that of an amplifier?

a)  Because the device parameters vary in time in response to the LO signal

The large signal LO drive causes the mixer to constantly move through the full operating range of the semiconductor junctions.  (see page 450)

8.  Why does a cascode amplifier provide higher gain than a single stage?

c)  Because of higher output impedance and higher reverse isolation

The cascode topology also offers a higher frequency of operation due to a reduction in the equivalent input capacitance.  (see page 457)

9.  What is the equivalent of a triply-balanced mixer?

b)  Combination of two doubly balanced mixers

A triply-balanced mixer is comprised of two doubly-balanced mixer in parallel, connected between baluns at each port, as shown in drawing to the right.  (see page 322)

10.  What is a major advantage of the Gilbert cell topology?

d)  It multiplies the input and output currents to avoid improve linearity

d)  It multiplies the input and output currents to avoid improve linearity

The Gilbert cell uses transistors to provide current gain for avoiding voltage limitations and thereby improving linearity.  (see page 480)

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