

General RF/Microwave Electronics Answers to RF Cafe Quiz #3  Return to RF Cafe Quiz #3
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RF Cafe Quizzes.
Note: Some material based on books have quoted passages.
1. What is a primary advantage to using 90 ° (quadrature) hybrid couplers in amplifier designs?
c) Input/output impedance not dependent on devices as long as device impedances are equal.
Due to the physical construction of the quadrature coupler, as long as the two devices between the couplers exhibit identical impedances the input and output impedances will exhibit the intrinsic coupler impedance. For example, if matched transistors with input impedances of 12  j5 Ω are connected between to quadrature couplers that have an intrinsic impedance of 50 + j0 Ω, then a 50 + j0 Ω impedance would be exhibited at the circuit input (similar for the output).
Why not always use quadrature couplers? The answer is that insertion loss, physical size and/or cost are often intolerable.
2. Why is there a frequency term in the equation for freespace path loss?
c) Antenna geometry requires it.
Antennas are an indispensable part of all wireless systems. There is no frequency dependency in the freespace power density equation as emitted from an isotropic radiator. Freespace power flux density decreases with distance due to energy being spread over the surface of a sphere, hence:
P[density] = P[transmitter] / (4 π * d^{2}) [W / m^{2}], where d is the distance in meters from the origin.
However, the gain of the receiving antenna, including its effective area (A_{e}) is:
G = G_{[receiver]} * l^{2} / (4 π)
Total path loss = 20 * log (4 π * d / l) [dB].
3. If an amplifier has a noise temperature of 60K, what is its noise figure for an ambient temperature of 290K?
c) 0.82 dB.
Conversion from noise temperature to noise figure is a straightforward process.
NF = 10 * log [(NT / T_{a}) + 1] dB, where T_{a} is the ambient temperature.
4. What is a primary advantage of offsetquadraturephaseshiftkeying (OQPSK) over standard QPSK?
c) More constant envelope power.
OQPSK shifts the inphase (I) and quadrature (Q) components of the digital data by half a bit so that the I and Q data never change at the same moment in time. This maintains a more constant output power.
5. A mixer has the following input frequencies: RF = 800 MHz, LO = 870 MHz. The desired output frequency is 70 MHz. What is the image frequency?
a) 940 MHz.
By definition, the image frequency for any combination of input and LO frequencies is:
f_{image} = 2 * f_{LO}  f_{input}.
For any mixer, there are two input frequencies that, when mixed with the LO frequency, will generate the desired output frequency. In this example, the 70 MHz output can be generated either by taking 870 MHz  800 MHz (desired), or by taking 940 MHz  870 MHz (undesired).
6. What is the spuriousfree dynamic range of a system with IP3 = +30 dBm and a minimum discernible signal (MDS) level of 90 dBm?
a) 80 dB.
Spuriousfree dynamic range (SFDR) is the maximum signal power above the minimum discernible signal (MDS) power level where two tones generate 2ndorder intermodulation products equal in power to the MDS. Input signals above that level will generate 2ndorder products that are greater in power than the MDS power level. MDS is generally defined as the noise power plus the minimum signaltonoise ratio (SNR)
One form of the equation is: SFDR = 2 / 3 * (IP3  MDS) dB.
7. A spectrum analyzer displays a component at 10 MHz @ 0 dBm, 30 MHz @ 10 dBm, 50 MHz @ 14 dBm, 70 MHz @ 17 dBm, and all of the other odd harmonics until they disappear into the noise. What was the most likely input signal that caused the spectrum?
a) A 10 MHz square wave (0 Vdc bias).
The Fourier series for a square wave with a 0 Vdc bias is the fundamental frequency and all of its odd harmonics. Amplitudes are scaled as the reciprocal of the harmonic number; in terms of power, the amplitudes are scaled according to 20 * log (1 / N) dB. A 10 MHz triangle wave also contains the odd harmonics, but amplitudes fall off according to the reciprocal of the square of the harmonic number, 40 *log (1 / N) dB.
8. On which side of a rectangular waveguide is an Ebend made?
b) The short dimension.
In a rectangular waveguide, the Eplane is in the direction of the short dimension while the Hplane is in the direction of the long dimension. The type of bend is determined by which side is curved for the bend. A useful mnemonic is the short dimension is the [E]asy side to bend, while the long dimension is the [H]ard side to bend.
9. During a network analyzer calibration, why are both a short circuit and an open circuit used?
b) To determine the characteristic impedance of the measurement system.
In order for the network analyzer (N/A) to make an accurate measurement, it must know what the impedance of the measurement system is. Characteristic impedance is mathematically the square root of the product of the short circuit impedance and the open circuit impedance. The S/A exploits this relationship.
10. What is the first harmonic of 1 GHz?
a) 1 GHz.
Harmonic number is often mistaken for overtone number. The second harmonic of 1 GHz is 2 GHz, while the first overtone frequency of 1 GHz is 2 GHz. In other words, N_{harmonic} = N_{overtone} + 1. The first harmonic of any frequency is its fundamental frequency.  






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