Spectrum Analyzer Circuits

182 Pages·2010·15.55 MB·English

by  Morris Engelson

Circuit
Concepts
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SPECTRUM
ANALYZER
C I R CU ITS
BY
MORRIS ENGELSON
Significant Contributions
by
GORDON LONG
WILL MARSH
CIRCUIT CONCEPTS
FIRST EDITION, SECOND PRINTING, AUGUST 1969
062-1055-00
@ TEKTRONIX, INC.; 1969
BEAVERTON, OREGON 97005
ALL RIGHTS RESERVED
CONTENTS
1 BACKGROUND MATERIAL 1
2 COMPONENTS AND SUBASSEMBLIES 23
3 FILTERS 55
4 AMPLIFI’ERS 83
5 MIXERS 99
6 OSCILLATORS 121
7 RF ATTENNATORS 257
BIBLIOGRAPHY 171
INDEX 175

1
a
BACKGROUND MATERIAL
Many spectrum analyzer circuits are based on
principles with which most engineers may not be
familiar. It is the purpose of this chapter t o review
some of t h i s specialized background material. This
review is not intended to be either complete or
rigorous. Those desiring a more complete discussion
are referred to the basic references and the
bibliography.
TRANSMISSION LINES*
A t low frequencies the basic circuit elements are
lumped. A t higher frequencies, however, where the
size of circuit elements is comparable to a
wavelength, lumped elements can not be used easily, i f
a t all. This accounts for the extensive use of
distributed circuits at higher frequencies.
A distributed circuit is a signal transmission path i n
which the basic circuit elements (resistance,
inductance, capacitance) are distributed over the
entire length of the transmission path. A transmission
line constitutes a distributed circuit.
*This section pertains to uniform transmission lines;
tapered transmission lines w i l l not be considered.
2
Transmission lines can be constructed in many
different ways. These will have different physical
characteristics and certain relatively minor
electrical differences. For example, the unshielded
two-wire construction is susceptible to pickup and
e I ectr i ca 1 radiation, whereas the shielded coaxial structure is
character- not. All transmission lines, however, have certain
istics major electrical characteristics as follows: The
transmission line is considered as consisting of an
infinite number of elements composed of infinitesimal
shunt capacitance (CAR) and conductance (GAR), and
series resistance (RAR) and inductance (LAR). For
most applications one can make the simplifying
assumption that the transmission line is lossless by
neglecting the effects of shunt conductance and series
resistance.
1
LOAD
I+----&-
Fig. 1-1. Transmission Line.
R
SECT IONS
AR
Fig. 1-2. Transmission-line equivalent
circuit.
3
QUANT I TY S Y“ BO L GENERAL EXPRESSION IDEAL LOSSLESS LINE
fi
CYARACTERISTIC IVPEDANCE
J
OC
PROPAGATION COUSTANT y= a t J3 jufi
-~
PHASE CONSTANT a (IMAGINARY PART OF y)
ATTENUATING CONSTANT a (REAL PART OF y) 0
Z + Z tan4 YL Z + jZ, tan
INPUT IMPEDANCE zi Z”. 5r t ZGr tanh YE. Zo Zr, f jZv tan En
I ‘IPEDANCE :
LINE SHORTED Zo tanh y: jZ, tan Be
LINE OPEN Z coth yP -jZ, cot 39.
0
LINE = ODD NUMBER Zo Z r t ZI; coth crL
OLlARTER WAVES Zo + Zr cotn a9-
LIYE = EVEU NUMeER ZD Z r + Z, tanh ai
QUARTER lnrAVES Zo t Zp tanh a?.
zr - z
VOLTAGE REFLECTION r
COEFFICIENT ’
‘r 0‘
VOLTAGE STANDING
VSWR
WAVE RATIO ~
R,L,G,C = DISTRIBJTED SESISTANCE, INDUCTANCE, CONDUCTANCE, CAPACITANCE PER UYIT LENGTH.
L = DISTANCE ALOhG LINE, VEASURED FROV LOAD END.
Zr = LCAd AT QECEIVING END OF LINE, Zr SHORTED LINE = 3, Zr OPEN LlYE = m.
I r I = MAGNITUDE OF VOLTAGE REFLECT I OY COEFF I c I ENT, r .
Fig. 1-3. Basic transmission-line
relationships.
When a transmission l i n e is terminated i n its
characteristic impedance (Z, = Zo), the combination
behaves as an infinitely long line. The energy
delivered by the generator propagates down the
transmission l i n e and is absorbed by the load without
reflections. When the load is other than the
characteristic impedance, Zo, then some of the
incident energy is reflected back down the l i n e
toward the generator, This gives rise t o peaks and
nulls of voltage (and current) along the l i n e known
as standing waves.
4
T
REACTANCE
xL
+
R = 0 AT POINT OF
SHORT C I RCU IT
xc
Fig. 1-4. Impedance of short-circuited
lossless transmission line.
t
REACTANCE
xL
.f- R = 0 AT POINT OF
OPEN CIRCUIT
xc
Fig. 1-5. Impedance of open-circuited
lossless transmission line.
The phenomenon of standing waves is easily
understood in connection with the impedance of the
stand i ng transmission line. Figs. 1-4 and 1-5 show the
waves variation of impedance as a function of line length
for shorted and open-circuited lines respectively.
It is obvious that the voltage has to go to zero
at a zero-impedance point and toward infinity at a
point of infinite impedance. In practice real
transmission lines are not entirely lossless, so
that the voltage along the line does not actually
go to zero or become infinite. Note that the line
impedance and, therefore, the standing-wave pattern
x
repeats at half-wavelength (-) intervals. This
2
characteristic is independent of the impedance of
the load.