By Wes Hayward, W7ZOI, and Terry White, K7TAU

A Spectrum Analyzer for

the Radio Amateur
Good tools are priceless when you
need them. Here’s a piece of test
equipment you’ve always wanted
for your workbench. Now you can
have it—without spending a fortune.

mong the many measurement ments in the 50 kHz to 70 MHz region. The We often read and hear about “simple

A tools sought by the amateur

experimenter, the most de-
sired—but generally considered
the least accessible—is the radio-frequency
design can be extended easily into the VHF
and UHF region with methods outlined
later. The instrument is configured to be
self-calibrating, or capable of calibration
designs.” Simplicity implies that some-
thing is eliminated to make the equipment
easier to build, use or afford. Unlike de-
signs that sacrifice performance for cost
spectrum analyzer or SA. This need not be. with simple home-built test gear.1 and simplicity, this one sacrifices only con-
Simple and easily duplicated, this home- venience, while retaining the capabilities
built analyzer is capable of useful measure- 1
Notes appear on page 43. needed for accurate measurements.

Figure 1—Block diagram of the spectrum analyzer. The circuit is a double-conversion superheterodyne design with intermediate
frequencies of 110 and 10 MHz.
August 1998 35
Figure 2—Time base for the spectrum analyzer. Refer to the text for a discussion of the various circuit functions. Front-panel controls
include SWEEP RATE, SPAN and TUNE. Unless otherwise specified, resistors are 1 / 4 W, 5% tolerance carbon-composition or film units.
Equivalent parts can be substituted.
U401, U402, U403—LM358 op amp R420, R423—PC-mount trim pots, 5 kΩ or control, 5 kΩ or 10 kΩ suitable. If a 10
D403, D406—6.2 V Zener diodes, 1 W 10 kΩ suitable turn pot is used for R3, R4 is not
C401—Metal film or Mylar, 1.0 µF R3, R4, R5, R6—Panel-mounted linear needed.
capacitor

Modern technology eases the construc- function. Hybrid and monolithic IC build- does not own an oscilloscope. With good
tion of this spectrum analyzer. The loga- ing blocks are employed extensively. These basic ’scopes available for about the price
rithmic amplifier uses an IF amplifier IC include mixers, amplifiers and VCOs—all of a hand-held FM transceiver, every ex-
found in cellular telephones and includes a vital elements in an analyzer. Finally, it is perimenter should have one. Our spectrum
received signal strength indicator (RSSI) a rare devoted experimenter today who analyzer uses a ’scope as the display. There
36 August 1998
are no special requirements for ’scope per- dow, while preserving system dynamic circuit commonly found in function gen-
formance other than an X-Y mode with dc range. The proper distribution of gain, se- erators. U401A operates as an integrator;
coupling in the X and Y axes. lectivity and signal-handling capability (in- current is pulled from the inverting input
tercepts) of the amplifiers and mixers is through a 56-kΩ resistor connected to the
Some Spectrum-Analysis Basics vital to achieving good performance in a SWEEP RATE control. This current must
The RF spectrum analyzer is essentially spectrum analyzer, and indeed, any re- flow through the capacitor (C401), creat-
a swept receiver with a visual display. The ceiver. A proper design will have the same ing a linearly changing op-amp output volt-
display shows the strength of all signals number of stages as a poor one, but will age. This ramp is applied to U401B, a re-
within a user-defined frequency span. Each probably use different components and generative comparator, which provides a
signal is represented by a line or blip that consume more current. reset signal to the integrator. The sawtooth
rises out of a background noise, much like The analyzer uses a ±15 V power supply. waveform (pin 1 of U401A) is asymmetri-
the action of an S meter. Commercial ana- The positive supply delivers about 0.5 A. cal: The positive-going ramp grows with a
lyzers are calibrated for signal power, with The negative supply current drain is under slope determined by the front-panel-
all signals referred to a reference level 50 mA. mounted SWEEP RATE pot, while the nega-
at the top of the screen. Our analyzer is Following sections present the circuit tive-going, faster reset ramp is determined
designed for a basic reference level of blocks in greater detail, in the order that by fixed-value components.
–30 dBm, a common value in commercial they should be built. The partial but grow- The U401 ramp is used twice. U402A and
analyzers. 2 ing system can then be used to test the other B process the ramp to generate a signal that
Signal levels are read from the display sections as they are built, turned on and drives the ’scope’s X axis. The signal has a
by noting that power drops by 10 dB for integrated. We strongly discourage build- 0 V-centered range with just over a 10 V
each major division on the ’scope. You can ing the entire analyzer before testing spe- total swing. Some of the “square wave” from
change the reference level. Adding gain to cific sections. Such an approach may work the basic time base (U401B, pin 7) is added
the analyzer moves the reference to lower for casual kits, but is not suitable when to the input of U402B to cause the sweep to
levels; introducing attenuation ahead careful control of signal levels is required. reset quickly, even though the return sweep
of the instrument moves the reference to That approach also robs you (the builder) for the VCO occurs in a more stable, smooth
higher power levels. of the excitement of the process: the learn- way. A slight overscan is generated for the X
ing that comes from detailed examination. axis, serving to hide an aberration occurring
Circuit Overview Before jumping into the circuit details, near the sweep beginning.
Figure 1 is a block diagram of our spec- we reemphasize that this analyzer—al- The sweep also generates the signal
trum analyzer. A double-conversion super- though simple—is intended for serious that controls the VCO. The sweep signal
heterodyne, it begins with a step attenua- measurements. This means that a normal (U401A pin 1) is applied to a SPAN control.
tor, followed by a low-pass filter and the maximum span display contains no spuri- When the analyzer is set for maximum
first mixer, where incoming signals are ous signals. When clean (well-filtered, har- span, the VCO voltage (about 2 to 10 V)
upconverted to a 110 MHz first IF. After monic-free) signals are applied to the ana- generates a sweep from 110 to 180 MHz.
some gain and band-pass filtering, a sec- lyzer, there should be no extra products as The VCO uses only positive sweep volt-
ond conversion moves the signals to a long as the signal level is kept on screen. ages, so the output of U403B is diode-
10 MHz IF. The resolution bandwidths This performance goal applies for a single clamped to prevent negative output. The
available are 30 kHz and 300 kHz. A video tone, or for two equal signals at the top of center frequency TUNE, FINE TUNE and a
filter smooths or averages noise. The avail- the screen. MAX SPAN calibration pot set up the proper
able frequency spans range from a per- sweep for maximum span. As the span is
division maximum of 7 MHz to about Time Base reduced with the SPAN control, the sweep
50 kHz. The center frequency can be ad- Figure 2 shows the analyzer time base, expands on (or zeroes in on) whatever ap-
justed over the entire 70 MHz range. An designed for basic functionality without pears at the center of the screen, determined
uncalibrated SPAN control allows expan- frills; the result is a circuit using only a by the tuning. The center frequency must
sion of the display about the screen center. handful of op amps.4 U401A and U401B be set for 35 MHz at maximum span, which
An uncalibrated SWEEP RATE control form a free-running sawtooth generator, a coincides with having the zero signal, or
allows the sweep to be controlled and
matched to a given span while avoiding
excessively fast scans that could introduce
errors.
Ideally, a receiver’s first IF should be
greater than twice the highest input fre-
quency, a design rule that we bend in this
application. The input tuning range in-
cludes all HF amateur bands and 6 meters.
(We’ll discuss higher tuning ranges later.)
We picked the 10 MHz second IF because
surplus-crystal filters and LC filters for this
frequency are easily built. You can easily
adapt the design’s IF to 10.7 MHz, or other
close, convenient values.
The swept LO tunes from 110 to
180 MHz with a commercial VCO module.
The VCO output is amplified to drive a
high-level-input mixer. The commercial
VCO is a recent modification to a design
that started with a homebrew oscillator. 3
Amplifiers are included at the 10 and Figure 3—An experimental logarithmic amplifier breadboarded to evaluate performance
110 MHz IFs. These establish signal levels prior to analyzer construction. You may want to duplicate this circuit and analyze its
that properly match the log-amplifier win- performance if you decide to use other log-amp ICs.

August 1998 37
 transforming networks. The bandwidth of
the AD8307 is about 500 MHz, so care is
required in its use.
Our analyzer uses the inexpensive and
readily available MC3356 log amp shown
in Figure 5. 8 An op amp, U303, used to in-
crease the signal output to 0.5 V per divi-
sion, follows the log chip, U301. The 0 V
level corresponds to the bottom of the
screen; a signal of 4 V brings the response
to the top of the screen. The op-amp output
is slightly higher than this, but is then at-
tenuated with a LOG AMP CAL control, R2.
This pot should be accessible from the out-
side of the instrument.
The log amp is preceded by an IF ampli-
fier, Q301 through Q303. These stages are
biased for relatively high-current operation
to preserve linearity. Gain is controlled
through variable emitter degeneration in
the form of a PIN diode, D301. Most com-
mon 1N4000-series power rectifiers work
well for gain control. The IF GAIN ADJ con-
Figure 4—Transfer characteristics for three different logarithmic amplifier ICs. Although trol (R1) should be available from the exte-
the MC3356 is used in our analyzers, use of the AD8307, shown in the lower curve, is rior of the RF-tight amplifier box. We have
recommended. Some curves have been linearly scaled to ease comparison. placed it on the front panel of our analyzers.
Calibration of the IF and log amplifier is
straightforward. First, set the ’scope’s Y
“zero spur” at the left edge of the screen. rithmic processing is used. axis to 0.5 V/division and short it. Set the
Setting up the time-base function is gen- The circuit element we use for this pro- now-working time base to drive the X axis
erally straightforward. The ’scope can be cessing is the log amplifier. 7 The term is a and adjust the ’scope’s vertical position
used to debug, check and study the circuits. misnomer, for the usual log-amp IC is both control to place the horizontal line at the
The X-axis signal is a ramp ranging from a logarithmic processor (amplifier) and a bottom of the screen. Inject a −10 dBm sig-
–6 to +6 V with a reset to –15 V during the detector. The chips provide a dc output nal from a signal generator into the log
retrace. A similar ramp appears (without a voltage that increases in proportion to the amplifier input, remove the short circuit
reset pulse) at the VCO output, but with an logarithm of the input amplitude. The cen- and adjust R2, LOG AMP CAL , for a full-
amplitude dependent on the SPAN control tral sensitivity specification for a log amp screen (reference level) response. The in-
setting. is a voltage slope that is equal to the volt- put level is next reduced in 10 dB steps.
Although the op amps are carefully by- age change (per decade or per decibel) of The horizontal sweep line should drop
passed, and the signal that tunes the VCO is input-voltage-amplitude change. down one major division for each 10 dB
shielded, most circuits are noncritical. An experimental log amp is shown in reduction over a 60 dB range. If this does
Normal op-amp circuit precautions are Figure 3. We breadboarded and tested this not happen, repeat the procedure with a
taken with resistors injecting signals into circuit to evaluate the log IC. To produce slightly different drive level. In our analyz-
inverting inputs positioned close to the the MC3356 curve shown in Figure 4, the ers, a typical drive level of –13 dBm pro-
op-amps.5 10 MHz output of an HP-8654 signal gen- duced good accuracy.
A 10-turn front-panel-mounted pot is erator was applied through HP-355 step Now, attach the IF amplifier to the log
used for the TUNE control (any value from attenuators. Exact dc output levels are in- amplifier and drive them with an input
5 kΩ to 50 kΩ is suitable). A single-turn significant, for they can be adjusted with level of –23 dBm. Peak the IF output filter
pot can be substituted if a 10-turn pot is not dc voltage gain in a following amplifier. for maximum response and set R1, IF GAIN
available. A fine-tuning function is in- The salient detail that we observe is the dy- ADJ , for a full-screen response. A true fil-
cluded in this design, but may be omitted if namic input window. The MC3356, with a ter peak can be confirmed by varying the
a 10-turn pot is used for the main tuning. 50-Ω input termination, produces a nearly generator frequency. There is considerable
straight-line output voltage versus input extra range in the IF GAIN ADJ, providing
Log Amplifier and Detector power for inputs in the –80 to –10 dBm extra flexibility during use.
Central to any spectrum analyzer is a range. Hence, the analyzer log amp should
logarithmic amplifier. The need for loga- operate with an input signal of –10 dBm for Resolution Filters
rithmic processing becomes clear if we signals at the top of the screen. Continuing the backward progression
consider the range of signals we want We evaluated two other ICs. One, the through the system, we encounter the
to measure: At the low end, we may want commonly available NE/SA604, shows resolution-bandwidth-determining filters.
to look at submicrovolt levels: under considerable ripple. The best performance Our analyzer uses bandwidths of 30 and
–107 dBm in a 50-Ω system. At the other offered came from a recently introduced 300 kHz, provided by crystal and LC fil-
extreme, we may want to measure the out- chip from Analog Devices: the AD8307. ters, respectively. The 300 kHz LC filter,
put of small transmitters, perhaps up to a This IC is designed specifically for mea- the crystal filter and the relay circuitry for
power of 1 W, or +30 dBm. The difference surement applications and offers outstand- bandwidth switching are shown in Figure
between the two levels is 137 dB. The hu- ing logarithmic accuracy, a dynamic range 6. Although shown as individual modules,
man ear is capable of handling linear exceeding 90 dB and better temperature they can be incorporated into one. The PC
ranges well over 60 dB. 6 This is a wide stability than found with the usual cellular- board for the filter includes the LC filter
dynamic range world and linear displays, receiver chips. The AD8307 requires a high and switching relays with room for a user-
such as our screen, are inadequate unless drive level, so it must be preceded with selected crystal filter. Builders may want
some form of data compression or loga- higher-power amplifiers or impedance- to implement their own scheme here. We

38 August 1998
Figure 5—The 10 MHz IF amplifier and log amplifier used in the analyzer. Refer to the text for adjustment details. Unless otherwise
specified, resistors are 1/ 4 W, 5% tolerance carbon-composition or film units. Equivalent parts can be substituted.
C309—Plastic dielectric trim cap D301—PIN diode; 1N4007 used R1—Panel-mount, 1 kΩ linear
(Sprague-Goodman GYD65000) L301—1.35 uH, 18 turns #24 enameled R2—Panel-mount, 5 kΩ linear
C307, C308, C310—Silver mica or NP0 wire on T-44-6 core, Q >150 U301—Motorola MC3356
ceramic capacitors, 10% tolerance Q301, Q302, Q303—2N3904 U302—78L05 +5 V regulator
C316—0.22 µF ceramic U303—CA3140 op amp

reasoned that builders would want to imple- front end. Accordingly, the second mixer Temporarily replace the crystal, Y201,
ment their own ideas. Maintain reasonable should have an intercept similar to that of with a 51 Ω resistor. Adjust the tuned cir-
shielding for this part of the system. Addi- the first mixer. This is the usual weak point cuit until oscillation occurs at the desired
tional attenuator pads can be inserted in line in all too many homebrew spectrum analyz- 100 MHz frequency. Then, replace the
with one filter or the other to approximately ers—as well as more than a few receivers! 51 Ω resistor with the 100 MHz crystal; no
equalize filter loss in the two paths. The second mixer, U202, uses a +17 dBm further tuning is required. Measure the
You may want to build crystal filters for level Mini-Circuits TUF-1H. This is not oscillator’s output with a power meter be-
your analyzer. 9 The VCO stability in this the place for a current-starved telephone fore applying it to U202. Adjust the pad
analyzer will support resolution band- component! The second mixer is terminated attenuation (R205, R206, R207) to realize
widths as narrow as 3 to 5 kHz. For a sim- in a high-pass/low-pass diplexer followed the specified LO drive level.
plified beginning, a very practical analyzer by an IF amplifier (Q202) biased at 50 mA. After the second LO is operating, attach
can be built with only one resolution band- This is a critical stage for dynamic range: it to the second mixer and the rest of the
width of 300 kHz. Don’t replace it with a monolithic substi- analyzer. With a second mixer input of
tute of reduced gain or intercept. –35 dBm at 110 MHz, you should obtain a
Second Mixer and Second Local The second LO begins with a 100 MHz, reference-level response.
Oscillator fifth-overtone crystal oscillator (Q201),
Figure 7 shows the second mixer and followed by a pad and a power amplifier. Voltage-Controlled Local Oscillator
related LO. The heart of this module—and The oscillator inductor, L201, in Q201’s and First Mixer
to some extent that of the entire analyzer— collector is made of five turns of #22 wire Figure 8 shows the analyzer’s swept LO.
is U202, a high-level second mixer. This wound on a 6-32 machine screw. (Remove The foundation for this module is a Mini-
mixer is bombarded by large signals that the screw before installing the coil.) Here’s Circuits POS-200 VCO module, U101.
are as strong or stronger than those at the an excellent way to align the oscillator: Similar VCOs are available from many
August 1998 39
Figure 6—Resolution filters: The upper schematic shows the 300 kHz bandwidth 10 MHz LC filter. If desired, that circuit can be
realigned at 10.7 MHz without other design changes. The LC filter is shown as a separate unit connected to the rest of the analyzer
with coaxial cable. However, the filter can be constructed on the board with the crystal filter and relays. Unless otherwise specified,
resistors are 1/ 4 W, 5% tolerance carbon-composition or film units. Equivalent parts can be substituted.
C501, C502, C504, C505, C507, C508, dielectric trim cap (Sprague-Goodman unused); values of associated dropping
C510, C511, C513, C514, C515—Silver GYD65000) resistors may need adjustment.
mica or NP0 ceramic, 5% K501, K502—SPDT relay; Aromat L501-L504—17 turns of #22 enameled
C503, C506, C509, C512—65 pF plastic TF2-12V used here (one contact set wire on a T-50-6 toroid (1.15 µH),
Q > 250

vendors. 10 The VCO output is about normal diode ring: The RF input is now circuits we built (or computer simulated)
+10 dBm, too low a level for the high-level attached to the dc-coupled port. This allows came close, but just didn’t cut it.
mixer. A MAV-11 amplifier, U102, pre- input frequencies as low as 50 kHz to be We described double-tuned circuits in
ceded by a pad to provide level adjustment, converted to the first IF. The low-frequency detail in a 1991 QST tutorial paper. 11 Those
increases the signal level. Confirm the out- end is limited by mixer LO to RF isolation, methods have recently been extended to
put power level before applying it to the which determines the LO energy that three-resonator filters. 12 One of the meth-
mixer, U103. reaches the first IF. The related on-screen ods presented in the later paper is a sequen-
Once the VCO output level is adjusted response is often termed the zero spur, a tial approach that begins with a double-
and confirmed, calibrate its frequency familiar “feature” in most RF spectrum tuned circuit (DTC). First, a DTC is built
against the VCO control voltage. If a VHF analyzers. for the desired 3 dB bandwidth and has its
counter is not available, you can obtain a This module (VCO and first mixer) is performance confirmed with a wideband
few points by tuning the VCO to hit local contained in a shielded enclosure with co- sweep (a vital requirement!) Then, a third
FM broadcast signals of known frequency. axial inputs and outputs, including coaxial resonator is inserted between the original
Calibrating the VCO is useful if the module routing of the VCO control voltage. The two. Coupling elements similar to the one
is used later as a signal source for align- front end is susceptible to any VHF and that produced the required DTC bandwidth
ment of the 110 MHz band-pass filter. UHF signals reaching it, making shielding are repeated in the triple-tuned circuit, but
Figure 8 also shows the input mixer, and decoupling especially important. end-section loading is not changed. The
U103, another Mini-Circuits TUF-1H, ter- center frequency of the three resonators is
minated in a 6 dB pad. Although the pad The 110-MHz IF Band-Pass Filter aligned to complete the filter.
degrades the noise figure, it presents a solid One of the more-critical blocks in the The schematic for the 110 MHz triple-
output termination for the mixer. This analyzer is the filter that establishes the tuned circuit is shown in Figure 9. The in-
termination is reflected, helping to pro- bandwidth of the VHF IF. The bandwidth ductors,100 nH, are made by winding 5
vide a good mixer-input impedance match, must be at least as wide as the widest turns of #18 wire on the shank of a 1 / 4-inch
important in a measurement instrument. 10 MHz filter, but must be narrow enough drill bit. These inductors typically have an
The pad is followed by a Mini-Circuits to reject the 90 MHz second-conversion unloaded Q of just over 200 at 110 MHz.
MAV-11 IF amplifier (U104) that restores images by 80 dB or more. This performance Larger-diameter inductors would have pro-
the gain lost in the mixer and pad. is only available with a three-pole or duced higher unloaded Q with the atten-
The mixer application differs from a higher-order filter. The best double-tuned dant lower insertion loss. However, the
40 August 1998
Figure 7Second mixer and second LO.
L203 in the output of U202 consists of 17
turns of #28 enameled wire on a T-30-6
toroid. The actual value is not critical and
a molded RF choke can be used in place
of the toroid. Unless otherwise specified,
resistors are 1 /4 W, 5% tolerance carbon-
composition or film units. Equivalent parts
can be substituted.
C201, C202—470 pF ceramic
C203, C206—15 pF NP0 ceramic or silver
mica
C204, C207—82 pF NP0 ceramic or silver
mica
C205—65 pF plastic dielectric trim cap
(Sprague-Goodman GYD65000)
C211—120 pF, silver mica or NP0 ceramic
L201—57 nH; 5 turns of #22 wire wound
on a 6-32 machine screw. Remove the
screw before installing the coil.
L202—1 µH molded RFC; any value from
100 nH to 2.7 µH is okay
L203—0.3 µH; 9 turns #24 enameled wire
on a T-30-6 core
Q201—2N5179
Q202—2N5109, 2N3866, 2SC1252, etc
U201—Mini-Circuits MAV-11
U202—Mini-Circuits TUF-1H mixer
T201—10 bifilar turns #28 on FT-37-43
ferrite toroid

stray coupling between coils would have

increased, which would have necessitated
shields between filter sections. The smaller
(10 nH) end-matching inductors are one-
inch lengths of #18 wire. The triple-tuned
filter, and its parent DTC, have bandwidths
of 2 to 3 MHz.
The filter alignment and experimenta-
tion is usually done with a sensitive power
meter, 13 a step attenuator and a signal
source. As mentioned earlier, the VCO can
serve the role of signal generator, if one is
not available.
The second-conversion image rejection
is easily measured with a finished analyzer.
Apply a 40 MHz signal to the analyzer and
adjust it for a reference-level response.
Don’t touch the analyzer tuning, but move
the signal generator to 60 MHz. An image
signal may appear at the same point on
screen as the original 40 MHz signal. The
rejection was only 66 dB with a DTC used
for some experiments. The triple-tuned fil-
ter produced 90-dB rejection. The slight
extra effort of the triple-tuned circuit is
easily justified. No PC board is available
for the IF filter.
Input Low-Pass Filter
A 70 MHz low-pass filter is shown in
Figure 10. This circuit and a step attenuator
are housed in separate shielded enclosures
in one of our analyzers. In the other, the fil-
ter and the attenuator remain outboard ele-
ments. Integral components are more con-
venient for routine analyzer applications,
but incorporation removes them from the
equipment pool available for other experi-
ments. Also, operation without an outboard
low-pass filter allows the instrument to be
August 1998 41
Figure 8—Front-end for the spectrum analyzer. The LO drive level is set to between +16 and +18 dBm by trimming the pad
attenuation. The 1 µH inductors used in the MAV-11 outputs can be molded RF chokes or made of 17 turns of #28 enameled wire
wound on T-30-6 toroids. Unless otherwise specified, resistors are 1 / 4 W, 5% tolerance carbon-composition or film units. Equivalent
parts can be substituted.
L101, L102—1 µH molded RFC; any U101—Mini-Circuits POS-200 VCO U103—Mini-Circuits TUF-1H
value between 100 nH to 2.7 µH suitable U102, U104—Mini-Circuits MAV-11

Figure 9—VHF band-pass filter used in the 110 MHz IF.

42 August 1998
 11
Wes Hayward, W7ZOI, “The Double Tuned
Circuit: An Experimenter’s Tutorial,” QST,
Dec 91, pp 29-34.
12
Wes Hayward, W7ZOI, “Extending the
Double-Tuned Circuit to Three Resonators,”
QEX, March-April, 1998, pp 41-46.
13
Denton Bramwell, K7OWJ, “The Microwatter,”
QST, Jun 1997, pp 33-35. Also, see the
sidebar in the referent of Note 11.

Wes Hayward, W7ZOI, and Terry White,

K7TAU, are not strangers to QST. This is also
Figure 10—Input 70 MHz low-pass filter. The filter started as a ninth-order not the first QST project on which they’ve
Chebyshev design, but was modified through computer manipulation to use equal-
value inductors and standard-value capacitors. The inductors each consist of 8 turns worked together. The first, “The Mountain-
of #22 wire. The wire is wound on the threads a 1/ 4-20 bolt as a form; remove the bolt eer—An Ultraportable CW Station,” described
after winding the inductor. See text. a QRP transceiver and appeared in the August
C701-C705—NP0 ceramic, 5% tolerance. L701-L704—8 turns #22 bare wire wound 1972 issue. At the time this spectrum-analyzer
in 1/4-20 bolt threads; see text. project evolved, the authors worked together in
the Advanced Circuits section of the Receiver
Group at TriQuint Semiconductor in Hillsboro,
Oregon. Over the years, Wes has provided
used well into the low UHF area by operat- 2
The usual unit for power measurement with a
readers of QST, The Handbook and other
ing the mixer with VCO harmonics. (No spectrum analyzer is dBm. This is an abso-
lute, impedance invariant unit, signifying ARRL publications with a wealth of projects
circuit boards are available for this filter.) power compared with 1 mW. Hence, –30 dBm and technological know-how. You can contact
is 30 dB below a milliwatt, or a microwatt. The Wes at 7700 SW Danielle Ave, Beaverton, OR
Construction and Adjustment Hints dBm unit is especially useful in spectrum
97008; e-mail w7zoi@teleport.com. Terry
analysis where two levels are often com-
The spectrum analyzer can be built using pared. The difference between two powers can be reached at 9480 S Gribble Rd, Canby,
a number of RF techniques. Our analyzers in dBm is a power ratio in dB. (See Jay OR 97013; e-mail twhite@tqs.com.
are collections of small boxes using Craswell, WBØVNE, “Converting Between
coaxial-cable interconnections. Power sup- dBm, Milliwatts and Watts,” Technical Corre-
spondence, QST, Jul 1998, p 70.— Ed.)
plies reach the interior of RF modules 3
Excellent homebrew VCOs can be built
through feedthrough capacitors. The only using the work of Allan Victor, WA4MGX,
open boards in the system are the time base “Wideband VCO Design,” Ham Radio Maga-
and (in one instrument) the log amp, both
constrained to low frequencies. The
110 MHz IF filter and all RF circuit boards
zine, Jul, 1984, pp 49-58. See also the recent
work of Colin Horrabin, G3SBI, as reprinted
in “Tech Notes,” Communications Quarterly,
Winter, 1996, pp 94-104.
New Products
4
are designed to fit in Hammond 1590B Many amateurs are uncomfortable with op-
amp circuits, a situation easily remedied by a ATV TRANSMITTER FROM
diecast aluminum alloy boxes. study of the excellent text by Horowitz and PC ELECTRONICS
The sensitivity of such RF measurement Hill, “The Art of Electronics,” 2nd Edition,
◊ The TX70-10 ATV transmitter from PC
equipment justifies the extensive shielding. Cambridge University Press, 1989. See the
golden rules presented on page 177. An op- electronics is aimed at operators who use
Spurious responses are readily seen on the amp integrator is presented on page 222. The cable-ready TVs (tuned to channels 57-60)
display. While a few can be tolerated, they information on sawtooth oscillators beginning
to receive amateur television signals in the
seriously detract from a measurement on page 288 is also useful.
5
PC boards for several of the analyzer circuits 70-cm ham band. The continuous-duty trans-
when, for example, spurious transmitter are available from Kanga USA. Details are mitter puts out 10 W AM (color or black-
products are being examined. available at the Kanga Web site, http:// and-white, just like broadcast TV stations)
Most of the adjustment has already been www.bright.net/~kanga/kanga/, or from
Kanga USA, 3521 Spring Lake Dr, Findlay, and features a built-in T/R relay, variable
discussed. Indeed, by the time the analyzer OH 45840. power output, front-panel RCA jacks for
is finished, there is little alignment left! The 6
The best digital audio recording and playback video and audio (with line and mic inputs), a
block diagram (Figure 1) has typical signal systems have a dynamic range around 90 dB. video monitor circuit and a PTL jack (push-
7
levels shown in parentheses. An outstanding fundamental treatment of log
amps is given in applications data from Ana- to-look) in parallel with the unit’s T/R
The finished analyzer is set up with a log Devices. See, for example, the data sheet switch.
–30 dBm signal from a stable-impedance for the AD8307, Analog Devices, Norwood, The TX70-10 is housed in a die cast alu-
source. Adjust the IF gain to establish the MA, 1997, http://www.analog.com/prod-
minum box measuring about 7×5×3 inches
reference level. If the 10 dB per division uct/Product_Center.html.
8
The log/IF board available from Kanga (see and is powered by 13.8 V dc at 3 A. The unit
tracking is not quite correct, change the log Note 5) is configured for the MC3356. Most comes with one user-specified crystal in the
amp gain, followed by a readjustment of of the 20 pins allotted to the IC are not used 440-MHz range. An optional second front-
the IF gain until tracking is correct. IF gain and their locations can be employed to bread-
board and retrofit the AD8307 (8 pin DIP) into panel-selected crystal is available for $20.
is adjusted to the reference level whenever the system, if desired. A repeater version, the RTX70-10, is also
the analyzer is used. 9
If crystal filters are built especially for this available.
Next month, we’ll present methods to project, we recommend a peaked response
Price: $439 shipped to the lower 48 states.
extend the analyzer to higher frequencies. shape (such as Gaussian-to-6 dB). We have
investigated wider bandwidth crystal filters as For more information, contact PC Electron-
We’ll discuss simple test equipment that presented in Wes Hayward, W7ZOI, “Refine- ics, 2522 Paxson Ln, Arcadia, CA 91007; tel
can be used in alignment and present some ments in Crystal Ladder Filter Design,” QEX, 626-447-4565; tomsmb@aol.com.
typical examples of spectrum-analyzer use. Jun, 1995, pp 16-21. See also the careful
work of Bill Carver, “High Performance Crys-
tal Filter Design,” Communications Quarterly,
Notes Winter, 1993, pp 11-18, and that of Jacob
1
Homebrew analyzers described in amateur lit- Makhinson, N6NWP, “Designing and Build-
erature in recent years have often used TV ing High Performance Crystal Ladder Filters,”
tuners as the front end. See, for example, Al QEX, Jan 1995, pp 3-17.
10
Helfrick, K2BLA, “An Inexpensive Spectrum See the Mini-Circuits catalogs and applica-
Analyzer for the Radio Amateur,” QST, Nov tions manuals. A family of viable VCOs are
1985, pp 23-29. A more recent example is also available from Synergy Microwave.
Fred Brown, W6HPH, “Build a 5- to 850-MHz These units are more expensive, but offer
Spectrum Analyzer,” Communications Quar- lower phase noise, which would be signifi-
terly , Winter, 1997, pp 91-96. cant for more-stringent applications.

August 1998 43