Chapter 4 - Electromagnetic Compatibility And Medical Devices: Bare-Bones Spectrum Analyzer

BARE-BONES SPECTRUM ANALYZER

While an ac voltmeter can provide an indication of the field strength to which a probe is
exposed, it does not provide any indication of the spectral contents of an emission. A spectrum
analyzer is a tool that certainly cannot be beaten in the search for offending signals.
Unfortunately, spectrum analyzers are often beyond the reach of a designer on a tight budget.
For near-field sniffing, however, even the crudest spectrum analyzer will do a magnificent
job.

Figure 4.9 shows a simple home-brewed adapter to convert any triggered oscilloscope
into a spectrum analyzer capable of providing qualitative spectral estimates with a bandwidth
of 100 kHz to 400 MHz. As shown in Figure 4.10, a voltage-controlled TV tuner IC1
forms the basis of the spectrum analyzer. Most any voltage-controlled tuner will work, and
you may be able to get one free from a discarded TV or VCR printed circuit board. The connection
points and distribution vary from device to device, but the pinout is usually identified by stampings
on the metallic can of the device.

Figure 4.9 A simple circuit can be used to convert any triggered oscilloscope into a 100-kHz to 400-MHz spectrum analyzer suitable for near-field EMI sniffing.

Varactor-controlled TV tuners receive signals on their RF input at a frequency determined
by the voltage applied to the VTUNE input. With power applied to the UHF section of
a tuner, typical control voltages between 0 and 32 V span a frequency range of approximately
450 to 850 MHz. The sensitivity of the tuner can be adjusted through the AGC
input. The output of the tuner is a standard 45-MHz IF. However, 450 to 850 MHz is not a
range that is directly applicable to the large bulk of EMI sniffing work. For this reason, the
more appropriate range 100 kHz to 400 MHz is up-converted to the tuner’s input range
through a circuit formed by IC4–IC7 (manufactured by Mini-Circuits). Here, signals from
the probe are low-pass filtered by IC6 and injected into the IF port of a TUF-2 mixer. The
LO input of the mixer is fed with the output of IC4, a self-contained voltage-controlled
oscillator tuned to 450 MHz by potentiometer R20. The RF port of the mixer outputs signals
with frequency components at the sum and difference between the IF input and the
LO frequency. This output is high-pass filtered by IC5 to ensure that only up-converted
components are fed to the tuner input.

Sweeping the tuner across its range is accomplished by a sawtooth waveform that
spans approximately 1 to 31V. The basic sawtooth is generated by IC3 and Q1 and
buffered by IC2B. The span of the sawtooth is set by attenuator R10, while the center of
the sweep is adjusted by introducing an offset on IC2A by means of R13. The output of
IC2A is amplified by transistor Q2. Q2 should be selected for a gain of 50 or less. The
final span and linearity of the sweep is adjusted in three ranges by R1, R2, and R3.

The IF output of the tuner is attenuated to a level suitable for processing by R7, R8, and
R9. The actual value of the resistors for this attenuator must be selected based on the output
level of the specific tuner that you use. Then, in the circuit of Figure 4.11, IC8, a NE/SA605

Figure 4.11 The intermediate-frequency output of the tuner is detected by IC8, a single-chip IF processor. The received signal strength indicator (RSSI) output as a function of the sawtooth signal driving the tuner is a logarithmic representation of the spectrum of the signal picked up by the probe.

single-chip IF processor, takes care of detecting the signal and producing a logarithmic output
of signal strength.

In this portion of the circuit, the 45-MHz IF signal is coupled to the input of a RF mixer
internal to IC8 by way of a tuned circuit formed by C18, C19, and L3. The LO input of
this mixer is fed from a 44.5-MHz crystal-controlled oscillator. The resulting 455-kHz IF
is filtered by two ceramic filters, FLT1 and FLT2. An internal received signal strength indicator
(RSSI) circuit is used as a detector and linear-to-logarithmic converter. The RSSI
output, as a function of the sawtooth signal driving the tuner, is thus a logarithmic representation
of the spectrum of the signal picked up by the probe. RSSI is a current signal that
requires conversion to a voltage by the network formed by R21–R23 and D3. C24 lowpass
filters the RSSI output to produce a smooth display, and IC2C acts as a buffer and
impedance transformer for the current-to-voltage converter. Finally, IC9A blanks the output
during retrace.

Figure 4.12 presents the power supply circuit for the adapter. Most of the circuitry, including
the up-converter, tuner, and sawtooth generator, is powered by 12 V; +5 V powers the IF
processor. The +32 V to drive the tuner’s varactors is obtained by multiplying the 12 V ac input
to +48 V, reaching the desired voltage through IC10, an LM317 adjustable linear regulator.

To operate the spectrum analyzer, the Y output of the adapter is connected to the vertical
input of the oscilloscope, and the TRIGGER output is connected to the trigger synchronization
input of the scope. The horizontal frequency of the oscilloscope is set such
that one full sweep caused by the sawtooth fits the full graticule on the oscilloscope’s
screen. Fine-tuning can be accomplished either by trimming the time base of the scope or
by adjusting the value of R18 appropriately. Alternatively, a two-channel oscilloscope can
be operated in the X–Y mode by injecting the sawtooth available at pin 7 of IC2A to the
appropriately scaled X-axis channel.

The comb generator circuit of Figure 4.13 can be used for calibrating the adapter. The
circuit is simply a TTL-compatible 40-MHz crystal-controlled oscillator module feeding a
synchronous binary counter. It is called a comb generator because the spectral pattern of
any of its outputs resembles an ordinary hair comb with its prongs pointing up. Because
these spectral components occur at harmonic multiples of the fundamental square-wave
frequency selected, it follows that the frequency difference between consecutive “prongs”
must be the same as the value of the fundamental frequency of the square wave.

Figure 4.14 presents the pattern obtained when the 20-MHz comb output of the generator
is probed by a commercial spectrum analyzer. Ac coupling was accomplished through
a series-connection 15-pF capacitor, and termination to ground of the output side of this
capacitor was performed through a 50-Ω noninductive resistor. This is the gold standard
against which the adapter should be calibrated.

Start testing the adapter by setting the sawtooth generator to vary the voltage at the
VTUNE input of the tuner between approximately 1 and 31 V. Initially, set R6 to apply
2.5 V dc to the AGC pin of IC1. The up-converter LO frequency should be adjusted to
450 MHz by trimming R20. 9.6 V dc at the VTUNE input of IC4 will typically result in the
desired LO frequency. L4 should be trimmed to achieve stable oscillation of the 44.5-MHz
IF LO oscillator. With a 40-MHz comb applied to the input of the adapter through a 15-pF
coupling capacitor and with 50-Ω termination, adjust L3 to obtain an approximation of the
expected 40-MHz comb pattern on the oscilloscope. After achieving a satisfactory display
for the 40-MHz comb, calibrate the linearity of the adapter using a 20-MHz comb by first
trimming R3 to produce equal spacing between spectral lines throughout the lower third
of the display. Then linearize the midrange by trimming R2, and finally, the high range by
trimming R1.

< Previous Excerpt Next Excerpt >

Purchase This Book

TABLE OF CONTENTS

Chapter 4 - Electromagnetic Compatibility And Medical Devices: Bare-Bones Spectrum Analyzer

Contact Preferences

This is embarrasing...

Customize Your GlobalSpec Experience

Select Your Free Newsletters

Industry Newsletters

Select Your Free Product Alerts

This is embarrasing...