Turn your CRO into
a spectrum analyser
for just $399
Have you ever dreamed of owning your own
spectrum analyser? If you're an amateur, TV
technician or electronics teacher, we'll bet you
have. The $20,000 plus price tag would
certainly have dampened your spirits. But now
there is a probe which turns your CRO into a
spectrum analyser.
By ALEX EADES
This little gem is called the
VOS-107 Spectrum Probe and at a
price of $399 it is a magic
accessory.
When an opportunity to review
the VOS-107 came my way, I
jumped at the chance. Having spent
years building ham radio projects
- where an analyser would have
been a wish come true - I needed
no convincing of the value of such a
device.
For the uninitiated, a spectrum
analyser is an essential test instrument for any equipment designE)d to
process frequency information and there is plenty of that! Take for
example entertainment electronics.
TVs, radios, VCRs, cameras and
music systems are all involved in
processing the frequency components of signals. The enormous
amount of this equipment in use
generates a growing need for spectrum analysers.
The VOS-107 Spectrum Probe
looks for all the world to be just
another logic probe, but it is
nothing like that at all. A normal
oscilloscope displays how a voltage
changes with time. A spectrum
analyser displays the amplitudes of
signals separated on the basis of
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their frequencies. The VOS-107
converts the latter into the former
(a handheld Fourier Analyser!).
And that's no mean feat, especially
for $399!
What is a spectrum analyser? It
is a device which looks much like an
ordinary CRO but with a quite different method of display. It shows
the frequency components in a
signal along with their strengths at
very good accuracy. To get an idea
of this, imagine a display like the
dial of your radio with vertical lines
showing all the strengths of the stations in their respective positions.
Sound over the top? Not really,
such devices have been around for
30 years or so. The snag has always
been the cost $20,000 to
$50,000! Winning Lotto was the only way the average bloke could get
one in the past.
Measuring frequency
There are several methods commonly used to measure frequency
and they are testimony to the ingenuity (desperation?) of souls
needing to quantify frequency information. The first is to display the
signal on the CRO and measure the
time taken for the pattern to repeat.
The frequency is then found by
dividing 1 by the time using a
calculator.
The second method is to use a
digital frequency meter (DFM)
which provides a reading in Hertz
directly, while the third technique
involves using a communications
receiver or absorption wavemeter.
Other devices such as slotted lines
and lecher wires are somewhat
esoteric and will not be discussed
further.
The CRO method will give the
fundamental frequency and an experienced operator a gut feel for
what others (harmonics or otherwise) may be present. The DFM will
latch onto the largest amplitude
signal and ignore the rest. The
receiver or wavemeter method is
probably the best as they can be
tuned to a certain frequency and
provide a measurement on a signal
strength meter. By tuning across a
band, an idea of the frequencies
present can be obtained. These
devices have been the mainstay of
frequency analysis by hams for
years.
The difficulty with all these
methods is that they display only
one frequency at a time. It's like
reading a newspaper one letter at a
time instead of in whole words. This
makes adjusting circuits a painstaking process when several frequencies need to be monitored simultaneously.
CROs & spectrum analysers
Most readers are familiar with
the display of an oscilloscope - a
pattern of how voltage on a circuit
changes with time. We have grown
to rely on the bounty of information
then brings up dozens of spikes on
the horizontal line, representing
short wave, amateur, TV and FM
signals.
Evaluation
This is the VS-107 Spectrum Probe in its case. It also comes with an AC
plugpack and a brief instruction manual. It can be used with virtually any
oscilloscope with a bandwidth of 5MHz or more.
these bright green traces provide.
Oscillation, switching, ringing, clipping, glitching, drooping, overshoot
spikes, rise time, fall time, lumps
and bumps are all familiar beasties
encountered on the oscilloscope
screen.
The CRO's main drawback is its
inability to provide detailed information on the frequencies of the
signals being monitored. As a great
deal of circuitry is designed to
manipulate signal frequencies, this
limitation is restrictive. With the
VOS-107 spectrum probe, these
waveform lumps, bumps, lines and
spikes become fundamentals, harmonics, sidebands and intermodulation products, allowing
spectral purity and bandwidth to be
easily seen and measured.
Connecting the probe
Virtually any CRO has adequate
performance for use with the
VOS-107. The normal vertical sensitivity setting is 50mV/div (or
50mV/cm), while the timebase setting is 0.5ms/div (or 0.5ms/cm).
Connecting the probe to the CRO
is simple. The probe has a figure-8
shielded output cable. One half of
the cable is terminated with a BNC
plug and this goes to the CRO's vertical input socket. The other half of
the cable is terminated in a 3.5mm
line socket and this is for the 12V
AC power input from a plugpack
transformer.
To monitor a signal, either the
probe tip is used or an adaptor for
coax cable is supplied - essential
for VHF work. Remember to use a
terminating resistor on the cable,
otherwise reflections will cause inaccurate results. The manual
recommends a simple arrangement
to achieve this. For high power
sources such as radio transmitters,
the earth lead can be clipped to the
input to form a "sniffer loop" which
can be placed near the transmitter
output.
When connected and powered
up, the screen displays a waveform
which looks like a video signal.
There is a negative-going sync pulse
on the far left, closely followed by
a vertical zero reference line, a
'noisy' horizontal line and, on the
far right, the beginnings of the next
sync pulse. The zero line represents
minimum frequency and its height a
level of 50dB a hove the noise floor.
The vertical trace position and
horizontal timebase controls are
adjusted so that the sync pulses are
off screen. Now the horizontal line
represents frequencies from 1MHz
to 100MHz. Touching the probe tip
The essential specifications for
the probe are: frequency range
1-lO0MHz; dynamic range 50dB;
vertical output 5mV/dB; IF bandwidth 180kHz; and horizontal
linearity ± 10%. These performance specs are moderate when
compared to a laboratory grade
spectrum analyser but still quite
useful.
To check these specs, I connected
the probe to a CRO, RF signal
generator and a spectrum analyser
so that comparisons could be made.
The overall frequency range of the
sample probe was 1MHz to
103MHz which is slightly greater
than the specifications. The horizontal frequency scale is linear
within the specification, covering
about 10MHz per division on a CRO
with 10 divisions.
Vertical scale accuracy was
tested at 10, 50 and 100MHz with
excellent results throughout, each
vertical division measuring l0dB.
You'll have more error just reading
the CRO than from the probe! The
easiest vertical set-up is to adjust
the zero reference line for 5 divisions, which automatically gives
lOdB per division.
Comments
The probe does not offer a direct
reading in say dBm, however the
vertical scale is useful for relative
measurements which after all are
the most common type. (For example, harmonics on a radio transmission are specified relative to the
carrier level). A reliable lOdB/div
scale is all you need.
To simplify use, I attached a
horizontal scale to the screen of the
CRO using masking tape and a ballpoint pen. It's a bit rough I guess,
but certainly effective, making frequency measurements a breeze.
Minimum sensitivity of below
40/lV is plenty good enough to sniff
out the majority of signals. The
most serious limitation in my opinion is the IF bandwidth as this
NOVEMBER 1990
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Turn your CRO into a spectrum analyser ..•
This is the normal oscilloscope display at 0.5ms/div and
50mV/div, here showing harmonics produced by an RF
oscillator.
determines the resolution of the
display. The 180kHz bandwidth
means that the spe.c trum monitor is
unable to resolve the sidebands of a
typical narrowband voice modulated signal from a CB or amateur
transceiver.
Sidebands determine the width of
the signal in the band and contain
the information in the transmission,
so the monitor's inability to display
them is an unfortunate limitation.
The manual specifies the probe's
resolution as 0.5MHz, a figure my
measurements support.
Amateur radio uses
To monitor the output of my SSB
transceiver, I placed the probe's
sniffer loop near the coax to the
dummy load. Only 5 or 10 watts
were needed to produce a useful
display - a very good result.
The object of an oscillator is to
have a pure single frequency at the
output. Viewing this output on a
CRO will give some qualitative idea
of the purity. Is it a good looking
sine wave or one containing lumps
and bumps? With the probe, the
main signal peak (the frequency we
are trying to generate) is visible
along with any other frequencies
(usually harmonics) - a greatly improved display. The effect of your
adjustments to the circuit can be
easily seen.
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This CRO photograph shows the video format of the
Spectrum Probe's output waveform. Notice the negative
sync pulses.
A "mixer" is a circuit designed to
change a signal of one frequency to
another and in the early days of
radio was called a "frequency
changer". The tuner in a TV set and
the "front end" of a radio are
usually mixers.
Mixers require careful adjustment to obtain the best results input levels, balance and output
tuning need to be spot on. The adjustment can only be performed accurately in the frequency domain,
hence the need for a spectrum
analyser. The aim is to maximise
the desired output and minimise the
unwanted signals - difficult with a
CRO alone but a breeze with the
VOS-107 probe.
TV technicians could also find
the VOS-107 probe a great tool in
their efforts to track down and nail
circuit gremlins. TV IF performance, especially bandwidth and
linearity, is clearly displayed. How
about viewing the frequency spectrum of the signal from video heads
or actually seeing the modulator
working?
Teaching Fourier Analysis
Fourier analysis is a mathematical process of taking a time
dependent signal (as on a CRO
displayj and turning it into its frequency components. For example a
square wave consists of the fun-
damental (or clock frequency), 3rd,
5th and all odd harmonics with
strengths inversely proportional to
the harmonics number. The 3rd
harmonic is 1/3 the strength of the
fundamental, the 5th, 1/5 the fundamental, and so on. If all these
signals could be generated and added together in the correct phase
relationship, the original signal
would be regenerated.
If you think this sounds all very
dry, you're not alone - thousands
of students would agree. Enter the
VOS-107. The device is of such low
cost that institutions could afford to
teach students "hands on". You
simply feed various signals into the
analyser and note what comes out
and compare it to the theory. I wish
these devices were around when I
did Fourier analysis!
Conclusion
All in all, I am impressed with the
performance of the VOS-107 and
am sure that it will become common in the near future. (I'll have to
ring Leo and tell him it'll take me a
few months to finish evaluating the
probe - should give me enough
time to complete a few ham
projects!)
Our review sample came from
David Reid Electronics, 127 York
Street, Sydney 2000. Phone (02) 267
1385.
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