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A signal tracer for au
Ever wanted to trace a signal through an
AM radio or amplifier? This simple
signal tracer will let you do it. It can trace
amplitude modulated RF signals right up
to the detector and after that, you switch
to audio mode to continue through to the
output stages.
I
F YOU ARE building projects published in SILICON CHIP and other
magazines, you probably seldom
have a need for a signal tracer. You just
wire the projects up and they work
first time. Well, mostly they work first
time. At those other times you have
to fall back on your troubleshooting
skills and actually figure out where
the trouble lies.
Often, you will be able to find faults
in circuits just by measuring DC volt
ages but there will be other times
when the DC voltages are correct but
the circuit steadfastly refuses to work.
Or perhaps you are called upon to
do the odd servicing job. Here again,
faults can often be found by careful
visual inspection, checking voltages
and so on. But finally, you will need a
signal tracer such as the one featured
here.
As well as being useful for radio and
audio circuits, it can be of use in some
digital circuits, as the varying logic
level signals will give an audible indication on the low-gain RF position.
Features
Our new Signal Tracer is housed in
compact plastic case with a 3-position
toggle switch on either side. On the
lefthand side is the power switch
which is Off in the centre position;
the other positions provide the RF and
audio modes. The righthand switch
provides three gain settings: hi, med
and lo. The Signal Tracer also comes
with a black wander lead which clips
to the earth or 0V point in the circuit
to be traced. And the Signal Tracer
has a prod fitted to one end which is
touched at each point in the circuit
to be checked.
The specified case for the project
has a battery compart
ment with a
slide-off lid. The small PC board is
mounted in the main compartment,
together with the switches and a
miniature loudspeaker.
Circuit description
Fig.1 shows the circuit which uses
two op amps, an LM318 (IC1) and an
LM386 (IC2). IC1 is a wideband op
amp which is wired in non-inverting
mode with gain switchable by one
pole of the switch S1; ie, S1a. This
varies the feedback to give the three
Fig.1: op amp IC1 works in both RF and audio tracing modes and is switched to provide three gain levels.
In the RF mode, diode D1 acts as a detector for AM signals. In the audio mode, the output of IC1 is passed
through a 30dB attenuator before being applied to amplifier stage IC2.
40 Silicon Chip
udio & RF
gain settings (hi, med and low). These
correspond to nominal gains of 85
(38.6dB), 10 (20dB) and 2 (6dB),
respectively.
The input impedance of the IC1, as
seen by the probe, is around 100kΩ
which is quite high and should lead to
minimal detuning in most RF circuits.
By the way, the input coupling capacitor for the probe is rated at 400VW.
This will enable it to be safely used
for signal tracing in valve radios and
amplifiers which may have plate volt
ages as high as 385V.
Now it might seem odd that we are
using a fairly common op amp as
the input circuit for a signal tracer.
After all, it should be good for at
least the lower shortwave radio
frequencies; ie, up to around
10MHz or more.
In fact, the LM318 has a
typical small signal bandwidth of 15MHz so it is quite
appropriate for this application. Unfortunately you can’t
get something for nothing, especially in electronics. The bandwidth
figure of 15MHz means that you can
get 15MHz at unity gain. If you want
higher gain, the bandwidth will be
correspondingly less.
Fig.2 shows the frequency response
of IC1 from the input to its output at
pin 6. The three graphs shows the
responses at the high, medium and
low settings.
The “low gain” graph, corresponding to a nominal gain of two (+6dB)
has been normalised to 0dB and as
you can see, the gain is usable to well
beyond 10MHz.
The “medium gain” graph shows
an increase of about 14dB above the
low gain setting, corresponding to its
nominal gain of 20dB. At this setting,
the response is usable to beyond
2MHz so the AM broadcast band is
well covered.
The “high gain” graph shows a
further increase of about 18dB and
By RICK WALTERS
PARTS LIST
1 PC board, code 04106971, 53
x 55mm
1 plastic case, 128 x 68 x 26mm,
Altronics H-0342 or equivalent
1 miniature speaker, Altronics
C-0606 equivalent
2 2-pole 3-position toggle switches
1 216 9V battery
1 battery clip
2 8-pin IC sockets
1 binding post terminal
1 4mm banana plug
Semiconductors
1 LM318 op amp (IC1)
1 LM386 audio power amplifier
(IC2)
1 1N914 small signal diode (D1)
Capacitors
3 100µF 16VW electrolytic
1 1µF 16VW electrolytic
3 0.1µF MKT polyester or
monolithic
1 .047µF 400VW MKT polyester
1 .01µF MKT polyester or
ceramic
1 15pF ceramic
Resistors (0.25W, 1%)
3 100kΩ
1 120Ω
3 10kΩ
1 56Ω
1 3.3kΩ
1 10Ω
2 1.2kΩ
June 1997 41
Fig.2: the frequency response of IC1 from the input to its output at pin 6. The
three graphs show the response at the high, medium and low settings. The “low
gain” graph, corresponding to a nominal gain of two (+6dB), has been
normalised to 0dB and as can be seen, the gain is usable to well beyond 10MHz.
once again, there is usable gain over
the whole of the broadcast band.
Pin 2 of IC1 is biased to half
the supply (nominally +4.5V) by a
voltage divider consisting of two
10kΩ resistors and a 100µF bypass
capacitor.
Mode switching
While the frequency response
curves of Fig.2 don’t show it, IC1’s
response extends down to around
200Hz, so it can be used for both RF
and audio (AF) signal tracing. In the
RF mode, switch S2 selects the output
of diode D1, so that the RF signals are
“detected” by the diode and filtered by
the .01µF capacitor before being feed
42 Silicon Chip
to IC2 via a 0.1µF capacitor.
Note that the cathode of diode D1
is taken to ground (0V) via a 100kΩ
resistor. As the DC voltage at pin 6 of
IC1 is around +4.5V this means that
this diode is permanently forward
biassed and conducting with about
40µA through it. This slight forward
bias enables the diode to detect lower
signal levels than if it was not biased.
An unbiased silicon diode needs a
peak signal level of about 0.6V before
it begins to conduct. So this measure
greatly enhances the circuit operation
for RF signal tracing.
While we use IC1 at the same gain
settings for both RF and AF signal
tracing modes, the high gain of the
LM318 could easily overload the following audio amplifier (IC2), which
itself has significant gain. Therefore,
for audio tracing, IC1’s output is fed
through a 30dB attenuator (made up
of the 100kΩ and the 3.3kΩ resistors)
before passing to IC2, an LM386 audio
amplifier. This prev
ents the audio
signals, which are normally at a much
higher level than RF signals, from
overloading the audio amplifier stage.
IC2 has its gain switched by the
second pole of S1. It has a gain of 20
(+26dB) in the lo position, 38 (+31dB)
in the med position and 147 (+43dB)
in the hi setting.
At the lowest sensitivity the overall audio gain is -2dB (+2 -30 + 26 =
-2dB) and at the highest setting it is
+51.6dB (+38.6 - 30 + 43 = +51.6dB).
This is sufficient to cover all normal
input signals.
Varying the gain of both ICs lets
Fig.3: the wiring details for the signal
tracer. Keep all the wiring as short as
possible and make sure that the ICs are
correctly orientated.
RESISTOR COLOUR CODES
No.
3
3
1
2
1
1
1
Value
100kΩ
10kΩ
3.3kΩ
1.2kΩ
120Ω
56Ω
10Ω
4-Band Code (1%)
brown black yellow brown
brown black orange brown
orange orange red brown
brown red red brown
brown red brown brown
green blue black brown
brown black black brown
5-Band Code (1%)
brown black black orange brown
brown black black red brown
orange orange black brown brown
brown red black brown brown
brown red black black brown
green blue black gold brown
brown black black gold brown
June 1997 43
Our prototype used a 4mm banana plug
with an old meter probe tip plugged into it.
Alternatively, a standard multimeter lead
could be used as a test probe, for reaching
difficult locations.
Fig.5: the full size etching pattern for
the PC board.
making sure that they have the correct
polarity.
Solder the 11 wires for the switches as well as the two wires for the
speaker into the PC board, leaving
each of them around 75mm long. As
well, solder the negative battery lead
(black) into its pad on the PC board.
Drilling the case
us boost the audio gain when the RF
signal is at a low level and reduce it
when the signal is higher.
Assembling the PC board
The assembly is quite straightforward and the component overlay is
shown on the wiring diagram of Fig.3.
Begin by checking the PC board
for shorted or open circuit tracks and
then make any necessary repairs. This
done, insert the resistors, diodes and
IC sockets, solder them, and cut off
the excess leads. If you align all the
resistors so the colour bands are in
the same direction (horizontally and
vertically) it makes it easier to read
the values and also makes the finished
PC board look better.
The same comment applies to the
values marked on top of the MKT
capacitors which should be fitted
next – make them all read in the same
direction. Lastly, fit the electrolytics,
At the opposite of the case from
the battery, drill a 4mm hole on the
centrelines and fit a binding post
terminal. Next, drill holes for the two
switches 16mm down from the top on
either side on the centreline (use the
label markings as a guide). This done,
mount the PC board in the case using
the two short screws and complete the
wiring as shown in Fig.3.
We used a 4mm banana plug with
an old meter probe tip inserted in it
as the probe but you could also use a
standard multimeter probe for reaching difficult locations. The earth lead
consists of a length of wire fitted with
a small alligator clip.
SILICON
CHIP
r
e
c
a
r
T
rf
off
audio
hi
lo
med
Testing the signal tracer
Fig.4: this is the full size front panel artwork for the signal tracer.
44 Silicon Chip
To test the unit, connect the battery,
switch to AUD and HI, and place your
finger on the probe. You should be
greeted with a loud screech. If you
live in the city and switch to RF &
HI, you should hear one or more AM
radio stations if you connect a length
of wire to the probe. The reason that
you hear several stations (if you hear
them at all) is that there is no selectivity and all frequencies are received
and are amplified equally.
For a good description on how to
use a signal tracer, refer to the articles on Vintage Radio in this and last
month’s issues of SILICON CHIP. SC
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