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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