Circuit: DIY Wideband RF Signal Tracer

You can learn a lot from old-school projects. Consider this wideband RF signal tracer circuit from 1988. If you're interested in RF technology and circuit design, you'll enjoy this design.

Simple RF Signal Tracer

The simple and versatile circuit can aid in troubleshooting defective RF amplifier circuits. The usable frequency range of the tracer is about 100 kHz to 30 MHz. Measured signals (0.5 mV to 500 mV) are amplified, detected and made audible with the aid of a small loudspeaker.

MOSFET T1 functions as an amplifier with a high input impedance to avoid loading the signal source. Transistors T2, T3 and T4 form a high-gain logarithmic amplifier that drives AM demodulator T5-D5. A single chip AF power amplifier, IC1, is included to make detected signals audible. Testing of RF equipment is carried out simply by "probing around" at suitable locations in the circuit and listening to the detected signal, whose relative amplitude can provide an indication of possible sources of malfunction. The tracer’s logarithmic amplifier obviates the need for frequent re-adjustment of the volume control, P1. The unit is so sensitive that it produces audible output when the input is only held near the circuit section under test.
  As to construction of the tracer, this is best fitted in a short length of ABS tubing to make a probe with three connecting wires for the supply voltage and the loudspeaker. Constructors are advised to strive for ample RF decoupling and short connections in view of the relatively large bandwidth. Current consumption of the tracer is about 100 mA from a regulated 6 V supply.

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This article first appeared in Elektor July/August 1988.

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The article, “Wideband RF Signal Tracer,” appeared in Elektor July/August 1988. Elektor Members enjoy ElektorMag, a 10% Elektor Store discount on many products, and 24/7 access to the online library, which includes this article and many others. If you don’t yet have an Elektor membership, register today and start reading through our thousands of projects and circuits.