Signal Injector

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A simple signal injector for troubleshooting radio receivers

A signal injector is a low power oscillator that generates a signal and lots of harmonics in the audio and low frequency RF spectrum. This allows you to test both the audio and RF stages of a radio receiver. They are very simple to built and every work bench should have one.

The circuit

The basic circuit for for signal injector is an Astable Oscillator - two transistors configured to alternately switch the state of the other transistor.

Basic circuit diagram of astable oscillator

The oscillator's operating frequency is set by R2, R3 and C1 & C2.

f = \frac{1}{T}

= \frac{1}{\ln(2) \cdot (R_2 C_1 + R_3 C_2)}

\approx \frac{1}{0.693 \cdot (R_2 C_1 + R_3 C_2)}


  • f is frequency in hertz.
  • R2 and R3 are resistor values in ohms.
  • C1 and C2 are capacitor values in farads.
  • T is the period.

The output approximates a square wave with the duty cycle controlled by the values of R2 and R3. In the case where R2 = R3, the duty cycle is very close to 50% and the circuit produces a reasonable square wave - this is not really what we want. A square wave is comprised only of odd harmonics, ideally we'd like both the odd and even harmonics so that there is a signal close to wherever the radio is tuned.


Some experimentation with the QUCS circuit simulator suggests reasonable values for the resistors and capacitors

Part Number Value
R1 & R4 22k Ohm
R2 470k Ohm
R3 100k Ohm
C1 10n F
C2 10 nF
Simulated oscilloscope output from oscillator
Simulated spectral output of the oscillator

You can see from the simulated data that the oscillator output is full of nice fast rising edges (good for harmonic generation) and does indeed contain harmonics out to beyond 20 kHz (that's as far as I plotted, the spectrum continues, but levels are too low to show on this plot). The low levels at higher frequencies are a good thing, as typically the circuit you'll be testing with this generator will be sensitive to tens-to-hundereds of micro-volts; you don't need big signals to test these circuits.

You can easily prove the circuit generates higher harmonics by building it and connecting the output to the antenna of an AM radio - you'll hear a buzzing noise repeating as you tune across the AM band.

Building the signal injector

The circuit is easily built on a scrap of veroboard.


The yellow Xs are where the copper strips are cut on the backside.

Using the signal injector

This circuit is only to be used on battery powered devices, NOT anything mains powered. There's no isolation from the mains provided by this equipment.

The ground clip is connected to ground on the radio receiver, and the output probe touched to the various stages' inputs, usually working back towards the antenna from the speaker / headphones. When you stop hearing a sound, you've found the non-working stage. From there it's a matter of faultfinding in the stage (usually a duff transistor or capacitor or broken wire).