A tone generator is a signal generator circuit which converts applied electrical signals to audio signals. It can be used to produce dial tones in telephones or produce sirens in ambulances or VIP vehicles etc or to generate melody tunes in toys, door bells etc.

Two-tone testing is a means of testing electronic components and systems, particularly radio systems, for intermodulation distortion. It consists of simultaneously injecting two sinusoidal signals of different frequencies (tones) into the component or system.

What does a two-tone test illustrate on an oscilloscope? Linearity of a SSB transmitter.

Intermodulation Distortion (IMD) occurs when two or more signals are used in a non-linear system. The products 2f1 and 2f2 are known as “Harmonics” which are nothing but replicas of a signal appearing at integer multiples of the fundamental signal. These sideband frequencies are considered to be undesirable.

linearity
What type of transmitter performance does a two-tone test analyze? A two-tone test measures the linearity of an SSB signal. A perfectly linear transmitter would produce those two tones—and only those two tones.

[′tü ¦tōn ‚mäj·ə′lā·shən] (communications) In teletypewriter operation, a method of modulation in which two different carrier frequencies are employed for the two signaling conditions; the transition from one frequency to the other is abrupt, with resultant phase discontinuities.

3rd order intermodulation products (D3 and D4) are the result of nonlinear behavior of an amplifier. Near saturation, each additional dB of input power results in proportionally less output power going into the amplified carriers and proportionally more output power going into the unwanted intermodulation products.

Filters. Noise that is the result of modulation, demodulation, and any nonlinear characteristics of the transmission medium or transmission system components. See also modulation and noise.

A two-tone test generator (see chapter 4.5 in this document) can reveal distortion in linear amplifiers so I decided to build the Elecraft 2T-gen on a piece of Veroboard. The 2T-gen user manual including a circuit diagram is found here.

A dual frequency tone generator will help you design your own IMD tests or experience the beat of two oscillators.

Two-tone generator: circuit diagram. I decided to create a new PCB for the two-tone generator. This PCB is visible in the photo below. The generator is mounted inside an alu-box. Front plate of generator. A small 12V 40 mA light bulb worked just fine as AGC. The settling time (stable amplitude) is about 2 seconds.

There are two sine wave oscillators in the Elecraft 2T-gen. The first one is at 700 Hz, and the second one at 1900 Hz. I tested both oscillators with an analog oscilloscope. The 1900 Hz tone appeared perfect on the screen, but the 700 Hz tone showed distortion (clipping), and this is not acceptable in a test generator.