Designing and Building a Two-Tone Generator

I wanted to measure distortion in my 2m SSB-transmitter and PA. 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.

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. Obviously, the problem was too high amplification in the circuit, and this pointed to poor AGC control.

I tried to solve the problem and eliminate the distortion. After some time, I gave up the MOSFET-based AGC and decided to implement a different method: Using a light bulb as automatic gain control. Here is the final circuit diagram:

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. If you are looking for a similar light bulb on ebay.com, I recommend searching for "12V 40mA".

OZ1BXM Lars Petersen
Homepage: oz1bxm.dk