The first component of the meter is a sine wave oscillator. There are several ways to generate a sine wave; the most common are:

• A Wein bridge oscillator
• A phase shift oscillator (either buffered or unbuffered)
• A Bubba oscillator
• A quadrature oscillator
• A digital synthesizer (such as AD9833)

I've found the following references particularly useful:

• SLOA060: Sine-Wave Oscillator
• Design of op amp sine wave oscillators 

Right now I'm currently evaluating the different oscillators, mostly based on price and distortion.