This is the battery-powered "pocket" oscillator PC board described in the Pro Audio design Forum.

Features

Low Distortion: The battery-powered oscillator generates 1 kHz tone with THD levels that are typically 0.00025%.

Floating Outputs: When battery-powered or when powered externally from a floating supply the outputs are galvanically-isolated.

Fixed Outputs: The oscillator runs at an internal +18 dBu level and provides two padded fixed level outputs. Line Level Output: The oscillator 0 dBu Line output is balanced with a 43Ω per leg output impedance using a balanced "L" pad. Mic Level Output: The mic output level is -40 dBu and is tolerant of phantom power. The output impedance is 150Ω obtained by a "U" pad. Other levels and impedance are available by changing resistors.

Variable Output Level: A variable unbuffered output of up to +16 dBu is obtained by using an external potentiometer. An optional stepped attenuator, "ATTO" is available to provide stepped output attenuation to -70 dBu.

Operating Current: 15 mA running; <2 µA standby. 

External Power: An external +24 VDC power input is available. Tip +.

Timer Mode: The oscillator can run continuously or in an auto-off timer mode to conserve battery life.  

Mechanical Specifications: Board Size: 4.0" x 1.9" Mounting holes: M3 on a 3.6" x 1.5" pattern. Assembled height: Approximately 1" not including stand-offs. 

Construction and assembly information can be found here: https://www.proaudiodesignforum.com/forum/php/viewtopic.php?t=1523

A Mouser Project Manager parts list is also available. https://www.mouser.com/Tools/Project/Share?AccessID=7b857b9335