A frequency reference for tuning up the RS-232 to 100 MHz RF desktop channel adapter elsewhere on this site, when I found this Saronix crystal oscillator in my junk box. A few minutes with AVRStudio produced an ATtiny12 to make a tone - even fewer parts than using an NE-555. With the addition of a transistor to gate the output, I was set to go. Quick and dirty, but it was useful. The Saronix oscillator has an open emitter output. When the 330 Ohm load resistor on the output is grounded, the RF out of the oscillator increases. Modulation is affected by driving the MPSH34 on and off to switch the 330 Ohm load resistor.
An Atmel ATtiny12 provides a square wave signal that can be used to drive the MPSH34, or if the SPDT switch is in the other position, an external modulation source can be used to modulate the signal, or if nothing is connected to the modulation input, the RF output is at maximum value because of the two 10K pull-up resistors on the base of the MPSH34. This technique should work with an oscillator that requires an external pull-down resistor on its output.