#1: . . . Quickie Audio Oscillator
by Joe Everhart, N2CX
I love quickies. That is, I like simple little circuits built for special purposes. The first was a simple single transistor RC audio oscillator built for a ham buddy. The breadboard circuit was literally a ball of components. Everything was just tack soldered together without a supporting circuit board.
When my buddy asked for the breadboard, I figured he wanted the parts so he could rebuild the circuit in a more permanent form. Being a frugal Pennsylvania Dutchman, he merely soldered on some connecting leads, plopped the circuit into a cardboard cup and filled it with a foam potting compound. It's probably still working!
The circuit here is a step up from the original quickie. Figure 1 shows the circuit. It's a bare bones twin-tee audio oscillator adapted from a variety of circuits in the last 20 years of hobbyist and ham publications. My wrinkle is that you really don't need to put it on a printed circuit board and enclose it in a fancy case. Mine is built on a scrap of Vector board. See Figure 2 for parts layout. Nothing is critical about the construction. Using a snap-on 9V battery connector almost eliminates the power switch.
It's been kicking around the shack for better than 10 years. Usually it's used as an audio source for testing homebrew receiver audio stages. The maximum output level is about one volt RMS at 500Hz. My ear peaks at this frequency, so I use it for my testing. Simply scale the capacitor values for other frequencies and, for accuracy, measure the output level with a scope or DVM.
To use the quickie, just tack solder on a resistive divider to get the level you need. Resistor values for various levels are given in Table 1. Connect the signal to the circuit you're checking with short leads and a 0.5uF Mylar coupling capacitor.
Start with a level of about 10mv at the input of the audio level of only a microvolt. Try doing that with a commercial AC line powered audio oscillator! To check gain again, note the audio chain's output level as you go backwards, knowing the calculated quickie voltage. This is very handy for verifying that each audio stage in a direct conversion receiver is working right.
You can contact Joe at firstname.lastname@example.org or send mail to: Joe Everhart, 214 New Jersey Road, Brooklawn, NJ 08030