Reverse linear ramp, sawtooth generator

by SV3ORA

Here is a very simple to build, reverse sawtooth generator, or as better known reverse linear ramp generator. It is made of discrete components and it can be used in oscilloscope, spectrum analyzer or other projects as a sweep generator. In comparison to a standard sawtooth generator, the reverse one can be used to trigger the oscilloscope and at the same time starting the trace scan. Of course the trace scan is reversed in this mode. This generator has been used in experiments like the coincident current gates.

The final two complement transistors, form a current amplifier so that harder loads may be driven, preventing the generator from being overloaded. The frequency of the generator can be altered by changing the 1uF capacitor. You would usually want to place a rotary switch at this point, to switch different capacitors to ground and therefore altering the frequency (sweep rate) of the generator.

The linearity of the generator is very good for such a simple circuit. Picture below, shows the measured waveform at 10Hz using a digital storage oscilloscope. At 12v input voltage, the output voltage is about 9v. The lower edge of the waveform shown, is at about 1v and the higher edge at about 10v.