Abstract: A variable frequency, fixed amplitude digital sweep generator for use with a cathode ray tube based instrument useful in troubleshooting, diagnosing, and servicing spark ignition internal combustion engines. A first counter (8) measures the period between the leading edges of a trigger pulse, V.sub.T, derived from the ignition system of an internal combustion engine by counting divided down clock pulses. A comparator (10) compares the count of the first counter (8) to the count of a second counter (11), which is clocked at a rate which is an integral multiple of the clock rate of the first counter (8). The comparator (10) produces an output pulse when the output of the two counters (8, 11) match. Each comparator pulse causes a monostable multivibrator (17) to produce a rectangular wave, V.sub.M. The constant width rectangular waves, V.sub.M, are integrated by an integration circuit (14) that produces a stair step waveform, V.sub.