When oscillator frequency drifts below 1 MHz, what kind of control voltage does the AFC circuit produce?

When the oscillator frequency drifts below 1 MHz, the Automatic Frequency Control (AFC) circuit generates a negative control voltage. This behavior occurs because the AFC is designed to compensate for frequency deviations from a desired reference frequency. When the oscillator frequency is lower than the target frequency (in this case, below 1 MHz), the AFC circuit detects this discrepancy and produces a control voltage that is negative to bring the frequency back up to the desired level.

The negative control voltage effectively acts to increase the oscillator's frequency, counteracting the drift and ensuring that the system remains aligned with the intended operational frequency. This control mechanism is crucial in maintaining stable performance in frequency-sensitive applications and assures precise tuning in radio systems, for example.

While positive control voltages might be used in scenarios where an increase in frequency is necessary, they are not relevant in this instance since the query specifically addresses a situation where frequency has decreased. The other options do not align with the intended function of the AFC in this context.