Linear Integrated Circuits

Step 1: Understand the function of a VCO. A Voltage-Controlled Oscillator (VCO) produces an output signal whose frequency is determined by an input control voltage.
Step 2: Trace the signal path in a typical VCO like the 566.

The process begins with the (D) Modulation Input voltage. This external voltage controls the frequency.
This voltage is used to control internal (A) Current Sources. The amount of current they produce is proportional to the input voltage.
These current sources alternately charge and discharge an external timing capacitor. The voltage across this capacitor ramps up and down linearly.
This ramping capacitor voltage is monitored by a (C) Schmitt Trigger. The Schmitt trigger switches its output state when the capacitor voltage reaches predefined upper and lower threshold levels. This produces a square wave.
The output from the Schmitt trigger is then passed through a (B) Buffer Amplifier to provide a stable, low-impedance output signal to the external world.

Step 3: Form the sequence. The correct sequence from input to output is D A C B.

Step 1: Recall the frequency formula for a 555 timer in astable mode. The frequency is given by:
Step 2: Identify the component values from the circuit diagram.
Step 3: Substitute the values into the formula. The calculated value is 640 Hz. The closest option is 644 Hz, which is likely the intended answer due to rounding of the constant 1.44 (more accurately 1/ln(2) 1.443).