The SG90 appears to be a standard servo motor included in most hobbyist kits these days (2017). While the following information mentions the Raspberry Pi specifically, the explanation is good for a variety of micro-controllers.
Different software libraries expose the PWM functionality with slightly different interfaces. Some specify a frequency as a value with a single call. With others you make several calls to set a clock divisor and a range. With this second type of interface, the frequency is calculated by taking the original clock frequency divided by a clock divisor divided by a range.
This posting on the Raspberry Pi StackExchange, Control Hardware PWM frequency provides several answers on using Pulse Width Modulation (PWM) to control a servo motor with the second type of interface (clock divisor and a range).
This post was quoted in the adafruit tutorial Adafruit's Raspberry Pi Lesson 8. Using a Servo Motor which uses an SG90 servo.
Also see RASPBERRY PI LESSON 28: CONTROLLING A SERVO ON RASPBERRY PI WITH PYTHON using the first type of interface, using a frequency value, which says:
We can now start the pwm sequence by giving a command to specify the
DutyCycle of the signal. Before we do this, we need to talk a little
bit about how servos work. A typical servo wants to see a frequency of
50 Hz on the control line. The position it moves to depends on the
pulse width of the signal. Most servos behave roughly as such, but you
will need to tweak these numbers for your particular servo. Typically,
the servo will go to the full left position when it sees a pulse width
of 1 millisecond, it will go the middle position when it sees a pulse
width of 1.5 millisecond, and it will go to the full right position
when it sees a pulse width of 2 millisecond.
Also see the answer to Servo position with no pulse which has this to say about the pulse protocol for commanding a servo:
Servo control signals are not actually PWM but a variant, pulse
duration modulation: Servo position is not defined by the PWM duty
cycle (i.e., ON vs OFF time) but only by the duration of the pulse. As
long as it is anywhere in a range of (typically) 40 Hz to 200 Hz, the
exact value of the frame rate is irrelevant. The servo expects to see
a pulse every so many ms, this can vary within a wide range that
differs from servo to servo.