Question E4A05:
What is the purpose of using a prescaler with a frequency counter?  

Choices:  
A: Amplify low-level signals for more accurate counting  
B: Multiply a higher frequency signal so a low-frequency counter can display the operating frequency  
C: Prevent oscillation in a low-frequency counter circuit  
D: Reduce the signal frequency to within the counter's operating range    

Correct Answer is  D: Reduce the signal frequency to within the counter's operating range  

To measure very high frequencies with a counter that can only handle lower frequencies, a prescaler divides the input frequency by a fixed integer before counting.

Why D is correct:
- A prescaler is a high-speed divide-by-N circuit. It outputs a reduced frequency that is within the counter’s input range. The counter then either:
  - Displays the divided frequency, or
  - Multiplies the count by N internally to show the actual frequency.

Key relationship:
$$f_\text{in} = N \cdot f_\text{counter}$$

Example:
- If $f_\text{in} = 1.28\ \text{GHz}$ and $N = 64$, then
  $$f_\text{counter} = \frac{1.28\ \text{GHz}}{64} = 20\ \text{MHz}$$
  which a typical counter can handle.

Why the other choices are incorrect:
- A: Amplify low-level signals for more accurate counting
  - That’s a preamplifier’s job. A prescaler divides frequency; it does not increase amplitude.
- B: Multiply a higher frequency signal so a low-frequency counter can display the operating frequency
  - Prescalers divide, not multiply. Any “multiply” happens in the counter’s math to restore the displayed value, not in the RF path.
- C: Prevent oscillation in a low-frequency counter circuit
  - Oscillation control is unrelated. Prescalers are for frequency division, not stability.

Useful rules of thumb:
- Common N values: 10, 16, 32, 64, 256.
- Ensure the prescaler’s max input frequency exceeds your signal’s frequency.
- The displayed frequency equals the counter reading times N if the counter doesn’t auto-compensate:
  $$f_\text{actual} = N \cdot f_\text{measured}$$