Understanding RF Frequency Counters: Types and Techniques

This page covers the basics of RF frequency counters, including a block diagram. It describes down-conversion techniques used in microwave frequency counters. We’ll also touch on vendors and manufacturers of RF/Microwave frequency counters.

As we know, the frequency of a repetitive or periodic signal is defined by the number of cycles per unit of time. If a total of n cycles appear in a time interval t, then the frequency is expressed as follows:

Frequency = n/t

Frequency is measured in Hertz (Hz) or cycles/second. It can also be represented as ppm (parts per million).

The relationship between ppm and Hz is:

Frequency (Hz) = { Frequency (ppm) x RF Carrier Frequency (Hz)} x 10^-6

Figure 1: Conventional Frequency Counter Block Diagram

Figure 1 depicts the block diagram of a conventional frequency counter. As shown, the total number of pulses counted by the counting register during a pre-defined gate time gives the frequency of the input signal being measured.

In this block diagram, the open and close condition of the main gate (a logical AND gate) is controlled using an oscillator. Typically, a crystal oscillator with a value of 1 MHz, 5 MHz, or 10 MHz is used. The calculated frequency value is then displayed on the numerical display.

For example, if the counting register value is 60,000 and the gate time is 1 second, then the frequency of the signal is 60,000 Hz.

Down Conversion Techniques Used in Microwave Frequency Counters

Frequency counters are digital instruments, and their frequency range is limited by the speed of the logic circuits used. With ongoing research in IC technologies, frequency counters up to 500 MHz or 1 GHz are achievable. To measure frequencies above 1 GHz, different down-conversion techniques are employed.

The following techniques are used in microwave frequency counters:

• Prescaling: Counters up to 1.5 GHz can be developed using this technique.
• Heterodyne Converter: Frequency counters up to 20 GHz can be developed.
• Transfer Oscillator: Frequency counters up to 23 GHz can be designed using this technique.
• Harmonic Heterodyne Converter: A combination of heterodyne and transfer oscillator techniques. Frequency counters up to 40 GHz can be designed and developed using this.

Generic Specifications of RF Frequency Counters

Consider the following generic specifications of RF frequency counters when selecting one for your needs:

• Number of channels supported
• Frequency range of operation
• Input signal bandwidth supported
• Number of digits of resolution
• AC Coupled, DC Coupled, or both
• Voltage input range and its sensitivity
• Accuracy of measurements
• Speed of measurements
• FM tolerance and AM tolerance
• External interface such as GPIB or IP connectivity
• Storage for measurements required for future comparisons

Vendors or Manufacturers of RF and Microwave Frequency Counters

Figure 2: RF Frequency Counter

The following table mentions RF and Microwave Frequency Counter Vendors or Manufacturers.

Figure 2 depicts an RF frequency counter (model 53181A) from Keysight Technologies. Note that this image is representative. Refer to the Keysight website for detailed technical specifications and dimensions of the product.