Frequency Measurement in Microwaves Using a Frequency Counter

Measurement of frequency in the microwave range involves determining the frequency of electromagnetic waves within the microwave spectrum, which typically spans from 300 MHz (0.3 GHz) to 300 GHz. Accurate frequency measurement is critical in numerous microwave applications, including telecommunications, radar systems, satellite communications, and microwave ovens. Several methods are commonly used for frequency measurement in this range.

These methods include:

• Frequency counters
• Heterodyne techniques
• Cavity resonators
• Waveguide filters
• Frequency synthesizers
• Phase-Locked Loops (PLLs)

Microwave frequency can be measured using a frequency counter and a cavity-based wavemeter. Frequency is also measured using the slotted line method, where the guide wavelength in a standing wave pattern is used for computation purposes.

The most accurate method for microwave frequency measurement is the down-conversion technique. Most modern automatic frequency counters employ down-conversion to measure frequencies above 500 MHz.

The techniques include:

• Prescaling (up to 1.5 GHz)
• Heterodyne Converter (up to 20 GHz)
• Transfer Oscillator (up to 23 GHz)
• Harmonic Heterodyne Converter (up to 40 GHz)

Figure-1 depicts a block diagram of a heterodyne-based down-conversion technique used for microwave frequency measurement.

Frequency Counter Block Diagram

A microwave frequency counter is an electronic instrument used to accurately measure the frequency of microwave signals.

The microwave frequency counter block diagram consists of:

• Input interface
• Signal conditioning circuitry
• Frequency divider
• Timebase oscillator
• Counter circuit
• Gate control
• Display and user interface
• Control and processing unit

These components work together to precisely measure the frequency of microwave signals. Let’s understand the functions of these components.

As shown in the diagram, a mixer is used. It beats the incoming unknown frequency to be measured with a known LO (local oscillator) signal. The LO signal is chosen such that the difference frequency falls within the 500 MHz bandwidth of the counter.

First, $F_{in}$ is generated using the instrument’s time base. This will typically be within the 100 to 500 MHz range. This $F_{in}$ is then fed to a harmonic generator, which creates a comb line of frequencies. These frequencies are spaced at $F_{in}$ and extend up to the complete range of the frequency counter.

A suitable processor is also incorporated into the design to facilitate high-performance measurement using advanced DSP algorithms. A microwave filter selects one line of the comb, say $K*F_{in}$, which is then fed to the mixer.

The mixer generates $F_x - K*F_{in}$. This video frequency ($F_x - K*F_{in}$) is amplified and provided to the counter. The display shows the sum of the video frequency and $K*F_{in}$. The processor generates the appropriate $K*F_{in}$.

The signal detector block determines the correct ‘K’ value. The processor starts with K=1 and steps through different K values across the comb line until the detector determines the K value where the video frequency is available. At this point, the acquisition process is stopped, and the measurement can begin.

There will be an AGC (Automatic Gain Control) circuit (not shown in the diagram). This AGC desensitizes the video amplifier such that only strong frequency components of the video signal enter the Schmitt trigger. These components are then accurately counted and measured.

Using a Microwave Frequency Counter

Consider the following specifications when purchasing a microwave frequency counter, as these parameters are also useful in using the counter:

• Frequency range
• Measurement Speed
• Accuracy
• Sensitivity and Dynamic Range
• Signal to Noise ratio
• FM Tolerance
• AM Tolerance
• Amplitude Discrimination

Let’s understand the definitions of frequency counter terminologies.

• Frequency range: This is the range over which frequencies can be measured by the microwave frequency counter.
• Measurement Speed: This is the time needed for a counter to perform the frequency measurement. It is divided into acquisition time and gate time.
  • Acquisition time: The time needed to detect the microwave signal and prepare the signal for measurement.
  • Gate time: The time needed to measure the signal with the desired resolution of the counter.
• Accuracy: This parameter defines how accurately the frequency is measured using the counter. It is limited by two factors: +/-1 count of quantization error and time base errors. The heterodyne converters are limited to an accuracy of $1 \times 10^{-9}$ resolution. With high-stability oscillators, an accuracy or resolution of $1 \times 10^{-10}$ can be achieved at microwave frequencies.
• Sensitivity and Dynamic Range: A good frequency counter should have a sensitivity of -25dBm. Dynamic range is the amplitude range over which measurements can be made successfully. It is the difference between the highest amplitude level and the sensitivity level.
• Signal to Noise ratio: When measuring a microwave signal, noise is always present. A frequency counter that measures the microwave signal with the least SNR is considered better.
• FM Tolerance: Usually, frequency is measured in pure carrier mode (i.e., without modulation). However, there is a need to measure microwave sources or carriers with inherent frequency modulation. In this situation, the FM tolerance of the microwave frequency counter is considered. FM tolerance is specified in Hz (peak to peak).
• AM Tolerance: Similarly, when measuring an amplitude-modulated signal, AM tolerance is considered. The AM tolerance for a heterodyne converter is limited to < 50%.
• Amplitude Discrimination: It is desirable to measure a signal in the presence of nearby low-level signals. The desired signal should be measured at least 2dB above the nearby undesired signals. Similarly, a 10dB level is maintained to discriminate far-end signals from the desired one.

Using a microwave frequency counter is very straightforward. Connect the output of the device under test (DUT) using an RF cable to the frequency counter’s input port. The counter will automatically display the frequency reading. Nowadays, all counters are digital, providing readings in numbers according to the resolution limit of the counter as explained earlier.

Microwave Frequency Counter Vendors

The following are popular microwave frequency counter vendors or manufacturers.

Conclusion

Overall, accurate frequency measurement in the microwave range is essential for ensuring the proper operation of microwave systems and devices in various applications. Engineers utilize a combination of measurement techniques and instruments tailored to the specific requirements of their applications to achieve precise and reliable frequency measurements.