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Microwave Frequency Counter basics

This page describes microwave frequency counter basics and covers application note on how to use microwave frequency counter.It mentions vendors or manufacturers of microwave frequency counter.

Microwave frequency is measured using frequency counter and cavity based wavemeter. Frequency is also measured using slotted line method wherein guide wavelength in standing wave pattern is used for computation purpose. The most accurate method of microwave frequency measurement is down conversion method.

The most of modern automatic frequency counter uses down conversion method to measure the frequencies above 500MHz. The techniques are as follows:
• 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 block diagram of heterodyne based down conversion technique used for microwave frequency measurement.

As shown, mixer is used in the design, which beats incoming unknown frequency to be measured with known LO signal. LO signal is choosen such that difference frequency falls within 500 MHz bandwidth of the counter.

First Fin is generated using instruments time base. This will be within 100 to 500 MHz range. This Fin is fed to harmonic generator which generates comb line of frequencies. These frequencies are spaced at Fin which extends upto the complete range of the frequency counter.

Suitable processor is also incorporated in the design to facilitate high performance measurement using advanced DSP algorithms.

Microwave filter selects one line of comb say 'K*Fin' which is fed to mixer. Mixer generates Fx-K*Fin. This video frequency( Fx-K*Fin) is amplified and provided to counter. The display contains sum of video frequency and K*Fin.

Here appropriate K*Fin is generated by processor. Signal detector block determines correct 'K' value. Here processor starts with K=1 and will step through different K values across comb line until detector determines K where video frequency is available. At this point, acquisition process is stopped and measurement can starts.

There will be AGC (Automatic Gain Control) part which is not shown here is used which de-sensitizes the video amplifier such that only strong frequency components of video signal will enter schmitt trigger and will be counted and measured accurately.

Application note-how to use Microwave Frequency Counter

Following specifications are considered while purchasing microwave frequency counter. 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.
These counter features are explained below.

Frequency range: It is the range over which frequencies can be measured by the microwave frequency counter.

Measurement Speed: It is the time needed for a counter to perform the measurement of the frequency. It is divided into acquisition time and gate time. Acquisition time is the time needed to detect microwave signal and prepare the signal for making measurement. Gate time is the time needed to measure the signal with desired resolution of the counter.

Accuracy: It is the parameter which defines how accurately the frequency is measured using the counter. It is limited by two factors viz. +/-1 count of quantization error and time base errors. The heterodyne converters are limited to the acuuracy of 1 X 10^{- 9} resolution. With high stability oscillators accuracy or resolution of 1 x 10^{- 10} can be achieved at microwave frequency.

Sensitivity and Dynamic Range: Good frequency counter should have sensitivity of -25dBm. Dynamic range is the amplitude range over which measurements can be made successfully. Dynamic range is the difference between highest amplitude level and sensivity level.

Signal to Noise ratio: When we measure the microwave signal , it is always with the presence of noise. The frequency counter which measures the microwave signal with the least SNR is considered to be better.

FM Tolerance:Usually frequency is measured in pure carrier mode i.e. without modulation. However there is need to measure microwave sources or carriers with inherent frequency modulation. In this situation, FM tolerance of microwave frequency counter is considered. FM tolerance is specified in Hz (peak to peak).

AM Tolerance: Similarly when measuring amplitude modulated signal, AM tolerance is considered. The AM tolerance for heterodyne converter is limited to < 50%.

Amplitude Discrimination: It is desired to measure signal in presence of nearby low level signals. Here desired signal is measured at least 2dB above compare to nearby undesired signals. Similarly 10dB level is kept to discriminate far end signals from the desired one.

It is very easy to use the microwave frequency counter. The output of device under test(DUT) is connected using RF cable to the frequency counter input port. The counter automatically displays the frequency reading. Now-a-days all the counters are digital and hence it will provide the reading in numbers as per resolution limit of the counter as explained.

Microwave Frequency Counter Vendors/Manufacturers

Following are the popular microwave frequency counter vendors or manufacturers.

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