Measurement Devices - Microwave Engineering

What is Measurement Devices?

Among the Microwave measurement devices, a setup of Microwave bench, which contains of Microwave devices, has a noticeable place. This entire setup, with few changes, is able to amount numerous values like guide wavelength, free space wavelength, cut-off wavelength, impedance, frequency, VSWR, Klystron characteristics, Gunn diode characteristics, power measurements, etc.

The output formed by microwaves, in determining power is usually of a little value. They differ with the situation in a transmission line. There should be an equipment to measure the Microwave power, which in general will be a Microwave bench setup.

Microwave Bench General Measurement Setup

This setup is a mixture of dissimilar parts which can be observed in detail. The resulting figure openly clarifies the setup.
microwave_bench_general_measurement_setup

Signal Generator

As the name suggests, it makes a microwave signal, in the order of a few mill watts. This uses speed modulation technique to transfer endless wave beam into mill watt power.

A Gunn diode oscillator or a Reflex Klystron tube might be an instance for this microwave signal generator.

Precision Attenuator

This is the attenuator which chooses the preferred frequency and limits the output around 0 to 50db. This is variable and can be attuned according to the requirement.

Variable Attenuator

This attenuator sets the quantity of attenuation. It can be agreed as a fine adjustment of values, where the readings are tested beside the values of Precision Attenuator.

Isolator

This eliminates the signal that is not essential to influence the detector mount. Isolator permits the signal to pass through the waveguide only in one direction.

Frequency Meter

This is the device which procedures the frequency of the signal. With this frequency meter, the signal can be adjusted to its resonance frequency. It also gives provision to couple the signal to waveguide.

Crystal Detector

A crystal detector probe and crystal detector mount are point out in the beyond figure, where the detector is linked through a probe to the mount. This is used to demodulate the signals.

Standing Wave Indicator

The standing wave voltmeter offers the reading of standing wave ratio in dB. The waveguide is positioned by some gap to regulate the clock cycles of the signal. Signals conveyed by waveguide are advanced through BNC cable to VSWR or CRO to measure its characteristics.
A microwave bench set up in real-time application would look as follows −

microwave_bench

Currently, let us have a look at the significant part of this microwave bench, the slotted line.

Slotted Line

In a microwave transmission line or waveguide, the electromagnetic field is measured as the sum of incident wave from the producer and the reflected wave to the generator. The replications show a bad fit or incoherence. The magnitude and phase of the reflected wave be influenced upon the amplitude and phase of the reflecting impedance.

The standing waves got are measured to know the transmission line inadequacies which are essential to have knowledge on impedance mismatch for effective transmission. This slotted line helps in measuring the standing wave ratio of a microwave device.

Construction

The slotted line contains of a slotted section of a transmission line, where the measurement has to be go through. It has a travelling probe carriage, to let the probe get linked everywhere required, and the ability for attaching and detecting the instrument.

In a waveguide, a slot is made at the center of the broad side, axially. A movable probe linked to a crystal detector is introduced into the slot of the waveguide.

Operation

The output of the crystal detector is related to the square of the input voltage applied. The flexible probe permits convenient and accurate measurement at its position. However, as the probe is moved along, its output is proportional to the standing wave pattern, which is formed inside the waveguide. A variable attenuator is employed here to obtain accurate results.

The output VSWR can be obtained by

Equation

The resulting figure shows the different parts of a slotted line labelled.
slotted_line_labelled

The parts labelled in the above figure indicate the following.

  • Launcher − Invites the signal.
  • Smaller section of the waveguide.
  • Isolator − Prevents reflections to the source.
  • Rotary variable attenuator − For fine adjustments.
  • Slotted section − To measure the signal.
  • Probe depth adjustment.
  • Tuning adjustments − To obtain accuracy.
  • Crystal detector − Detects the signal.
  • Matched load − Absorbs the power exited.
  • Short circuit − Provision to get replaced by a load.
  • Rotary knob − To adjust while measuring.
  • Vernier gauge − For accurate results.

In order to get a low frequency modified signal on an oscilloscope, a slotted line with a tunable detector is employed. A slotted line carriage with a tunable detector can be used to measure the following.

  • VSWR (Voltage Standing Wave Ratio)
  • Standing wave pattern
  • Impedance
  • Reflection coefficient
  • Return loss
  • Frequency of the generator used

Tunable Detector

A detector mount which is used to detect the low occurrence square wave modulated microwave signals the tunable detector. The resulting figure gives an idea of a tunable detector mount.
tunable_waveguide_detector

The resulting image signifies the practical application of this device. It is dismissed at the end and has an opening at the other end just as the above one.

detector_mount

To deliver a match among the Microwave transmission system and the detector mount, a tunable stub is often used. There are three different types of tunable stubs.

  • Tunable waveguide detector
  • Tunable co-axial detector
  • Tunable probe detector

Likewise, there are fixed stubs like −

  • Fixed broad band tuned probe
  • Fixed waveguide matched detector mount

The detector mount is the final stage on a Microwave bench which is terminated at the end.

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