MICROWAVES CHAPTER 8

DIRECTIONAL COUPLERS

A device that consists of two transmission lines with no direct connection, placed very close to each other so that a portion of the energy in one line is present in the second line.

directional

means, basically, that energy is passed in one direction only. Any energy from a reverse direction will not pass or allow the component to operate properly.

insertion loss

• The energy path from ports 1 to 2

• determined by how far the coupled line is from the main input line

coupling loss

• The energy path from ports 1 to 3

• amount of input signal that appears at port 3 (in decibels) compared to the total energy that is at the input.

isolation

• The energy path from ports 1 to 4

• tells how good a directional coupler you have.

Applications of directional couplers

A. Monitor circuits
B. Leveling circuits
C. Power measurements
D. Reflectometers

QUADRATURE HYBRID

What we have in this are the two output signals. They are 90° out of phase with one another in quadrature; that is, there are two outputs from a quadrature hybrid, and if you measure the phase relationship between them you will find a 90° difference.

Quadrature

Defined as “any two objects that are at right angles to one another.”

quadrature hybrid

a specialized coupler that takes an input signal and equally divides it between its two outputs.

There is a high degree of isolation between the two output ports and between the two input ports

Amplitude balance

a comparison of the power levels at the two output ports.

phase balance specification

measure of how well the phase difference tracks over the frequency range of operation.

applications of quadrature hybrid

A. Matched detector
B. The process of combining amplifiers
C. Switches using hybrids

MATCHED DETECTOR

designed to take an RF signal at the input and use the matched diodes to convert it, very efficiently, to a video or dc signal, as needed.

quadrature hybrid and matched diodes

these components are what make this circuit work as well as it does.

The combination of basically equal output power levels from the hybrid, and the isolation between input ports and output ports make this a very good component to use when a detector is needed.

COMBINING AMPLIFIERS

• This employs back-to-back quadrature hybrids. The outputs of the amplifiers similarly are applied to the two input ports of the output quadrature hybrid.

• If we apply two signals to the input, the hybrid combines the signals at the output port.

• As a result, we have a small input power to the entire component and an output that is the result of the two amplifiers being used.

SWITCH

• When the diodes are forward biased, the energy goes from the input port, through the hybrid combination, and out the output 1 port with a very low loss.

• When the diodes are reverse biased, the energy coming into the device will be reflected back toward the input. But because a hybrid junction is present, the energy does not get back to the input but is sent to the other port, output 2.

POWER DIVIDER

Amplitude balance tells how closely the output ports track one another.

The phase balance is ideally 0 degrees.

One application in which both quadrature hybrids and power dividers are used is an image-rejection mixer.

Input-matching circuit

DETECTOR
matches the diode impedance to the characteristic impedance of the system (usually 50Ω).

Detector sensitivity

refers to the minimum input signal level required to make the detector operate and the amount of output voltage obtained for a certain amount of input power.

the input power and the output voltage.

Two factors affect sensitivity:

Diode (or a set of diodes)

DETECTORS
performs the detection process.

Low pass filter circuit

DETECTORS
removes any RF from the output if either a dc or video output is desired.

Applications of detector

A. Monitoring circuit
B. leveling circuit
C. Spectrum analyzer

The input coupling network, The diode circuit, Filter circuit

Elements of a mixer

input coupling network

Elements of a mixer:
combines the RF and LO signals

diode circuit

Elements of a mixer:
takes the RF and LO signals and mix them together to form new signals.

Fixed attenuator
Variable attenuator

Types of attenuator

step attenuator

a series of fixed attenuators that are switched in and out as needed to give the selected value of attenuation.

continuously variable attenuator

continuously varies the attenuation to any value within the range of the attenuator.

Band pass filter (BPF)

passes a specific band of frequencies and rejects frequencies below and above that band.

Passband

the area where there is a minimum loss in the filter response.

Filter skirts

on either side of the passband. This is where the signal is greatly attenuated.

Insertion loss

the internal loss through the filter within the passband.

The ripple

a result of the of sections (or poles) used to construct the finished filter.

rejection

tells how much an undesired frequency is attenuated on the skirts of the filter.

Side-coupled half-wave resonator filter

It has a series of resonators side by side with designed gaps between them for coupling purposes to obtain the final response.
- Types of Bandpass filter

Short-circuited quarter-wave stub filter

In this type of filter, all the elements are a quarter-wavelength long. The ends of the extended resonators are shorted to ground, which results in the proper bandpass response.
- Types of Bandpass filter

Inter digital filter

This type of filter has a series of quarter-wave resonators grounded at alternate ends to form the required bandpass response.

Applications of BANDPASS FILTER

• To reduce noise and harmonic content of a system by limiting the bandwidth of the signals seen by that system.

• To a receiver for improved selectivity.

• To “clean up” the system response.

• They are used at the outputs of signal generators in the laboratory to obtain purity of the signals being sent to a test setup.

LOW PASS FILTER (LPF)

LPF passes frequencies below a certain frequency with very little loss and attenuates frequencies above that frequency.

Applications of lowpass FILTER

• To provide the difference between the RF and LO inputs at the output of a mixer.

• To remove the spurious signals and harmonics from generators or systems in which too many signals may be present. Many times, an LPF is placed at the output of a generator or source that is rich in harmonics.

HIGH PASS FILTER (HPF)

passes frequencies above a specific cutoff frequency.

Applications of highpass FILTER

• To attenuate low-frequency signals that cause problems in a system and help the system operate properly.

• It also may help when a test generator has a high degree of low-frequency signals.

• To reduce the low-frequency gain that some high-frequency transistors have.

Duplexer

Applications of a circulator
- used when a transmitter and a receiver have to use the same antenna.

CIRCULATORS / ISOLATORS

- It is used with a tunnel diode to produce an amplifier.

Applications of an isolator

• To keep the high level of an LO in a mixer circuit in the mixer itself and not have it radiate out through the incoming antenna.

• It is designed to supply a constant load to an oscillator circuit.

AMPLIFIERS

A device that increases the level of a signal to produce the required amount (voltage or power) at the output.

Low noise amplifier

• An amplifier that exhibits very low noise characteristics.

• It used many times at the “front end” of a receiver.

• It provides sufficient gain to allow it to be the controlling factor in determining the noise characteristic of the entire receiver.

• It usually has a very low-noise figure.

• Hence, the overall noise characteristic of such a receiver will be excellent.

Low-level linear amplifier

It is sometimes called the intermediate amplifier.
The currents that these amplifiers draw are higher than the low-noise amplifiers.

Power amplifier

• It is an amplifier that produces much higher amounts of output power.

• It is not a linear device. There has to be a certain amount of power at the input to produce an output power.

OSCILLATOR

• a device that provides a sustained unstable signal that can be used as a frequency reference in a testing system or communications system or to provide a certain frequency to produce a required tone.

Components of an oscillator

DC voltage
Gain
Positive Feedback
Frequency-determining components

Barkhausen Criterion

To sustain oscillation

1. The total Gain must be equal to 1

2. The total phase shift must be equal to 0 or 360 degrees

Colpitts oscillator

LC Oscillators which has the capacitance portion of the LC tank circuit split into two capacitors with one in the feedback path.

Clapp oscillator

LC Oscillators which is very similar to the Colpitts except that the Clapp has an additional capacitor in the inductance side of the circuit that is used as a fine adjustment for the frequency of the oscillator.

Hartley oscillator

LC Oscillators which is an LC oscillator with the inductance portion split instead of the capacitance.