London Digital PR

Digital PR

Latest News

Microwave Attenuators and Their Uses


Microwave attenuators diminish the power of a signal while keeping the waveform of the signal intact. An attenuator introduces a known amount of loss when functioning between two resistive impedances: Input (Load) Impedance Zin and Output (Source) Impedance Zout. This intentional loss in the circuit could be in order to protect delicate electronics, or as a trade-off to maximize power transferred by the circuit.


Uses of Attenuators

Fixed attenuators in circuits have a number of purposes:
1) Use in reducing voltage and in dissipating power
A measuring device will employ an attenuator when measuring signals to lower the amplitude of a signal. This protects the device from possibly damaging signal levels. They can also be used to reduce the volume of a high-wattage sound amplifier, as well as for tone-shaping the sound coming from an electric guitar.

2) Use in improving impedance matching
When the impedance of the load (Input Impedance Zin) is set equal to the impedance of the source (Output Impedance Zout), the power transfer in the circuit will be maximized, and reflections from the load are minimized. This is also called also called reflectionless matching or broadband matching.

On the other hand, a reflective attenuator reflects some portion of the input power back to the driving source. The amount reflected is a function of the attenuation level. The efficiency of the circuit would be higher if the Output Impedance were higher than the Input Impedance: a higher percentage of the source's power would be transferred to the load. Higher efficiency also means that the total circuit resistance also is increased. Attenuation is obtained by purposely introducing impedance mismatch in the transmission line. This causes some of the power to be reflected back toward the power source.

3) Temperature compensation
Ideally, a circuit's performance should not be temperature dependent. Amplifiers may exhibit reduced gain as their temperature rises. A temperature compensating attenuator can dissipate power in response to temperature change—with minimal attenuation at high temperatures, and maximal attenuation at low temperatures--to set off the amplifier's temperature dependence. The resulting circuit will thereby behave more uniformly under a wider range of temperatures.

4) Measuring gain/loss of a device
An attenuator can be used to measure the insertion loss (=loss of power upon insertion of a device).


Reflective Attenuators

A reflective attenuator will feature high VSWR (= Voltage Standing Wave Ratio). VSWR is the ratio between the maximum and minimum voltages in a standing wave, which indicates a loss of signal power resulting from the reflection. The VSWR is a figure of merit for impedance match (or mismatch). A perfect impedance match is represented by a VSWR of 1.0:1; an infinite VSWR will indicate a purely reflective attenuator. A VSWR that is slightly higher than 1.0:1 represents a slight mismatch from the ideal match, and is generally the goal sought by adding attenuators to a multiple-component design or test system.

A high reflection loss indicates that there is low insertion loss and high attenuation. A reflective attenuator will operate at a broad range of frequencies, and can reach very high levels of attenuation, depending upon the number and spacing of the PIN diodes.

Advantages of a Reflective Attenuator:
• They are inexpensive to design and build,
• They generally has very low insertion loss, and
• They can operate at high switching rates.

Disadvantages of a Reflective Attenuator:
• Large impedance mismatch limits the attenuator's usefulness.
• May be undesirable in many applications, because it may cause frequency pulling (= a change in the frequency of an oscillator due to a change in load) and power instability.


Balanced Attenuator

If the input of the attenuator is matched to load impedance Z1 and the output to source impedance Z2, the circuit is a matched attenuator, and the loss is entirely due to transmission loss and not to reflection loss. The source (input) and the load (output) may be reversed, since resistive networks are reciprocal. In the case where input load Z1 equals output load Z2, the resulting matched attenuator design is said to be symmetrical, or to exhibit network symmetry.

A special case of the matched attenuator is a balanced attenuator, where source and load have equal impedances with respect to ground.

Advantages of a Balanced Attenuator:
• The balanced attenuator features the simplicity of a standard reflective attenuator, with the added feature of providing low VSWR under all conditions of attenuation.
• It can handle twice the power of simple diode attenuators, because the incident power is divided into paths by a hybrid circuit consisting of many PIN diodes.
• It has a wide bandwidth, moderately fast speed and excellent linear behavior.

 

Bookmark and Share
© 2009 London Digital PR Ltd - Designed by The Creative Clinic