Question

Please give brief explanations of antennuations of RF boards and give through examples how it could be applied through voltage regulators filters with certain frequencies.

Answer 1

Higher signal levels are generally a good thing in RF circuits. They result in increased signal-to-noise ratio (SNR) and reduce the problems caused by both internal circuit-component noise as well as external-signal noise. As a result, higher signal levels generally simplify many challenges of RF-circuit design.

However, in many systems, the RF signal can unavoidably have a wide dynamic range spanning 30, 40, or more dB; some designs must deal with signals having a range of over 100 dB. Examples include radar or long-range wireless, or even short-range LANs where one or both of the link nodes are moving and there are obstacles and interference.

If a system is designed to function properly with lower-level signals, it may not have the headroom for the higher-power signals (in RF, power and signal level are usually closely correlated). The result is overload, saturation, and even possible damage to sensitive analog components such as front-end amplifiers. Even if there is no permanent damage, the system cannot function properly as long as elements of the signal chain are “maxed out.” In these cases, it may take a relatively long time for the components to come out of saturation so that the signal chain is once again viable. In other cases, the attenuator matches the maximum value of a signal at one point to the more-limited maximum of another stage in the chain.

For these reasons, it’s often necessary to manage and attenuate the signal level by a known or controllable amount, and that’s where RF attenuators play a role. There are three types of RF attenuators:

1) Fixed-value attenuators, providing values such as one or two dB, or 10 dB, 20 dB, or more dB.

2) Voltage-variable or voltage-controlled attenuators, where an analog voltage sets the attenuation level over a continuously variable range, such as between 0 dB and 30 dB or 0 dB and 60 dB.

3) The digitally controlled attenuator or digital-step attenuator (DSA), where a multibit code establishes the attenuation in discrete steps over a range of 0 dB to 32 or 64 dB, for example, in steps of one or two dB/bit; some products offer step sizes as small as 0.25 dB.

(Note that there are also mechanically-controlled attenuators, where the user sets the attenuation with a rotary knob. These are used almost exclusively in test situations or high-power one-off designs.)

The controllable attenuator is the complement of the variable-gain amplifier (VGA), which boosts a signal to match the range of components in the chain. For designs which need additional flexibility, there are even VGAs available which span both gain and attenuation such as from -10 to +40 dB; internally, these are built from a variable attenuator (voltage- or digital-control) in series with a gain block.

Key attenuator parameters guide selection

As with all components, attenuators have many design specifications which define their suitability in a given application. The primary ones for attenuators are frequency range and attenuation value.

Some attenuators are designed and specified for relatively broadband use such as from 1 to 4 GHz, while others target narrowband situations such as the nominal 2.4 GHz ISM band. Although wider bandwidth may seem to be a benefit, it is more difficult to design and costly to achieve performance over a wider band. A comparable situation exists for variable attenuators. A wide-range unit which functions over a range of 60 or more dB is more costly than one which must do so over a more limited range of 30 dB.

The two parameters of frequency range and attenuation combine for other critical specifications: attenuation deviation or flatness across the frequency range, and across operating temperature. Flatness is typically on the order of 0.25 dB or 0.5 dB and is generally harder to maintain at higher frequencies.

If the attenuator is a fixed-value unit, the flatness needs to be specified only at that single dB value. However, for variable attenuators, the flatness may vary with the amount of attenuation, so its variations must be specified at different attenuation values, such as every 5 or 10 dB. Finally, as with nearly all passive and active components, performance must be specified at nominal temperature (25⁰C) as well as the device’s rated low and high temperatures; some vendors offer temperature coefficient data as well.

Power-handling capacity for attenuators ranges from milliwatts to thousands of watts. The power rating is one factor which determines the possible package, which can be a tiny surface-mount technology (SMT) device to larger coaxial and even waveguide packages, as power levels go to upper levels. Regardless of package type, most vendors specify power ratings for both continuous-wave (CW) operation as well as peak value for pulsed operation. Depending on the duty cycle of the pulse rating used, the ratio between CW- and peak-power ratings can be a factor of ×10, ×100, or more.