This document provides an overview of RF control products, specifically RF switches and RF attenuators. It begins with an introduction to RF control products and their applications. It then discusses RF switches in more detail, covering naming conventions, technologies, characteristics like isolation and power handling, and application considerations. RF attenuators are also discussed, including fixed attenuators, digitally-controlled attenuators, and analog attenuators. Example applications of RF switches and attenuators are provided. New RF switch and attenuator products are introduced at the end.
3. Introduction
► Control Products are defined as impacting the signal chain performance
By configuring the Chain
By adjusting the signal level
NOT amplifying or converting the signal (i.e. ideally linear and passive)
Operating Frequency above 1GHz to close to 100GHz
► RF Switches (SPST, SPDT, SP3T, SP4T, etc.)
► RF Attenuators (Digital, i.e. DAT, Analog)
► TX/RX Switches
3
15dB
24dB9dB
1.9-
2.34GHz
1.25-
1.8GHz
1.8-
2.5GHz
870-
1250MHz
2.5-
3.4GHz
600-
870MHz
450-
600MHz
3.4-
5GHz
10-
450MHz
176
8
250
0
125
0
353
6
88
4
625a
500
0
625
b
4. Application and Marketing Considerations
►ALL RF Systems/PCB include RF Switches and often RF
Attenuators
Some systems (PCB) have 30 to 50 RF switches and few attenuators
Examples:
Inserting a calibration signal in the Signal Chain
Bypassing fixed-gain amplifiers
Routing a signal to different (frequencies) downconversion chains
Routing an LO signal thru alternative frequency selective gains, before the
mixer
Optimizing the RF signal level for best noise performance, after a fixed gain
stage (LNA) or between fixed gain stages
4
5. RF Control Product: Common Basic Knowledge
► RF Devices, packaged for PCB usage
► RF “Connectorized” Components, to be used as stand alone
► Passive, i.e. always have a transfer attenuation
► RF Switches key function is to route Signals to different Signal Chains
► RF Attenuators key function is to maintain the RF Signal Levels to the
planned ranges for best system performance
► Controlled Signals are
Information signal (receive, transmit)
Clocks, Local Oscillators
5
7. RF Switches: Naming
► Single Pole Single Throw (SPST)
► Single Pole Dual Throw (SPDT)
► Single Pole “X” Throw (SPXT)
► Notes
Double Pole (e.g. in a DPDT) normally called “Differential”
RF circuits are normally single ended
7
(Ex: SP8T)
8. RF Switches: Grouping
► Electromechanical (EM) switches
Lower Reliability and Life Time
High Electrical Performance (after switching transitions)
► Solid State (SS) switches
High Reliability and Life Time
Good Electrical and Switching Performances
► Micro-Electro Mechanical switches (MEMS)
Promising both EM and SS like characteristics
Lower Operating frequencies than SS
Part of ADI IP
8
9. RF Solid State Switches: technological options
► FET-based switches
GaAs – Fast, High Frequency capability (100Hz), less robust (ESD)
Silicon – getting faster, good settling time, med Frequency (20-40GHz), robust
► Pin Diode switches
High frequency (100GHz), robust, difficult to drive/control
► Hybrid (FET and Pin Diode)
Combination of both the above
May not allow a monolithic solution
9
10. RF Switches: System Level Characteristics 1 of 2
► Architecture
Absorptive – includes a matched impedance (Typically 50 Ohms) on
the input when the switch is open, so the circuit driving the input will
see a matched impedance at all times (across the operating
frequency range)
Reflective – has no matched impedance when open, so the driving
circuit needs to be able to handle the reflected waves and power
► Control Signals (FET)
Direct Control and low power control signals
May suffer from RC delay
May require negative voltage control signals to operate the switch
► Power Supplies
Need most often to have both polarities supply voltage
The negative supply voltage can be internally generate, at the
expense of a higher system noise, from the integrated charge pump
10
11. RF Switches: System Level Characteristics 2 of 2
► DC coupling
ALL Traditional RF switches do not like DC signal through them
DC decoupling caps are needed, unless a 0Vdc can be guaranteed
Decoupling Caps will impact the low frequency performance
New Technology in development with DC handling capability
► Power consumption
Low static power consumption (100-300uA)
► Reliability (SS)
Highest among available technologies
GaAs switches have low ESD threshold, requiring specific care
11
12. RF Switches: Electrical Characteristics
► Frequency Range (from 1-2GHz to 80GHz+)
► Insertion Loss (normally lower than 0.5dB, or max 1dB)
► Return Loss (normally higher than 10-15dB)
► Power Handling (the higher the better, now around 30dBm, moving to
40dBm+)
► Isolation (normally in the 25-50dB, frequency dependent)
► Distortion/Linearity (normally high, above 50dBm)
► Switching Speed (from 10ns to 1us depending on used processed, GaAs
is faster, Si is slower)
► Settling Time (from 100ns to 5us depending on the fabrication process
used, Si is slower, but has a shorter settling time)
► Noise (Leakage, switching)
12
13. Operating Frequency Range – Insertion Loss (IL), Return
Loss (RL)
► Frequency Range is commonly the FIRST selection criteria for
switches
► Insertion Loss is mostly impacted by
The switch’s direct intrinsic resistance
Reflected Loss as of the switch resistance
Leakage paths, which reduce the useful signal power to the load
IL must be compensated for by other circuits or taken into account
when performing level planning (as it impacts noise performance)
► Frequency dependent IL
Low Frequency IL is affected by
Any decoupling Cap at the In/Out leads
Power handling (see later) capability
High Frequency IL is affected by
Intrinsic parasitic capacitances
In band IL variation is affected by
Return Loss, caused by impedance mismatching
► Return Loss (RL)
Indicates the amount of power not transferred over the switch (but
reflected back)
Depends on the impedance matching (50OHM) of the switch with
the externally connected devices
13
Return Loss
Insertion Loss
14. Signal Power Handling and Distortion
► Switch capability to handle high power signals
Allows to deploy switches closer to the system RF
connectors
Decreases at lower frequencies, as limited by process
technology and switch architecture
Defined while the switch is static or switching (Hot
switching power)
Described by P1dB (or PSAT)
► Distortion
Measured by IP3
Related with Power handling
Approaching P1dB non linearity increases (IP3 decreases)
Normally the Switch is not the critical Signal Chain block
on distortion
14
Power De-rating compared to the
required nominal power
the switch can operate at
IP3
Decrease at low frequencies
15. Switching Speed & Settling Time
► Switching Speed (or Time) – Time from 50%
of switching control Input to 90% of the RF
signal out.
► Settling Time – Referred to the time the RF
signal has set to 0.05dB or 0.01dB from its final
time
Settling Time is a particular challenge for GaAs
switches
15
Switching
Time
16. Isolation
► Signal going through the switch when it is OFF
From the common I/O to any other port
Between two different ports
► Normally NOT the coupled noise from the control
ports (see later)
► FET switches have very high low frequency
isolation
Drain-Source Cap is limiting high frequency
isolation
This can be improved by shunting the input port to
ground when the switch is open (implemented
inside the switch itself)
► Pin Diode Switches have good high-frequency
isolation (and poor isolation at low frequencies)
16
RF1
RF2
RFC
17. Other Noise and Sources of Interference
► Video leakage or feedthrough
Describes the noise from the control ports to the
RF ports
Normally measured when no RF is present
(reported in mV)
Lower in FET switches
Critical is some applications (for example when a
high gain AGC amplifier follows the switch)
► Power Supply Noise
From external supplies – to be managed by proper
filtering
From internal bias voltages (normally negative)
generated by an integrated charge pump
Typically avoided by high end applications (T&M)
17
18. RF Switches - Application Topics
► RF Ports Coupling - Traditional RF switches are not taking any DC signal
Requiring DC decoupling at the ports.
Impacting the IL at low frequencies. Proper selections of the Caps would depended on the
desired low frequency performances.
Decoupling Caps can be avoided if the operating signals have no DC component (this is
mentioned often in datasheets as “not needing any decoupling Cap”!!!)
► Power Supplies
Switches can operate from single supply or from dual supplies
A competitor has integrated the Vneg supply, with the related system noise drawback
► Power Handling
A critical system parameter, in many application, as the switch might need to be protected,
especially at low frequencies (below 10-100MHz)
Improved handling is achieve with Si based switches
► Control Signals – voltage ranges, drive
Control voltage polarity depends on the switch supplies, and can be also negative voltages
Latest switches operate with positive voltages (compatible with standard logic levels), and have
bipolar supplies
Solid state switches are easy to drive (unlike some PIN Diode based switches)
18
20. RF Attenuator Typicales
► Fixed Attenuators (“Pads”)
► Digitally-Controlled Attenuators (DAT)
Serial or parallel Control
Series of switched-in/out fixed attenuators
Resolution from 1bit to 7bit
► Analog (Voltage) Controlled Attenuators (VVA)
More complex control architecture
Preferred when in AGC loops or for high signal
level accuracy
20
21. RF Attenuators: WHY?
► Key justification for Attenuators
Achieve a more optimized signal level plan (for noise and distortion)
RF amplifiers have commonly fixed gain
RF amplifiers may not like high power input signals
21
G G
A A
Input range Pin:
From Pout-G+A
To Pout-G
Desired Output
Range: Pout
Signal Chain SNR
Improved by G-NF-IL
Max PinMax Pin=Pin+A
22. RF Attenuators: Main and Common to the RF switches
Electrical Characteristics
► Frequency Range: Operating Frequency Range
► Attenuation Range: discrete or continuous
► Attenuation Resolution (DAT): minimum nominal attenuation step, in dB. Related to the bit count and
Attenuation Range
E.G., 32dB range, 6 control bits (64 levels) gives 0.5dB resolution, to a max attenuation of 31.5dB (0dB attenuation
included)
► Attenuation Accuracy: nominal accuracy. Normally reported across frequency and attenuation range
► Insertion Loss: Attenuation across the device, when 0dB is selected (ideally no attenuation applied)
► Return Loss: reflected Power at the Input/Output ports, related to the device impedance matching
► Power Handling (P1dB, P0.1dB): maximum input signal power, the device can handle and keep operating
linearly. Normally reported across the frequency range.
► Distortion/Linearity (IIP3): see RF Switch description, normally reported across the attenuation range
► Switching Speed: see RF Switch description
► Settling Time: see RF Switch description, normally reported across the attenuation range
► Overshoot Free DAT: the Attenuator output presents no overshooting voltage, when switching between any
attenuation steps, as a consequence of how the internal attenuation switches are operated
► Power Supplies, Control Voltages: see RF Switch description
22
23. RF Attenuators: State Error
Absolute Attenuation Error at each attenuation level, across the frequency
and attenuation range
23
24. RF Attenuators: Step Error
Relative Attenuation Error at each attenuation level, across the frequency
and attenuation range
24
25. RF Attenuators: Phase Variation
Relative Phase variation from the output to the input signal, as it goes
through the different internal attenuation steps
Normally reported as max value, across the frequency and attenuation range
Also reported as graph
25
26. Analog Attenuators (VVA)
• Similar Function as with VGA, but
implemented with an Attenuator (as
“programmable wideband RF Amplifiers
are rare”).
• Key technical challenges
• Maintain Insertion Loss and Return Loss
performances in the attenuation range, across
frequency range
• Linear relationship with the control voltages
• Simplify control circuitry
26
Reference Attenuator Circuit
Discrete Control CircuitPlease see Ref.4 from the Reference List
28. 28
LO Generation
LPF
LPF
LPF
SP3T
Test In
LO
Fout
SP4T
• Remove Spurious and Harmonics
• High Insertion loss (6dB+)
• Could be Band Pass on selected Paths
Brings LO level to Max allowed
And desired by the follow-on
Circuit (eg mixer)
• Amplifies Fout
• Isolates Fout from downchain
• May be omitted, with certain Fout
Critical Parameters
• Low insertion Loss
• High Return Loss
• High Linearity
End Application Dependent
• Low video Leakage
• Fast Switching/Settling
• Power Handling
Non Critical Parameters
• Isolation
29. 29
RF Input Stage
DAT
G1
G2
LNA
IL, RL impacts SNR Sets the signal level
IP3 impacts
System linearity
Input Protections
Required for high Pin
IN
Test in
Or TX
OUT
SW1
SW2
SW3 SW4 SW5
30. HMC1118LP3DE 9KHz-13GHz SPDT (New Product)
High Isolation Silicon Switch
► Features
Non-Reflective 50 Ω Topology
Wideband solution with excellent Isolation 56dB @8GHz
Fast 0.1dB Settling Time of 7.5us(Critical for T&M apps)
Optimized for Low Frequency operation down to 9KHz
No DC Blocking Cap is required on RF pins.
Flat Insertion Loss across Frequency : Less than 0.2dB
variation up to 9GHz.
High Input IP3: +62 dBm @ 3.0 GHz
Optimum for High power apps: High P1dB: >+37dBm
High Power Handling: +36dBm through, +27dBm
terminated/hot-switching
Positive Control, 0/+3.3 V
Excellent ESD Rating: 2 kV HBM
RoHS Compliant 3x3 mm 16 Lead QFN Package
► Device Pin-out
30
► Electrical Specification
► Availability
Loose parts abd evaluation boards available, pre-production
Full production and general availability Q3’2015
Parameter Spec Units
Frequency Range 9 kHz - 13.0 GHz
Insertion Loss @ 0.1 GHz 0.45 dB
Insertion Loss @ 8.0 GHz 0.60 dB
Insertion Loss @ 10.0 GHz 0.90 dB
Isolation @ 0.1 GHz 81 dB
Isolation @ 8.0 GHz 56 dB
Isolation @ 10.0 GHz 35 dB
Input P1dB @ 3.0 GHz +37 dBm
Input IP3 @ 3.0 GHz 62 dBm
Switching Speed 2.7 μs
Settling Time 0.1dB 7.5 μs
Settling Time 0.01dB 12 μs
Bias Voltage VDD +3.3 V
Bias Voltage VSS -2.5 V
ESD Rating Class 2 (2kV) HBM
31. 31
HMC1119LP4E 0.1-6GHz 0.25dB LSB 7-bit (New Product)
Overshoot Free Digital Attenuator
► Features
7-bit 0.25 dB LSB Steps to 31.75 dB
High Input IP3: +55 dBm
Overshoot Free Operation
Low insertion loss of 1.2dB @2GHz
Typicalical step error of ±0.2dB
TTL/CMOS compatible contol interface
High ESD robustnest of 2KV HBM
Single +3.3V to +5V supply
RoHs 4x4mm SMT compliant package
► Device Pin-Out
► Availability
Loose parts and evaluation boards available. X-Grade production.
Full production and general availability Q3’15
► Electrical Specification
Parameter Spec Units
Frequency Range 0.1 – 6.0 GHz
Atenuation Resolution (LSB) 0.25 dB
Attenuation Accuracy : ± 0.25 (3%) dB
Insertion Loss @ 0.1 GHz 1 dB
Insertion Loss @ 2 GHz 1.2 dB
Insertion Loss @ 4 GHz 1.7 dB
Phase var. over attenuation range @ 2 GHz 16 deg
P0.1dB @ 0.1 GHz 32 dBm
P0.1dB @ 4 GHz 32 dBm
Input IP3 @ 0.1 GHz 55 dBm
Input IP3 @ 4 GHz 52 dBm
Input Return Loss < 6 GHz 17 dB
Output Return Loss < 6GHz 18 dB
Supply Voltage +3.3 to +5 V
Control Interface Ser./ Par -
Control Voltage 0/3.3 or 0/5 V
Switching Speed
tRise, tFall (10 / 90% RF)
tON , tOFF (50% LE to 10 / 90% RF)
270/190
320/210
ns
ESD Rating Class2 (2kV) HBM
32. Bibliography
1. Agilent Technologies, “Understanding RF/Microwave Solid State
Switches and their Applications”,
http://cp.literature.agilent.com/litweb/pdf/5989-7618EN.pdf
2. D. Fischer, R. Lourens, P. Bacon, “Overview of RF Switch Technology
and Applications”, Microwave Journal, July 2014
3. National Instruments, “Complete Switching Tutorial”,
http://www.ni.com/tutorial/3118/en/
4. “Designing with the HMC346MS8G Voltage Variable Attenuator”, Hittite
Microwave, Product Note
5. Gary Breed, “A Review of RF/Microwave Switching technologies”, High
Frequency Electronics, May 2010, pag.70
32
Prepared by Salvatore Napolitano, Salvatore.Napolitano@analog.com
Control Products, by ADI, include mostly Switches and Attenuators. So strictly speaking they do NOT enhance signals (performance) characteristics, but ideally they do not change them. They are though very important building blocks on any RF systems, as they introduce flexibility in the signal routing (Switches) and in the signal levelling (Attenuators), which are key system level characteristics of any RF system.
In the picture above, two simple examples are shown, where on the left side a combination of switches, attenuators and amplifiers are producing an output signal with many possible signal levels. On the right side, the input signal can be filtered thru a set of low pass filter, in order to optimize its harmonic content, as required by system specs.
The number of switches and attenuators in an RF system is most often underestimated. Application examples are listed above, but there many more possible ones. So it is always a good question asking RF designer whether they need RF switches or attenuators, which kinds they need and how many they are considering using.
RF Control products, like other RF products, can be packaged in different way, depending on the operating frequency range (most often very high frequency products are used as die, on PCB or hybrids) and on their system usage (in sealed and connectorized, they could be used as input devices, for selecting and adapting inputs signals.
RF Control Products are, from the signal handling point of view, passive devices, i.e. they do not add any power to the processed signals. These can be information signals (receive or transmit signals), system control (digital configuration signals) or timing signals (LO, clocks).
This again to stress the broad usage and presence of Control signals in RF systems.
The switches can have different configurations.
The SPST is a simple open/close switch, which will break or close a signal path.
The SPDT is router from 2 paths to 1 path (in analog language, it would be a bidirectional 2-1 mux/demux). Similarly for the SPxT versions.
They could be multiple switches in a package, with independent control, as an option. In case of a dual device, it could be considered a differential device, as in the analog language.
It must be noted than RF signal chains, at PCB level, are often single ended. Also crosstalk in RF system is a quite complex topic, when dealing with multiple devices.
The Switch function can be implemented with different technologies and each has its own advantages and disadvantages. In ADI, we focus on Solid State RF switches, where the switch element is a semiconductor device.
There are also different kinds of solid state switches. ADI is focusing on Transistor (FET) based switches.
An RF switch can be implemented with a properly biased (RF) diode, which traditionally requires complex control/biasing configurations.
In RF signal chains, impedance matching is very important, as it impacts directly power conservation (i.e. signal levels), minimizing reflections and interferences. So quite commonly a switch includes an integrated 50 OHM impedance on the interrupted signal path (it is assumed that matching on the switched in path is provided by the component down the signal chain after the switch). In this case the name Absorptive switch is used, rather than Reflective.
Controlling the switching element is also a key functionality, as this impacts the switching speed and required driving circuitry. In some case for a robust switching negative polarities are used. So the switches might have a negative power supply too.
There are competitor’s, which have integrated the negative supply generation (with a charge pump), providing a system level cost saving. The drawback is an increased noise level (as the charge pump is clocked), which is very undesired by most RF designers.
RF signal chains are normally DC decoupled and so are RF switches. It is expected that decoupling caps are used at the input/output of the device. A compromised between their size and RF performance, and their impact on the lower frequency range is normally required.
Some (from competitors) datasheets reports NOT to need any decoupling caps. It must be noted that this is true just when no DC voltage is applied at the switches.
New switches are being developed, with the capability of handing also DC signals, but these are normally application specific switches.
Switches are passive devices, so they consume very low power, when not switching, beyond the small bias supply. Dynamic power is consumed during the switching.
Switches are very robust devices, when used within the prescribed signal power levels.
These are some (of the most important) parameters used to characterized switches. Their importance would of course depend on the specific application scenario.
For example, Signal Power handling can be a key feature, when a switch is deployed at the input of a Test equipment, as the actual input signal can be unpredicted, so proper protections are needed, to protect the system signal chain. The protections could be placed at the input or at the output of a switch, depending on the switch Power Handling capability, and this would impact their cost (the switch would eventually introduce a fix attenuation, as required by the chain signal handling requirements).
The most common (and first) selection criteria is the operating frequency range of the system, or signal chain of interest. Within the frequency range, the switch shall have ideally no loss. In reality a minimal loss is present and increasing toward the upper side of the operating frequency range. The absolute loss is in the 0.5-1dB range, with an additional 0.5-1dB along the frequency range,
The Return Loss (and the input and at the output) is a measure of the Input/Output Impedance matching, so it must be related to a reference Impedance (typically at 50 OHM). In the RF world, a mismatch is related to reflections to the coupled components, with a loss of signal power (along the operating frequency range_. Only in extreme cases, impedance mismatches can generate power handling issues.
Return Loss is also presented along the operating frequency range, as effectively its impact can be different, depending on the actual handled signal (its bandwidth and frequency range).
An ideal switch will carry any signal power and with no distortion. A real RF switch can actually handled only a limited signal power and this would depend on the signal’s operating frequency. As RF switches are optimized for high frequencies, their signal power handling capability is lower at low frequencies. This is sometime described at power derating values, compared to the nominal power handling at high frequencies. More and more customers are wanting higher power handling capabilities at low frequencies, so this is one of the new design focus areas.
The handled signal power can when the switch is in a static position (i.e. signal power going through the switch) or while the switching is transitioning from one state to the other (Hot switching). Both values are normally reported, as they are normally different and have different impacts, depending on the application usage scenarios.
The signal power is represented by the P1dB value, i.e. a signal power (expressed in dBm) 1dB lower than the power level, where the device starts distorting the signal (i.e. not being linear any longer). It can also be represented by the Psat value, which is the value after which the signal is distorted.
NB dBm is the signal power, referred to 1mW of power, in most cases operating with reference impedance of 50 OHM.
Signal distortion is represented by the IP3 value (Intercept Point of third order), which is also a frequency varying number. Switches have normally quite high IP3 values, compered to other RF building blocks, so they are not a limitation at system level (this actually depends on where the switch is positioned in the system, so it is adviceable to briefly discuss the matter with customers and assure no issue is foreseeable).
Switching speed is important is application, where signal paths are changed dynamically (for routing option and/or level management). Similarly Settling time is critical in some applications, like measurement equipment, where the switch attenuation range cannot no be calibrated/compensated in real time and its settling must be completed, before the signal can be considered valid.
Signal Isolation is a very important parameter, it is related to crosstalk and interferences from other signals. It normally refers to coupling from other signal paths, and not from control signals, which tend to be more of a the wideband type. As expected, isolation would depend on the frequency ranges, so a graph is always provided, as its impact would depend on the signal frequency range.
As shown, isolation performances are different with the various switches technologies, so it is important to discuss the topic with the designer, and understand which competing technology he/she would consider.
Anyhow, Pin Diodes are in general very competitive at very high frequencies.
Noise and interferences from control signals, are named video feedthrough. Their nature is mostly as wide band noise and are normally there only during transitions. At system level, there could be coupling noise from the control pins, in case those line were very noise. So designers have to make sure the control lines are not noisy, but still keeping control signals fast.
Some of the important system level application issues, once a switch has been selected are
RF ports coupling, as related to the low frequency insertion loss
Required power supply, which can be positive and/or negative
Power handling, which may impact how to protect the switch, and in which position can be used. Power handling can be static (RF power) or during switching (Hot Switching), i.e. the switch is changing state, while an RF signal is applied at its inputs
Control signals polarity and driving complexity (as related to the switching speed)
RF Attenuators share many of the system level considerations and electrical parameters with the RF Switches. So in the following pages, only those specifically related to Attenuators will be addressed, after a short mention of those share with the RF Switches.
The RF Attenuators can be fixed, digitally controlled and analog voltage controlled. The most common ones are the digitally controlled one (DAT), as they are easier to use/control and more flexble.
Fixed attenuators are used for specific usages,
Analog controlled Attenuators (VVA) are most commonly used when the Attenuator is part of a control loop and good accuracy is desired.
Analog Devices has products in all 3 of categories, most new products are in the DAT family.
Attenuators are used very often as input stage of an equipment, to manage (limit) the signal level to the active electronics (for example a Low Noise Amplifier, LNA, will not work properly or will break, when subjected to a high input signal).
Also when the signal level must be accurately set, being RF amplifiers mostly with fixed gain, an attenuator is used to set the gain accurately. Attenuators, by definition, deteriorate the signal chain signal-to-noise performance.
NF is the Amplifier Noise Figure (noise added by the Amplifier)
IL is the attenuator Insertion Loss, acting also when 0dB attenuation is programmed.
This is the list of most important Attenuators specifications, used by designers to select an Attenuator. These are quite common to switches and will not be developed in the following pages.
1. Frequency Range: is the operating frequency range, where the attenuator maintain its specified characteristics (most often the Insertion Loss)
2. Attenuation Range: is the (the difference of) max and min attenuation values for the device, not including the intrinsic device attenuation (Insertion Loss)
3. Attenuation Resolution: for a DAT is the min attenuation step size (in dB)
4. Attenuation Accuracy: nominal error on each attenuation level, across frequency and attenuation ranges
5. Insertion Loss: the device attenuation, when set at no attenuation (0dB), across the frequency range. Ideally it shall 0dB.
6. Return Loss: at each port (Input/Output). Attenuators are always matched to the reference RF impedance (50 OHM)
7. Power Handling: Similarly to the switches, it is the input max power level the Attenuator can handle, conserving its characteristics (Linearity, Insertion Loss) and its power dissipation capabilities.
8. Distortion/Linearity: shown as IP3
9. Switching Speed: similar to Switches, it refers when switching between Attenuation Levels
10. Settling Time: similar to Switches
11. Overshoot Free DAT: when switching among different attenuation levels, depending on the digital coding/decoding implementation, the attenuator could present attenuation spikes, which will eventually transferred thru the Signal Chain. This specification is very important, when Attenuation is changed on the fly, with a signal through the device.
The Attenuation Error is normally shown for each attenuation value and across the operating frequency range, or viceversa (at specific frequency values, across the attenuation range). Depending on the specific usage scenario, the designer will be able to calculate the max signal chain error. Here the absolute attenuation error is shown.
This is the relative error at each attenuation level, across attenuation range and frequency.
As RF devices are complex, when implementing different attenuation levels, the signal could go through different processing blocks. This could imply a different phase delay at each attenuation level, which combined with a natural phase distortion, could make phase equalization quite a difficult (by feasible) task. So minimizing phase distortion is of great value to RF designers.
Analog Attenuators are more complex device to make and use. One challenge is to control the attenuation and maintain a proper impedance matching, so special measures need to be taken, within the device and with control voltage driving circuits.
This is a simple and common switches application, where a filter bank is needed to clean the harmonic content of a frequency generation block. Ideally the switch shall not introduce any attenuation (impact on the SNR) and have very high return Loss (impact on the SNR).
Similar configurations can be found in signal generators, where a programmable sinusoid generator is followed by a filter band (LPF or BPF) to generate a clean signal. This could be followed by broadband amplifiers to reach the desired output level.
The diagram here may represent the input stage of a Comm or Test Equipment.
Input protections are normally used, though higher switch(SW1) power handling will make those cheaper or reduntant.
The first stage is a bypassable Low Noise Amplifier, which amplifies the input signal, if required. The switch will impact directly the SNR, with its Insertion Loss and Return Loss. Normally the switch linearity is not an major issue, as it is higher and better than that of the LNA. Also linearity on blocks early in the Signal Chain has less impact on the overall signal Chain linearity.
The second stage is also a level adjust stage, expanding the signal level range across the signal chain. In this blocks noise performance (i.e. insertion Loss) is not the most important, but linearity could be relevant (IP3), as more impacting the overall Signal Chain Linearity.
The second stage with a ATT and a Gain in parallel could be replaced by the same block in series.
These blocks implements the following functions
Max signal gain of (in dB) G1+G2-5xIL (sum of all switches Insertion Loss)
Max signal attenuation (in dB) A+5xIL
All switches are critical, except for SW5 on Power handling, Linearity