Allowing the state of the art of automatic test equipment

The challenges facing the industry test and measurement of RF can be equated to the classic problem of the chicken and the egg. To make it more efficient automatic test equipment (automatic test equipment, ATE), the need for better RF IC (RFIC) and RFIC are the best, the test equipment must be able to test the limits of the CI develop. The result is that more performance RFIC drive test equipment capacity of "prior art" used for its development, opening the way to turn to the next generation of instrumentation to be used to push the boundaries once again. All this calls to maintain a cooperative relationship between the IC design centers of higher performance and ATE manufacturers market leaders.

One of the major functional components constituting any portion of a solution of RF ATE high-end RF switch is a single pole, double-position (SPDT). These devices are used in many critical circuits and can exert a powerful influence on the performance of the ATE solution. Among other important functions, the switches are used to build the digital step attenuators, high capacity (digital step attenuators, DSA, Figure 1), filter bank switching (Figure 2) and the selection of the RF signal path of ATE instrumentation.
The limitations of GaAs and SiGe to provide a repeatable switching capability in these applications for a wide frequency range are well known. As a result, the test equipment manufacturers have adopted CMOS semiconductor technology with the expectation of higher benefits. Their main expectations of silicon-based devices are that they will get a time of rapid development, superior performance at low frequency, high linearity, high performance of electrostatic discharge (ESD) and reliable quality circuit to circuit and from batch to batch.

Switching transients and fast settling time
One of the most important aspects of the ATE is a quick setup time, because it allows the instrumentation take steps more quickly, improve performance, usability and cost reduction of manufacturing test. All these enhancements add up to measurable benefits for ATE manufacturers need to succeed in a highly competitive environment. The challenge is well known that a typical switch based on GaAs shows a gate delay time of establishment (see Figure 3), resulting in a drift loss and insertion phase.
In general, ATE manufacturers specify the insertion loss in the switch for 0.05 dBm at 20μs. Simply put, the switch is stable within this limit of 0.05 dBm, the faster the ATE to provide the measure. Figure 3 shows a typical GaAs MESFET switch with a final settling time of 83μs. Note also that the GaAs switch also has an overshoot of ~ 1dB after a switching event. This overshoot means that the dynamic stability can be up to 26% higher than the final value, and if you use multiple switches, the dynamic stability will be even greater. ATE engineers have had to compensate for these transients in their designs for many years.
In contrast, Figure 4 shows the performance of the switch UltraCMOS ™ PE42552 Peregrine Semicon-ductor. Note that this device is stabilized within the insertion loss of 0.05 dBm at 13μs (faster than the ATE manufacturer's specification of 20μs). And no overshoot.
In applications where switches are used for attenuators (Figure 1) the settling time delay in switching transient (as in Figure 3) can result in errors in the attenuators or the switching function. In the DSA, for example, is important to have a precise amplitude and phase signal, so the rest of the instrument knows the correct signal level.
Unfortunately, the settling time of transients in high-performance switches, GaAs is unpredictable, which makes the "design and manufacture based on them" are a challenge to ATE companies. The use of an alternative device, like a switch silicon design eliminates these problems. To date, only the technology UltraCMOS silicon on sapphire (Silicon-On-Sapphire, SOS) has been able to offer support to these exacting requirements on the switching function.
Features & Low Linear Frequency 
The test and measurement equipment takes advantage of broadband services, making it a more attractive investment to manage multiple communications protocols. As a result, the components that comprise them must also be broadband. Many GaAs switches are specified to work from DC. GaAs switches have a typical corner frequency of 100MHz, and its operation below this corner frequency significantly degrades the linearity and problems arise in the noise factor. Currently, designers can use a single switch that provides high performance in the kilohertz range and up to 7500MHz. For example, the PE42552 SPDT switch works between 9kHz and 7500MHz with a high linearity performance for the frequency range.
Linearity at lower frequencies affects the ability of a switch used as a broadband component. In general, any non-linearity of the components in the test and measurement equipment can cause intermodulation distortion (Intermodulation Distortion, IMD), which can inhibit the ability of equipment to provide an accurate measure. This is especially complicated when the linearity of the ATE IQ is as good (or bad) as the device to test. As a result, ATE designers seeking the best linearity available. Figure 5 shows that PE42552 presents a significantly better performance than that of a GaAs MESFET switch at low frequency.

ESD Protection
GaAs MESFET switches are characterized by typical Class 0 (<250 V) or Class 1A (250 to 500 V) HBM ESD and can be damaged even by small events electrostatic discharge (ESD). The detection of this damage can be particularly problematic, so are best avoided. In response, designers have added traditional ATE ESD protection to the switches. Unfortunately, this may limit external protection circuit performance (power and dynamic range) and degrade the performance of the instrument. The benefits of silicon integration allows you to integrate ESD protection devices in the silicon switch. For example, switches UltraCMOS provide benefits HBM Class 1C Arms with RF (1000V to 2000V).


Reliable performance
Precision is a key step in the design of ATE. To achieve the required levels of performance, ATE manufacturers need to minimize the variation in the performance of the switches they use. An inefficient way of accomplishing this is shielding each batch of switches, and discard those that are of specifications. Alternatively, MEMS switches can be used, but that technology presents problems of reliability and repeatability. The ready availability CMOS switches provide performance repeatable from batch to batch due to the nature of the silicon process. When trust in the reliability of supplies of switches, ATE manufacturers can eliminate the phase preapantallamiento and accelerate the production and supply.
In addition to reliable performance in broadband and linearity mentioned above, another important parameter for RF switches that are used in the higher range of test equipment is the reliability on the insertion loss. The insertion loss is important because when there are many switches in the signal path loss for each switch is multiplied. The total loss, especially in higher power routes, resulting in a higher power consumption. The insertion loss (and hence the noise factor) of the switches can also limit the dynamic range in a receive path. Figure 6 shows the typical insertion loss for a broadband switch UltraCMOS is <1dB at 7.5 GHz, which is about 50% less than equivalent GaAs switches.
The insertion loss performance can be closely linked to linearity. For example, the RF switches based on GaAs tend to bring about an increase in insertion loss and size of the tablet with improved linearity. This is because conventional circuits require multiple FET stacked or multi-gate FET widths and large doors to achieve low distortion, which results in a high parasitic capacitances with the degradation of insertion loss. Instead, UltraCMOS technology consists of a stack of FET fabricated on a perfectly insulating sapphire substrate, which provides the ability to allow passage of signals of high power RF.


CMOS: a control
Have a CMOS interface on a device makes it easier to use for any designer, and this is also true for the design of test and measurement. Normally a logic function is performed in CMOS system, so if the switch is manufactured using CMOS processes, the chipmaker can easily include logic in the circuit itself.
For example, the PE42552 SPDT switch is designed with an integrated CMOS control logic which is controlled by a control input low voltage CMOS single pin. Another way to improve the functionality of the switch is a table with user-defined logic. Thus, a switch UltraCMOS selection includes a pin which inverts logic polarity of the logic for continuous switching applications, changing into practice the logical definition of the control pin.
As manufacturers of test equipment and measurement tools for mobile phones ready LTE (Long Term Evolution), WiMAX and possibly a convergent version of the two, the performance of their test equipment to be stretched to the max. Fortunately, they can provide the benefits of bandwidth, repeatability and accuracy demanded by its customers because the technology is commercialized UltraCMOS years, its production is massive and devices have been tested in the ATE market. Ultimately, the current RF test instrumentation must make extremely precise measurements and repeatable, and the good news is that suppliers of test and measurement equipment have access to devices that help them achieve their goals.

1 Baker, Ray. "CMOS-Based Digital Step Attenuator Designs," Wireless Design & Development Magazine, May 2004. http://www.psemi.com/articles/2004/2004_ar_5.pd

Author:

Mark Schrepferman, Peregrine Semiconductor