Breakthrough instruments and products RF/Microwave power amplifiers for electromagnetic compatibility testing

Historically, amplifier selection for Electromagnetic Compatibility (EMC) testing was based mainly on internal knowledge and information scattered across the EMC industry. However, proper EMC amplifier selection is an important step in achieving required radio-frequency (RF) levels. Considerations are numerous: Amplifier RF output power level, frequency coverage, linearity, and harmonic performance are all important performance parameters for amplifiers in EMC usage. AR RF/Microwave Instrumentation is a company specializing in EMC testing. The origin of the company stemmed from the recognition that EMC testing presents a unique challenge with amplifier operation due to conditions that can cause substantial power to be reflected back into the amplifier. If unprotected, this reflected power can cause the amplifier to reduce its output power or can even cause damage to the amplifier. Building amplifiers with rugged mismatch-tolerant class A amplifier design, where the RF output transistors are biased for the full 360° of the input signal, allows continuing operation despite high reflected-power conditions, driving loads of any magnitude or impedance, without reducing output power, and thus became the start of AR’s amplifier product line. The U series family of amplifiers are the most recent product family to be introduced by AR. The U series amplifiers are a breakthrough product for the company as it provides a very wide instantaneous bandwidth of more than 16 octaves, 10 kHz to 1000 MHz. This exceedingly wide bandwidth makes this amplifier series very versatile for many uses where wide frequency coverage in a single amplifier band is desirable. The U series of amplifiers extended frequency range combines the frequency coverage of the A series and W series amplifiers designed by AR RF/Microwave Instrumentation.

Amplifier output power is always an important aspect when discussing amplifiers, and especially so when combined with the mismatch tolerance characteristic provided by AR amplifiers, including the U series family of amplifier models. The U series includes ten models, covering 1 up to 500 W power levels. The U series is only one of a number of amplifier families offered by AR, with a broad range of frequency coverage and power levels, some catalog models of 10 kW Continuous Wave (CW) power rating, and specialized custom models reaching 50 kW CW and above.¹ 

The U amplifier series is a family of amplifiers with available power levels up to 500 W power ratings and a broad frequency range and power levels. When comparing amplifier power levels, an understanding of the terminology is needed, as well as a realization that manufacturers differ in their power rating methods. Therefore, specifications of the amplifier should be reviewed closely. An amplifier output power can be specified in various manners/ways. Since the amplifier power will vary across the frequency band, a single power rating will not fully capture the power profile. A single power rating may be the maximum power, minimum power, or average/nominal power. Additionally, the rating may be indicative of the saturated power or the linear power. The power rating can provide guidance to the amplifier power output level; however, what this number conveys, and the details of the output power profile, need to be understood. CW power is shown in Fig. 1. This is the power the amplifier will produce at a given frequency with 0 dBm input signal level.

Figure 2 shows compressed power vs frequency performance for two levels of input, 1 and 3 dB. Ideally, an amplifier will operate linearly, where a 1 dB increase in the input signal will result in a 1 dB increase in the output signal. At signal levels where this is the case, the amplifier is considered operating in its linear range. The test for linearity is commonly performed by increasing the input level by 10 dB and verifying that the output level also increases by 10 dB. As input signal levels increase, however, the amplifier will begin to transition into compression: it is said to be at 1 dB compression when an input increase of 10 dB produces an output increase of 9 dB. This is referred to as the 1 dB compression point or 1 dB power level. Similarly, the power level where a 10 dB increase of the input level produces a 7 dB increase in output power is the 3 dB power level. The amplifier is said to be fully compressed or saturated or at maximum power when there is no increase in the output power when the input power is increased.

Variation of amplifier gain across the frequency band is termed gain flatness. Minimizing this variation can be beneficial for some amplifier usages. Designing an amplifier that produces a flat gain response increases in difficulty as the frequency range of the amplifier increases. AR has achieved breakthrough gain flatness of its 250U1000A amplifier encompassing a frequency spread of over 16 octaves.

With a wideband amplifier, harmonics (Fig. 3) of the fundamental frequency will fall within the frequency range of the amplifier over a substantial portion of the bandwidth. Minimizing harmonics in a wideband amplifier requires careful circuit design. AR amplifiers are designed to provide excellent harmonic suppression performance.² 

The 250U1000A amplifier (Fig. 4) is a solid-state design that provides improved reliability over TWT (Traveling Wave Tube) amplifiers and is characterized by air-cooling with no internal liquids. The amplifier will faithfully reproduce amplitude modulation (AM), FM, or pulse modulation appearing on the input signal, and with 0 dBm input, no preamp is needed, thereby reducing the need for additional equipment. It includes adjustable gain, local or remote operation, and a graphical digital color touch-screen display for amplifier control and status display. The amplifier also provides status and fault indications to provide information on operational state and detected faults. It also has GPIB/IEEE-488 format, RS-232 9-pin D and fiber optic, universal serial bus (USB), and Ethernet communication.³ 

U series amplifiers have been specially designed to excel in the challenging environment that is found in EMC testing usage where they face difficult operating conditions while the amplifier must continue to operate and meet its performance criteria. High on the list of requirements is the ability to operate into high VSWR (Voltage Standing Wave Ratio) conditions. When performing immunity EMC testing, the amplifier will often be subjected to high levels of reflected power due to reflections from the test object or due to impedance variations when using injection clamps. Amplifiers must be rugged to allow operation under these conditions and continue to produce RF power.⁴ Single-band amplifiers with wide instantaneous bandwidths are beneficial, as a single wide-bandwidth amplifier can replace two or more narrow frequency amplifiers, thereby simplifying test setups and reducing equipment cost. The U series amplifiers provide RF performance into varying impedances and reflected power levels, very wide bandwidth, and excellent linear power and harmonic performance, making these amplifiers well-suited for use in EMC testing.

The wide frequency bandwidth, combined with class A fully mismatch-tolerant designs, increases the capability and flexibility of the U series amplifier models. In addition to uses in EMC testing, these features make the U series amplifiers useful in applications and uses in medical and physics research, laboratory testing, antenna and component testing, and watt meter calibration, among others. Wideband amplifiers are well suited for research and development laboratories and general lab use, where wide frequency capabilities provide maximum flexibility, for example, in driving various impedance loads such as piezo-electric devices. Amplifiers used for plasma generation or energizing/ionization of gases present challenging conditions to an amplifier, as the load impedance seen by the amplifier will be highly variable and will vary substantially and rapidly depending on the state of a given gas. Amplifiers such as the U series are designed to operate under these types of challenging applications into various and changing load impedances. In the wireless industry R&D, where a wideband amplifier can span multiple cell system bands, use would primarily be for development purposes, as dedicated narrow-band amplifiers would be more cost effective for deployment.

If a pulse signal is inputted into an amplifier RF input, the rise/fall time of the signal on the amplifier output will be the fastest rate the device can provide, which will be determined by the highest frequency it can attain. In the case of U series amplifiers, this limit is 1 GHz (1 ns period leading to 0.25 ns rise/fall time maximum). During the fixed steady-state portion of the pulse, the amplifier output will not hold at a steady fixed DC level but will rather fall toward 0 V at the rate of the lowest rated frequency, which for the U series is 10 kHz (100 µs period). To create flat pulse tops, the lower the frequency rating of the amplifier the better. The wider the bandwidth of an amplifier, the better it is in achieving both of these characteristics. As an example, the wideband U series amplifiers allow performing diathermy treatment, the controlled production of deep heating in the body for therapeutic purposes at various desired frequencies.

The flexibility of an amplifier is most reliant on the frequency range, its bandwidth, and the number of applications in which the amplifier can be used. AR RF/Microwave Instrumentation’s Universal “U” series is a family of amplifiers with such a wide bandwidth that significantly increases the range and versatility in its application usage. Specifically designed to excel in challenging environments found in EMC testing, AR’s U series of amplifiers provide RF performance into varying impedances and reflected power levels, exceptionally wide bandwidth, and superb linear power and harmonics performance.

The data that supports the contents of this article are available by contacting the product manufacture at [email protected].