Signal amplification is a key function in high-frequency radios. Many newer wireless-communications systems rely on maintaining stable amplitude within the radio's downconversion circuitry even when faced with a wide range of input signal levels. Because of this, the variable gain amplifier (VGA) has become an essential building block in many wireless radio transceivers. Traditional VGAs have varied the DC voltage to set the amplifier's gain, although additional circuitry and printed-circuit-board (PCB) space were needed for driver circuitry capable of providing precise control voltages. Hittite Microwave (www.hittite.com) has developed an alternative: the digital VGA (Fig. 1). It features the performance of traditional analog VGAs, with the simplicity of a digital control interface and the added benefits of wide bandwidth, minimal need for additional components, and minimal use of PCB space.
The new DVGAs were born out of the market need to finely control the gain increments and maintain increased stability over a wider frequency range. Important design requirements included wide dynamic range, with high output third-order intercept point (OIP3) and low noise figure, but also a wide gain control range with fine-resolution steps, uniformity in performance across frequency and different gain levels, and monotonicity in gain tuning across a wide operating bandwidth. The resulting DVGAs, models HMC625LP5E, HMC626LP5E, and HMC627LP5E (see table), are designed to cover all of the major wireless standards/bands, including Global System for Mobile Communications (GSM), code-division-multiple-access (CDMA) cellular, wideband CDMA (WCDMA), Universal Mobile Telecommunications Service (UMTS), wireless local-area networks (WLANs), and WiMAX applications. All three variable-gain amplifiers are housed in a tiny 5 x 5-mm plastic QFN leadless surface-mount packages (LP5E leaded versions are also available for all models) so that users have the flexibility to interchange devices for specific system needs. All three DVGAs offer total gain-control range of 31.5 dB, achieved through combinations of signal attenuation and positive gain.
For example, model HMC625LP5 provides the widest bandwidth, at DC to 6 GHz, with typical gain of 18 dB from DC to 3 GHz and 13 dB from 3 to 6 GHz for the maximum gain setting. The 6-b DVGA provides gain settings of -13.5 to 18 dB in 0.5-dB steps. When set for maximum gain, the HMC625LP5 achieves a healthy dynamic range, with noise figure of only 6 dB and OIP3 of +33 dBm from DC to 6 GHz. Model HMC625LP5E also boasts typical input return loss of 15 dB and typical output return loss of 12 dB. The output power at 1-dB compression is typically +19 dBm from DC to 3 GHz and typically +16 dBm from 3 to 6 GHz.
The DVGA's digital control feature is flexible as well as precise, allowing for a serial or parallel interface as well as the selection of a preferred power-up state. Traditionally, achieving a certain level of gain in a transceiver front end involved setting multiple amplifiers to high or low gain states and then cascading them to achieve fine increments of gain variation. This same level of gain control and range can be achieved with one DVGA.
The use of a DVGA rather than an analog VGA offers some significant advantages to front-end designers as it allows the flexibility to operate the DVGA's integrated amplifier at a constant gain state, limiting its nonlinear behavior while controlling the gain output using other circuitry within the DVGA. As a result, the nonlinear contributions of the integrated amplifier's distortion or OIP3 are minimized, extending the upper limits of the available dynamic range. At the other end of the dynamic range, the noise figure of the DVGAs is very stable with frequency, also due to operating each DVGA's integrated amplifier at a constant gain state and adjusting gain with additional circuitry. In contrast, the noise figure of a traditional analog VGA tends to vary across frequency and with gain setting.
In the 6-GHz model HMC625LP5 DVGA as well as in the 1-GHz model HMC627LP5, the integrated digital control feature includes a dual-mode interface that is CMOS and TTL compatible and accepts either a three-wire serial port interface (SPI) or a 6-b parallel word. A user selectable power-up state and a serial output port provide addition flexibility for bench-top evaluation as well as cascading with other devices employing serial control.
Figure 2 shows maximum gain for the HMC625LP5 DVGA as a function of frequency for the major control states. In Fig. 3, the major gain states versus frequency are normalized to the maximum gain state as a reference, to highlight the uniformity of performance for all gain (and attenuation) states across the wide 6-GHz bandwidth.
Figure 3 not only indicates the relative state flatness over the frequency range but the overall dynamic range that can be realized. Figure 4 presents data for bit error versus frequency for the major gain states of the HMC625LP5 DVGA. As the plots show, bit errors never result in more than +0.6 dB amplitude deviation over frequency and the 32-dB dynamic range. It is specified for gain step errors of typically ±0.25 dB across the 6-GHz frequency range.
For applications where optimum performance below 1 GHz is of primary importance, the lower-frequency model HMC627LP5 provides a wide dynamic range with OIP3 of typically +36 dBm and noise figure of only 4.3 dB from DC to 1 GHz. It offers a 31.5-dB gaincontrol range courtesy of its 6-b digital control, with typical gain of 20 dB from DC to 500 MHz in the maximum gain setting, and typically 17.5 dB from 500 to 1000 MHz in the maximum gain setting. The output power at 1-dB compression is typically +20 dBm across the frequency range. The input return loss is typically 17 dB while the output return loss is typically 12 dB. Model HMC627LP5 draws 90 mA from a single +5-VDC supply.
The remaining device in the new DVGA family, model HMC626LP5, is the high-gain member of the trio. It provides twice as much gain as the other models, but maintains the 6-b controllable 32-dB dynamic range. As with the HMC627LP5, it covers DC to 1 GHz, but can deliver anywhere from 8 to 42.5 dB gain with only 2.8 dB noise figure. Model HMC626LP5 achieves an OIP3 of +36 dBm across the frequency range with +20 dBm typical output power at 1-dB compression. As with its DVGA counterparts, it promises gain accuracy within ±0.25 dB for each digital gain setting. It operates with only 176 mA current from a single +5-VDC supply.
A key design challenge for most DVGA vendors is the ability to maintain a monotonic (gain states do not overlap over frequency) and uniform (gain states do not separate by a large amount over frequency) state relationship across the entire frequency band. Figure 5 shows that the new DVGAs maintain precise gain levels with each digital gain setting, with the maximum step error never exceeding the least-significant bit (LSB) of +0.5 dB across the entire frequency range for any amplifier model. This ensures a monotonic relationship between the different gain states. On the other side, uniformity is maintained provided the step error does not exceed a negative level greater than the LSB or in this case -0.5 dB. It is critically important that the DVGA maintains a monotonic state relationship over its operating bandwidth otherwise gain states will overlap and a digital logic input may not result in the desired gain output. As Fig. 5 shows for the HMC625LP5 DVGA, the versatile amplifier exhibits consistent performance across a wide frequency range.
Figure 6 shows a block diagram for a base-station transceiver, highlighting where the DVGAs fit. The three DVGAs are supplied in 32-lead 5 x 5-mm surface- mount-technology (SMT) packages. The 50-ohm amplifiers are simple to use, requiring no external matching components. Hittite Microwave Corp., 20 Alpha Rd., Chelmsford, MA 01824; (978) 250-3343, FAX: (978) 250-3373, Internet: www.hittite.com.
