Improve System Integration and Performance with a New Family of Variable Gain Amplifiers (VGA)
By Richard Houlihan, Hittite Microwave Corporation

A fundamental function of the radio portion of all wireless systems involves signal amplification. Naturally, there are many different amplification requirements as one navigates the modern transceiver, but as a result of new system demands and the requirement to utilize critical building blocks within their optimum range, the need for signal level stability has become more and more critical. This heightened importance has focused attention on the variable gain amplifier (VGA) and the use of its primary function, to maintain a constant output over a specific range of input signal intensity. As a key contributor to component and system evolution, Hittite Microwave Inc. is proud to announce the introduction of new digital and analog VGAs to its product family that address this system requirement. This new VGA family exhibits the desired gain control accuracy while improving traditional product performance and increasing bandwidth capability, as well as conserving board space and reducing overall component cost.

New Analog VGA
The typical operation of analog VGAs is to use dc voltage as the “variable” to control the amplification and hence, overall signal level. This type of VGA typically requires a driver circuit to control the voltage swing, but benefits in the ability to achieve finer degrees of attenuation and typically, a much wider dynamic range. Hittite’s recent product release, the HMC640LP5E, accentuates these qualities by delivering a gain control range of 40 dB over a frequency range of 0.4 to 2.0 GHz. Frequency bandwidth, usually a trade-off in analog VGAs, is minimized in this design with the careful utilization of wideband amplifier and attenuation techniques. The result allows the HMC640LP5 to operate across a wide range from 0.4 to 3.0 GHz and allows users the flexibility of utilizing one or both of the internal attenuator stages. As with most analog VGAs, this architecture can be used in a closed loop circuit, ensuring continuous control and self adjusting signal leveling. Another attractive feature of Hittite’s analog VGAs is shown in Figure 2. The stability of Output IP3 over frequency and tuning range is often a differentiating factor when comparing VGAs as these parameters are often difficult to maintain.

New DVGA Family
These newest types of VGA offerings were born out of the market need to predictably control the gain increments and maintain increased stability over a wider frequency range. Key factors in performance not only include OIP3, noise figure and dynamic range but also gain increment, maximum gain, uniformity, monotonicity and bandwidth of operation. Hittite Microwave’s DVGAs are designed to cover all the major wireless bands and standards, such as GSM, CDMA, WCDMA, UMTS, WLAN and WIMAX and most are housed in the same 5x5mm QFN leadless package, giving the user the flexibility to interchange parts in a effort to address specific system needs. The digital control feature is also user friendly as it allows for a serial or parallel interface as well as the selection of a preferred power-up state. Historically, amplifiers were set to a high gain or a low gain state and then cascaded to achieve even finer increments of gain variation. This can now be achieved in one insert. This method includes a digital gain control feature in unison with the amplifier that can be varied to achieve the required output level. There are some significant advantages in this approach as it affords the component designer the flexibility of operating the integrated amplifier at a constant gain state, limiting its non-linear contribution, while controlling the gain output using other circuitry. Consequently, the contribution from distortion or OIP3 is minimized, therefore maximizing the upper level of the available dynamic range of the end product and ultimately, its range of use. The other closely monitored parameter in DVGAs is noise figure as unlike its analog counterpart, represents an accurate measure of noise contribution within the system. This is once again due to the fact that the gain from the integrated amplifier itself is not varying and therefore, delivers a more consistent and stable contribution to circuit noise over the intended frequency range.

Expanding on this concept, Hittite Microwave has introduced four new DVGAs to help maximize system performance and flexibility of use throughout the RF chain. (See Table 1 to review the key parameter performance of all five VGA products.) The HMC625LP5E 6 Bit Digital VGA provides the user with 31.5dB of dynamic range, a gain control range of -13.5 to +18dB with the ability to step in 0.5dB increments, providing the user with a fine tune capability. In its maximum gain state, the HMC625LPE delivers a noise figure of only 6dB and an output IP3 of +33dBm across all states. The flatness of OIP3 across such a wide frequency band is unique in the industry and offers the performance stability required by most modern transceivers.

The integrated digital control feature includes a dual mode interface that is CMOS and TTL compatible and accepts either a 3-wire serial port interface (SPI) or a 6 bit parallel word. A user selectable power-up state and a serial output port provides additional flexibility for bench-top evaluation as well as cascading with other Hittite serial controlled devices within the system. A unique product in the market, this design feature eases the transition between engineering evaluation and high volume production. A chart of maximum gain versus frequency for the major states can be seen in Figure 3. To highlight the uniformity achieved over frequency, a similar chart of major gain states normalized or referenced to the maximum gain state is shown in Figure 4. This chart not only indicates the relative state flatness over the frequency range, but the overall dynamic range that can be achieved. A chart of bit error versus frequency for the major gain states is presented in Figure 5. As can be seen, the bit error never exceeds +0.6dB over the frequency and 32dB of dynamic range.

For applications below 1 GHz, the HMC627LP5E might be the correct choice as the OIP3 is typically +36dBm and the noise figure is only 4.3dB. This part lends itself very well to VGA requirements further down the receiver chain or early in the transmit chain where low noise and high linearity are paramount.

For extremely accurate step requirements, the HMC628LP4E delivers a gain accuracy of +/- 0.05dB below 250MHz, +/- 0.2dB from 250 to 500MHz and +/- 0.4dB from 500 to 800MHz. This BiCMOS MMIC operates with a +5V supply and consumes only 65mA of supply current.

Another member of the DVGA family is the HMC626LP5E, which delivers twice as much gain and 32dB of dynamic range that can be varied using a 6-bit, parallel logic control. Similar to the HMC627LP5E, its frequency range covers DC to 1GHz, however it can provide anywhere from 8dB to 40 dB of gain with only 2.8dB of noise figure contribution. The single positive control line per bit digital control VGA achieves near DC operation, making it suitable for a wide variety of RF and IF applications. It delivers an output IP3 of +36 dBm and requires a low supply current of only 176mA.

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. This occurrence is more easily understood with reference to the actual product performance presented in Figure 6. This chart displays the maximum step error over the entire frequency range for all states. As can be seen, the maximum step error never exceeds +/- 0.5dB (the LSB) over the entire frequency range, therefore ensuring a monotonic relationship between the 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, -0.5dB. It is critically important that the DVGA maintains a monotonic state relationship over its operating bandwidth; otherwise gain states will overlap and the digital logic input may not result in the desired gain output. The HMC625LP5E exhibits excellent consistency in performance across the characterized frequency range and displays another key differentiation between Hittite’s offering and other competing products.

Hittite Microwave’s design philosophy involves the careful selection of the best semiconductor process and design techniques available for each component solution, ensuring the delivery of market leading product performance. This performance improvement allows the systems designer to maximize the true potential of the transceiver design and ultimately deliver a superior product. Product differentiation is epitomized in the HMC625LPE, which is the widest bandwidth surface mount DVGA on the market and consumes approximately half the current of other narrowband market alternatives. A typical block diagram for a base station transceiver is shown in Figure 7 and highlights possible components and locations where the DVGA can effectively be used. In all cases, the introduction of new variable gain amplifiers enables the system designer to reduce component count and overall board real estate while improving the circuit performance. Whether your requirement demands the high resolution and closed loop control provided by the analog VGA or the wide dynamic range and high accuracy of the digital VGA, Hittite has superior product solutions to match your gain control needs.

Hittite is continually expanding its product portfolio and designers can choose from more than 630 standard products, including amplifiers, attenuators, data converters, frequency dividers and detectors, frequency multipliers, modulators and demodulators, phase shifters, passives, mixers and converters, power detectors, VGAs, oscillators, high speed logic, sensors, synthesizers and switches. Data sheets and detailed product information for all Hittite products are available online.

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