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For wireless infrastructure applications, IDT has developed a family of transmit (Tx) variable gain amplifiers (VGA) with high gain and high linearity. Incorporating a new constant linearity KLIN™ RF VGA, the family provides a nominal 32 dB maximum gain with 29.5 dB gain control, and an output IP3 of +38.5 dBm. The devices incorporate a digital step attenuator (DSA) with 0.5 dB step resolution. Three parts cover the bands from 700 to 2950 MHz (see Table 1), all with similar gain, OIP3 and P1dB performance. Device linearity is optimized over the respective frequency ranges for each VGA; however, gain flatness is optimized over a broader frequency to account for systems with extended digital predistortion (DPD) bandwidth requirements.

In wireless infrastructure base stations and remote radio units (RRU), RF VGAs are used in the transmit paths connecting the data converter to the power amplifier (PA) stages. The typical requirements for the components in the Tx path are:

  • Low distortion
  • High efficiency
  • High ACLR
  • Low spectral noise
  • High signal-to-noise ratio
  • Low DC power consumption
  • Small PCB footprint
  • Low cost

Table 1

Figure 1

Figure 1 Functional block diagram of Tx VGAs with the KLIN constant linearity feature.

The IDT Tx VGAs meet or exceed wireless infrastructure requirements. Each Tx VGA in the F145x family consists of a KLIN RF amplifier, DSA and a PA driver amplifier (see Figure 1) that are configured to provide an optimum balance between noise and linearity performance over the gain control range.

Common features of the three Tx VGAs include:

  • Broadband operation
  • 32 dB maximum gain, flat and temperature invariant
  • 4 dB noise figure at maximum gain
  • 23.4 dBm P1dB, which is constant for the first
  • 12 dB of gain reduction
  • 38.5 dBm OIP3, also constant for the first 12 dB of gain reduction
  • 29.5 dB total gain control range in 0.5 dB steps
  • Serial peripheral interface (SPI) for gain control
  • Internal matching and biasing
  • High reliability MSL1 package with low thermal resistance
  • Single +5 V supply voltage and 220 mA of DC current
  • Standby feature for TDD applications
  • High reliability, due to the assembly of a monolithic silicon die in a QFN package

Figure 2

Figure 2 Output IP3 vs. gain, showing the flatness of the KLIN feature compared to a competing product.

KLIN FEATURE

Unique to these Tx VGAs is the KLIN feature. The KLIN amplifier extends the OIP3 and output P1dB performance over a wider range of input and output powers than conventional competitive devices. Unlike other products, the F145x Tx VGAs separate the gain control between both the KLIN amplifier and the DSA. The first amplifier incorporates circuitry to improve the linearity of the device as gain is reduced from its maximum value. This results in an output IP3 characteristic that remains constant for approximately 12 dB of gain reduction (see Figure 2), providing greater dynamic range for the system. With this added dynamic range, the device is able to operate over a wider range of input power levels while maintaining a constant output P1dB and OIP3. A conventional VGA, whose OIP3 characteristic falls off more steeply as gain is reduced, has a more limited range of acceptable input power levels, i.e., for which the desired intermodulation performance is met.

The same applies to the P1dB performance. The first F145x stage tolerates higher input power as a function of gain reduction before contributing to the overall compression performance of the device. In contrast, the first stage amplifier of some competitive parts compresses early. With degrading P1dB and OIP3 as a function of gain reduction, these competitive devices only allow a relatively small window of acceptable input power.

Integrated Device Technology Inc.
San Jose, Calif.
www.idt.com