Cellular base station performance and the performance of other wireless infrastructure systems depends on how well the systems can receive wireless signals, both from numerous smartphones and other wireless devices that link to each tower. Improving the signal-to-noise ratio (SNR) of the receiver, plus providing a wide range of gain control will ensure the base stations can deliver consistent performance while minimizing dropped connections due to signal loss or noise. One key to improving performance is to optimize the receiver signal path with the aid of latest-generation digitally controlled variable-gain amplifiers (VGAs).
In cellular base stations, the demodulation and decoding of the intermediate-frequency (IF) signal can be done in either the analog or digital domains. Advances in digital technology and high-speed analog-to-digital converters (ADCs) now let the receive channels employ a direct-to-digital approach to IF conversion. Commonly referred to as IF subsampling, this conversion approach can significantly reduce receiver parts count, power consumption, and cost versus receiver systems that employ analog demodulation.
IF subsampling systems typically consist of a digitally controlled VGA driving an anti-alias filter which, in turn, feeds a high-speed ADC. The ADC output is digitally processed to form a mixed-signal feedback loop to the VGA to provide gain adjustment based on received signal strength (Fig. 1). The feedback from the digital signal processor (DSP) adjusts the gain to keep the signal in the "sweet spot" for optimal signal-to-noise ratio and minimal distortion.
The digitally controlled VGA is at the heart of the IF subsampling approach. A number of different commercial digitally-controlled VGAs are available from various suppliers, optimized for different IF ranges. The model F1200 VGA from Integrated Device Technology will be used as an example in this report to demonstrate how this technology can serve modern cellular base stations. Model F1200 is the first in a series of digitally controlled VGAs from the company, with an IF range of 40 to 160 MHz. Models F1206 and F1207 are additional models with higher frequency ranges of 150 to 260 MHz and 230 to 300 MHz, respectively, for use in systems employing higher IFs.
The F1200 has a 7-b control interface, which offers higher tuning resolution than the 5- or 6-b control typically used in commercial digitally controlled VGAs. The 7-b control allows the gain to be set in increments of 0.25 dB over a gain range of -1 to +22 dB. That finer granularity allows designers to better optimize the system performance. High accuracy is maintained from gain step to gain step, with typically less than 0.1 dB variations.
Additionally, the digitally controlled VGA chip has a considerably lower noise figure than existing VGA solutions, at only 2.6 dB. The lower noise figure also helps simplify system design since the gain setting can be kept slightly lower, and that translates into less distortion in the signal. The chip itself also offers very low distortion, with an output third-order-intercept point of +48 dBm. This low noise figure and low intermodulation distortion allow for simpler post-VGA filtering circuits, and in turn reducing the component count and cost in a cellular base-station receiver. The low distortion yields increased spurious-free dynamic range (SFDR), which makes the circuit suitable for a wide range of applications.
Figure 2 shows the typical performance characteristics of the signal channel for second-generation (2G) and third-generation (3G) cellular communications systems. In the graph, the output signals are shown for 2G Global System for Mobile Communications (GSM) input signals. The gain would be adjusted in such a way that the 2G signals would be amplified to just below the ADC's maximum input level. The intermodulation between these signals causes the third-order intermodulation products close-in on either side of the desired, amplified output signals. Much further away in frequency, the second-order intermodulation products are shown. The output noise is also shown in the plot. The second-order products and noise must be filtered prior to the ADC to avoid aliasing into the wanted signal band. The F1200's second-order intermodulation distortion and added noise are very low, meaning only minimal filtering is necessary. The third-order products and in-band noise cannot be filtered, so it is critical to choose an IF VGA with very low third-order distortion.
In an actual receive signal path, the received signal typically passes through a mixer to downconvert the frequency and then through a surface-acoustic-wave (SAW) filter to eliminate unwanted spurious signal products (Fig. 3). The resulting signal is then processed by the digitally controlled VGA. The input of the F1200 presents a 200-Ω balanced input impedance, which eliminates the need for external impedance matching components. The output differential impedance of 200 Ω is also a good match for the ADC's interface and the differential nature of the device rejects even-order spurious response. Higher-order harmonics are easily rejected by the anti-aliasing filter before they feed into the ADC. Once the adjusted signal is converted by the ADC, it is usually sent on to a host processor or DSP that executes various algorithms to further optimize the feedback to set the optimum gain setting on the VGA.
When designing a board for the VGA, the board traces typically have a 100-Ω characteristic impedance. By keeping the traces as short as possible and the exact same length, noise, bandwidth, and signal losses can be optimized. The low 500-mW power consumption of the F1200 also makes board design and thermal path layout simple. The leadless package includes an exposed thermal pad that can dissipate heat through the copper on the printed-circuit board (PCB), eliminating the need to add a heatsink.
CHRISTOPHER STEPHENS
Director of Marketing, RF Products
Integrated Device Technology
6024 Silver Creek Valley Rd.
San Jose, CA 95138
(408) 284-8200
FAX: (408) 284-2775