National Semiconductor Corporation introduced a high-performance DVGA (digitally controlled, variable gain amplifier) for use in a broad range of wireless communication products, including PCS base stations, paging base stations and mobile radios. The CLC5526, National's 350MHz DVGA, expands the dynamic range of IF-sampled communications receivers by 42dB. It has eight digitally programmable gain settings in 6dB steps. A very low 2.7nV/rt.Hz input noise voltage assures integrity of the system dynamic range. With AC-coupled differential inputs and outputs allowing large signal swings on a single 5V rail, the CLC5526 is optimized for use with a National chipset designed to enable the software radio. The chipset consists of the CLC5956 12-bit/65MSPS A/D and soon-to-be released CLC5902 digital downconverter with integrated automatic gain control (AGC).

"The CLC5526 was designed to dramatically improve the performance of base station receivers by scaling signals dynamically," said Kurt Rentel, marketing director for National's Data Conversion Systems Product Group. "However, we have found that many applications are looking for the wide range of programmability and the combination of both DAC and VGA functions that the CLC5526 incorporates," he added.

The CLC5526 maintains a 350 MHz bandwidth over the entire gain and attenuation range from -12dB to +30dB. Narrowband filtered signals are scaled (amplified or attenuated) by the DVGA to an optimized input amplitude for the A/D. By scaling signals to near the full scale level of the A/D, the DVGA allows the A/D to operate at its highest dynamic range. In effect, seven additional bits are added to the conversion process, allowing the system to operate with the dynamic range of a 19-bit A/D in narrowband applications. Since the 6dB step of the DVGA gain is a factor of two, digital correction for the gain steps is implemented by a simple binary shift rather than an addition.

The company name may have been banished but Comlinear's part numbers endure to remind us where this part, for example, originated. A standalone, programmable gain amplifier (PGA) suitable for frequencies up to several hundred megahertz has been needed for some time, and this CLC5526 at least partially satisfies that need.

The complete gain range of the part -- at 42 dB -- is good but I would have preferred to see smaller steps in gain change than the 6 dB offered. Actual circuit arrangements use a differential input resistive ladder which provides the attenuation followed by a fixed 30-dB gain amplifier. This is quite satisfactory for most arrangements but in fringe applications it should be remembered that the noise figure will be directly increased by the attenuation at the input. For example, at 30 dB gain, the noise figure is typically 9.3 dB; if you go to 12 dB attenuation, the noise figure will slide to 51.3 dB.

The 200 ohm differential input impedance is fine but the differential output impedance is a little high at 1000 ohms. No doubt this is for good matching to National's intended bandpass filter in their software receiver design but it will mean that unless you can accommodate a higher load the maximum gain will drop. With a 200 ohm differential load, for example, the maximum gain will drop to about 20 dB. I particularly like the fact that unless a new gain value has been latched into the part it will wake up in the same gain mode after a shutdown.

This will be a volume part with most of the output going into base station designs; it should also sell into the imaging market and into instrumentation, but in both cases and in its possible use with high-frequency video -- such as analog HDTV signals -- it will be problematical for the design engineers to come up with solutions that achieve what they want while also ac coupling into the part.