FROM EDN EUROPE: I/Q modulator eases 3G designs
By Graham Prophet -- EDN, February 3, 2005
Key specification parameters of Linear Technology's LT5528 modulator aim at designers of 3G (third-generation) cellular radio basestations (Picture). Engineers can use the part in a variety of other high-performance radio applications. The 5528 is a direct-conversion I/Q modulator; differential baseband I and Q signals directly modulate an RF carrier in the range of 1.5 to 2.4 GHz. It supports all commonly used cellular radio standards, and engineers can use it as an image-reject upconversion mixer by applying signals with a 90° phase shift to the I and Q inputs. As digital modulation in later cellular radio systems requires, modulator linearity is high, with output third-order intercept quoted at 20.5 dBm at 2 GHz. Matching the gain in the I and Q paths to ±0.1 dB yields carrier suppression of –47 dBm at 2.14 GHz, with image rejection at the same frequency of 45 dBc. All signal ports have 50Ω impedance. The noise floor is –159 dBm/Hz and adjacent channel-power ratio is –66 dBc.
The part's architecture uses voltage-to-current converters on the I and Q inputs, driving double-balanced mixers whose outputs are summed and applied to an RF transformer that performs the conversion to a 50Ω, single-ended output. The 5528 operates from 4.5 to 5.25V and will cost $5.95 (1000).
Linear Technology, +44 1628 477066, www.linear.com.
