Active frequency mixers offer conversion gain where needed, but often represent a compromise in dynamic performance. Those traditional tradeoffs can be neglected. However, in the case of the LTC554x family of passive downconverting mixers from Linear Technology, they combine low noise figures with high input third-order intercept points to achieve outstanding dynamic range from 700 MHz to 4 GHz.
The LTC554x family of mixers includes models LTC5540 (600 to 1300 MHz), LTC5541 (1300 to 2300 MHz), LTC5542 (1700 to 2700 MHz), and LTC5543 (2300 to 4000 MHz). Although the integrated circuits (ICs) contain passive mixer circuitry, they also contain intermediate-frequency (IF) amplification and require bias energy from a +3.3-VDC supply. The mixer circuits, which are supplied in compact 20-pin housings (see figure), contain an IF amplifier, local oscillator (LO) buffer amplifier, LO switch, and RF balanced-unbalanced (balun) transformers. Each mixer offers a single-ended RF input port matched to 50 O, while a pair of single-ended LO input ports are provided for frequency-hopping applications. Each of the LO ports is matched to 50 O, whether the mixer is powered on or off to avoid unlocking of phase-locked-loop (PLL) circuitry. Each mixer is designed to operate with only 0 dBm LO drive power.
Individually, the 600-to-1300-MHz model LTC5540 operates with conversion gain of 8.0 dB and noise figure of 10.1 dB. As with the other members of the LTC554x family, the LTC5540 is designed to maintain a low noise figure in the presence of strong blocking signals, achieving a noise figure of 17.7 dB in the presence of a +5-dBm blocking signal. The LTC5540 exhibits an input third-order intercept point of +25.8 dBm and consumes 0.66 W of power during normal operation.
Model LTC5541 operates from 1300 to 2300 MHz, attaining conversion gain of 7.8 dB at 1950 MHz. It has a noise figure of 9.5 dB at 1950 MHz, which reaches 16.0 dB in the presence of a +5-dBm blocking signal. The LTC5541 consumes only 0.63 W of power while operating on a single +3.3-VDC supply and exhibits an input third-order intercept point of +26.4 dBm at 1950 MHz.
Model LTC5542 is designed for frequencies from 1700 to 2700 MHz and provides conversion gain of 8.0 dB with a noise figure of 9.9 dB. With a +5-dBm blocking signal, the noise figure elevates to a still-respectable 17.3 dB. The LTC5542 consumes only 0.66 W of power and boasts an input third-order intercept point of +26.8 dBm.
The highest-frequency member of the downconverting mixer family, model LTC5543, operates from 2300 to 4000 MHz with conversion gain of 8.4 dB and noise figure of 10.2 dB. The noise figure reaches 17.5 dB with a +5-dBm blocking signal. The LTC5543 consumes 0.66 W of power when running from a single +3.3-VDC supply and has an input third-order intercept point of +24.5 dBm.
The mixers have identical pin configurations for flexibility. They draw 200 mA quiescent current from a +3.3-VDC supply and have a power-down feature with turn-on and turn-off times of 2 and 3 s, respectively. P&A: $6.50 each.
Linear Technology, 1630 McCarthy Blvd., Milpitas, CA 95035-7417; (408) 432-1900, FAX: (408) 434-0507, Internet: www.linear.com