hf/rf ZONE Products for the week of July 26, 2004
Linear Technology Says . . .
LT5521: High Linearity Active Mixer Simplifies Design
Of Cellular Basestations
A new RF active mixer from Linear Technology provides linearity and signal-to-noise performance matching that of passive mixers, allowing designers to build 3G cellular basestations that are more compact and at substantially reduced cost. The LT5521 streamlines basestation design by resolving the high LO (Local Oscillator) drive level, poor LO suppression and low conversion gain issues common to passive solutions. In addition to applications in wideband CDMA, UMTS, GSM, and PHS basestation transmitters, the LT5521's low distortion is ideal for cable downlink infrastructure.
The LT5521 incorporates a differential input and output architecture, supporting superior linearity over a wide operating frequency range from 10MHz to 3.7GHz. It also provides flexible operation in either an upconverting or downconverting application. As a transmit (upconverting) mixer, the input third-order intercept (IIP3) is +24.2dBm at 1950MHz, and the Noise Figure (NF) measures 12.5dB. These two parameters contribute to a high spurious-free dynamic range.
Unlike passive mixers, which require a high LO drive level of as much
as +17dBm, the LT5521 needs only a modest -5dBm LO signal. This saves one
or two stages of external LO amplifier, resulting in a more compact solution
size and lower cost. Moreover, the LT5521 offers substantially reduced local
oscillator leakage from the LO to other input and output ports. This solves
a major design headache by removing undesirable LO signal from the transmit
path. Unlike passive mixers, which typically have high conversion loss in
the range of 6 to 10dB, the LT5521 has only 0.5dB loss, contributing to
improved signal-to-noise performance. Taken as a whole, these attributes
result in a solution that has far fewer external blocks, markedly reduced
system costs, and overall improved performance.
analogZONE Says . . .
When you look at the primary numbers at about 2 GHz for the LT5521: IIP3 at +24.2 dBm -- a good 7.5 dBm improvement on competing parts -- noise figure at 12.5 dB (1.5 dB better) it becomes obvious this is a part that demands to be taken notice of. There is no conversion gain (a 0.5 dB loss, in fact) but the linearity performance makes up for that, and more.
Linear has provided both the input circuitry and LO with wide bandwith capability, 10 MHz to 3 GHz for the RF input and 10 MHz to 4 GHz for the LO allowing for the output frequency to be between 10 MHz and 3700 MHz. Theretically, at least, it might be possible to use one LT5521 for both the transmit and receive chain, though engineering that in practice would be really tough.
Active mixing reduces the demands on the LO signal level quite dramatically compared to passive mixing, and here Linear has reduced the need for the single-ended LO input to be a typical +0.5 dBm. The RF path is fully differential and a 1:1 impedance-matching transformer is required for a single-ended RF input signal to be handled. This could be incorporated on-chip at a later stage if the demand level for a particular IF sustains it. The output balun is recommended to be 4:1 and matching at particular frequencies would be achieved with series inductors.
The LO input return loss at 1700 MHz is a typical 12 dB, while the input and output return losses can be matched to the same 12 dB. The important LO to RF leakage is an extremely good -42 dBm at 2 GHz output (from a 250 MHz IF) with a 5-V rail and -36 dBm with a 3.3-V rail, greatly reducing the demands of the band-pass filter required on the output. The supply range is 3.15 V to 5.25 V with a typical quiescent of 82 mA. Shutdown is 20 µA.
This is a very serious competitor in the RF mixer market and its flexibility will allow a myriad of designs in cellular, cable, fixed-wireless and general wireless arenas. It is priced to sell.
The LT5521 is in production in a QFP-16 and is priced at $4.95 in 1000-piece lots.