hf/rf ZONE Products for the week of September 8, 2003
Linear Technology Says . . .
LT5515/6: Direct Conversion Quadrature Demodulators
Simplify Design for RF Receivers
Two new direct conversion quadrature demodulators from Linear Technology Corporation simplify radio design for cellular basestations and microwave and satellite links. Linear Technology's LT5515 and LT5516 Direct Conversion Quadrature Demodulators deliver exceptionally high linearity, providing more flexible system design and wide spur-free dynamic range. These devices significantly reduce costs compared to discrete solutions by integrating on a single chip the functionality of a signal splitter, two high linearity down-converting mixers, a precision local oscillator quadrature generator (0°/ 90°), and 260 MHz bandwidth output buffers with single-pole, low pass filters on each of the outputs.
The LT5515 and LT5516 have applications in cellular infrastructure and in microwave and satellite links, where they directly convert an RF signal to baseband In-phase (I) and Quadrature-phase (Q) components. The devices' matched I and Q channels ensure precise gain and phase matching, so that significantly less calibration is required. These direct conversion receiver ICs eliminate the need for additional Intermediate Frequency (IF) stages, local oscillators, and associated filtering.
The LT5515 operates over an input frequency range of 1.5GHz to 2.5GHz,
and the LT5516 operates with RF input from 0.8GHz to 1.5GHz. Both devices
are designed for high linearity applications. These include wireless infrastructure
of all types, such as basestations for GSM, CDMA, W-CDMA and fixed wireless
communications, as well as for satellite and microwave receivers, high-performance
radios, and instrumentation.
analogZONE Says . . .
Development of Linear's RF parts has been very much a stealth operation with little to no exposure publicly. This release is a major jump in showing the direction the company is taking in the wireless arena.
The LT5516 is a direct conversion quadrature demodulator for the 800 GHz to 1.5 GHz band, while the LT5515 covers the 1.5 GHz to 2.5 GHz spectrum. Between them they cover all the cellular frequencies. The architecture of the two ICs is the same and for the most part we will talk here of the more difficult to design and manufacture, the LT5515.
Conversion gain in the LT5515 is, not surprisingly, -0.7 dB at 1.9 GHz (the corresponding number for the LT5516 at 900 MHz is +4.3 dB) while the noise figure is 16.8 dB (12.8 dB for the LT5516). That might seem high but in all the designs where the part would be used it will be preceded by an LNA and suitable band-pass filtering. It is the gain and noise figure of the LNA which will contribute most to the overall noise equation.
The RF amplifier at the input of the parts is differential, and should be coupled differentially to minimize the amount of LO signal that might be fed back out of the RF input port. Linear recommends a single-ended to differential coupling transformer and, hopefully, they will be able to implement this in later versions on chip -- as at least one other vendor has been able to do. They also show on the typical application block that a transformer is also used to feed the LO signal input, but that can be implemented with a little less cost using an RC network.
The internal RF amplifiers for the signal and the LO feed two quadrature mixers -- with the LO precision split into quadrature -- to produce differential I and Q outputs. Internal low-pass filters set the 3-dB bandwidth of the output at 260 MHz. The channels are matched within a typical 0.3 dB in gain and 1° in phase. The LO signal needs to be between -10 dBm and 0 dBm.
The RF input pins are biased internally at +1.54 V so they need to be dc blocked if they are connected other than to an isolated transformer secondary. The differential S11 magnitudes range from 0.698 at 1.5 GHz to 0.637 at 2.5 GHz with corresponding angles from -24.9° to -33.7°. These capacitive elements should be easily matched out.
The I and Q channel outputs are internally connected to Vcc through 60 Ohm and the output bias voltage is Vcc - 0.85 V. They can be either ac or dc coupled to their loads (though there may well be linearity problems if dc coupled and the bias is incorrectly set on the mixers) while the output impedance is set to give a -3 dB point at 260 MHz with 120 Ohm in parallel with an internal 5 pF capacitor. The differential output impedance is 120 Ohm.
With a 5-V supply rail the LT5515 consumes 125 mA typical while the shutdown current is a maximum of 20 µA. Turn-on time from shutdown is a very fast 120 ns.
The characteristics show that linearity has been very deliberately maximized for 1.9 GHz operation with the best numbers for NF and IIP2 (>50 dBm) set just below and just above the "sweet" frequency, as is I/Q phase mismatch. Return loss is better than -10 dB at 1.9 GHz on the RF input and is about -18 dB on the LO input.
These are major introductions for Linear and deserve to achieve a high degree of market success. There is a lot of competition in this arena but they have made a big enough jump into direct conversion that if they get a hearing from customers they will win sockets.
The LT5515 and LT5516 are in production in 4 mm x 4 mm thermally-enhanced QFN-16 with sensible pin-outs with the LT5515 priced at $6.75 and the LT5516 at $7.40 both in 1000-piece lots. (Unusual for the higher-frequency version of any part to be the lower priced.)