wirelessZONE Products for the week of September 26, 2011
Linear Technology Says…
LTC6946: Low Noise & Spurs 373 MHz to 5.79 GHz Integer-N Synthesizers
Integrated VCO Improves System Performance
Linear Technology announced the LTC6946, the first device in a family of high performance integer-N frequency synthesizers with integrated VCO, delivering -226dBc/Hz normalized closed-loop in-band phase noise, superior -274dBc/Hz normalized in-band 1/f noise, and best-in-class -103dBc spurious output. In a typical 900MHz application, these performance attributes help to achieve a closed-loop phase noise of -100dBc/Hz at 1kHz offset. The device is available in three frequency options: the LTC6946-1 tunes from 2.240GHz to 3.740GHz; the LTC6946-2 from 3.080GHz to 4.910GHz; and the LTC6946-3 covers 3.840GHz to 5.790GHz. In addition, each part has an on-chip output divider that is programmable from 1 through 6 to extend frequency coverage to as low as 373MHz.
This family of devices integrates a low noise 5.7GHz phase-locked loop (PLL), which includes a reference divider, phase-frequency detector (PFD) with phase-locked indicator, ultralow noise charge pump, and integer feedback divider to attain very low noise PLL operation. The PLL circuit is tightly coupled to a low noise VCO as well as internal self-calibration to ensure optimum VCO resonator tuning for best phase noise performance. The VCO requires no external components. The on-chip SPI compatible bidirectional serial port allows frequency tuning and control, and read back of register and loop status information.
The low phase noise and spurious capabilities of this family of frequency synthesizers enhance the performance of multiband basestations supporting LTE, W-CDMA, UMTS, CDMA, GSM and WiMAX standards. Its high frequency capability also supports point-to-point broadband wireless access, military, avionics, and high performance test and measurement applications.
The LTC6946 is an ultra-low phase noise integer-N frequency synthesizer being offered in three band versions. -1 offers an internal VCO that can be operated from 2.240 GHz to 3.740 GHz, the -2 from 3.080 GHz to 4.910 GHz and the -3 from 3.840 GHz to 5.790 GHz. With internal dividers of 2, 3, 4, 5, and 6 any frequency between 373 MHz and 5.790 GHz can be produced.
The architecture of the LTC6946 consists of a PLL with temperature-compensated, self-calibrating, VCO and N divider (32 to 65535) feeding into a phase/frequency detector and external loop filter. The detector is also fed with an external reference between 1 MHz and 250 MHz which can be single-ended or differential and can be either a sine wave or a logic signal. A buffered reference output is also available. The buffered reference is divided by an R divider of 1 to 1023 so that both reference and VCO-derived signals are below 100 MHz at the detector. The output of the VCO is also taken to the output divider for the main RF output signal which always has a 50% duty cycle. The output can be ‘muted.’
Operational bandwidths of LTE systems are increasing as competing carriers share the same infrastructure with multiple groups of signals, often with different modulation standards, and the phase noise floor becomes increasingly more significant to the overall performance. (Other unwanted effects are also going to be a problem in modulators and demodulators as bandwidth increases.) The LTC6946 is Linear’s answer to achieving phase noise floors which overcome the premium lost – a theoretical 3 dB for every doubling of bandwidth – with the expansion of bandwidth. The levels are such that they will be increasingly difficult to accurately measure but Linear is claiming typical integrated noise numbers of, for example, 0.215º rms for the LTC6946-1 outputting 900 MHz with an output division of 3. In-band closed-loop normalized phase noise is claimed at -226 dBc/Hz and in-band 1/f noise (starting below 10 kHz) at -274 dBc/Hz. The spurious levels are claimed at below -100 dBc.
On chip is an SPI and a status indicator that includes PLL lock. The parts are specified over a -40ºC to +105ºC temperature range. An on-chip charge pump provides from 250 μA to 11.2 mA source or sink current pulses (with a ±6% accuracy) to the loop filter to move the frequency up or down.
The LTC6946 is a 3.3 V part with supply current depending on operation but with a maximum of about 122 mA.
A Windows-based Wizard is available for download from Linear which suggests recommended parts for the frequency plan you are using, and help to design a loop filter optimized for minimum noise. The Wizard will also read and write from/to all the device registers over the SPI and to troubleshoot common set-up problems and programming errors. Also featured are simulations for loop frequency response and phase margin, as well as source noise and output noise characteristics.
There will also be available the LTC6945, which has apparently not yet been released. This will have the same architecture as the LTC6946 but without the internal VCO. An external VCO with frequencies above 9 GHz will be usable with the LTC6945. That part is probably going to be rather more difficult for the average designer to use but the extended frequency range opens up a lot of non-commercial applications in military communications and test and measurement.
The most popular version of the LTC6946 will be the LTC6946-2. The availability of the new frequency synthesizer fills another one of the gaps in Linear’s RF offerings for mobile telephony and data. What remains is the most important arena of high-speed DACs. At some stage, with the right commercial opportunity offered, we should also expect to see Linear get into the LNA business as well. The parts are a welcome addition to Linear’s RF arsenal and will prove to be equally attractive to designers who do not have to cope with large bandwidth issues straight away.
All three frequency range versions of the LTC6946 are in production in QFN-28 with pricing at $5.75 in 1000-piece lots.