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Wideband demodulator booster receiverens ydelse

Linear Technology introducerer wideband I/Q demodulator med IIP2 optimering og DC offset cancellation, hvilket forbedrer receiver ydelsen markant (in english).

The exponential growth of smart phones, iPADs, tablet PCs, notebooks and netbooks has contributed to an insatiable demand for wireless network data speed and capacity.
 
Wireless service providers everywhere are scrambling to accelerate the deployment schedule for 4th generation wireless technology, LTE, which promises a significant increase in data rate.

To help reduce their cost of deployment, the base station industry is trending toward building few platforms – ultimately moving to a single platform – that can be field-configured for different frequency bands and different standards for specific markets and countries.


In effect, the goal is to develop a flexible, software-configurable base station that can cover all the frequency bands and can seamlessly communicate with all cellular standards including LTE, W-CDMA, UMTS, CDMA and GSM. Doing so will have a positive financial impact, as it provides efficient use of equipment design resources and reuse of technology, hence improving their cost structure.

This goal illuminates a number of difficult design challenges.

First, to make a base station configurable, it must be capable of operating in all cellular frequency bands including the low bands of 700MHz and 880MHz, as well as the high bands of 1.7GHz, 1.8GHz, 1.95GHz, 2.14GHz and 2.4/2.6GHz. This necessitates that base station transmitters and receivers are able to tune over all of these bands. Frequency matching over such a wide frequency range is difficult and can have a negative impact on a radio’s performance. Additionally, wider band receivers open up the noise band, resulting in a signal-to-noise performance hit.

The second challenge is that within each band, there is at least 60MHz bandwidth available, split into three 20MHz spectrum. Each is typically licensed and auctioned to different service providers in various geographic markets. So a configurable base station must have the flexibility to transmit and receive in the entire 60MHz band. Opening up the basestation bandwidth to 60MHz wide in itself isn’t too difficult to implement.

However, the burden is in the transmitter’s DPD (digital predistortion) receiver to correct the transmitter’s harmonic spurs to 5th order, which correspondingly pushes its receiver bandwidth out to at least 300MHz. To perform this function properly and to meet the requirements of the cellular standard, the passband flatness must be better than ½ dB, which is difficult to attain. Within this bandwidth, the receiver distortion spurs must be less than those generated by the transmitter.

The common DPD receiver employed is a heterodyne architecture which downconverts the RF signal to an IF frequency and then filters and digitizes the signal. But a 300MHz bandwidth signal riding on an IF frequency requires an exceedingly high speed A/D converter pushing sampling speeds over 500Msps, which is costly and at a performance level that is difficult to attain.

Linear Technology introduces an attractive alternative solution to this problem.  LTC5585 is an ultrawide bandwidth direct conversion I/Q demodulator with outstanding linearity performance (IIP3 = 25.7dBm and IIP2 = 60dBm at 1.95GHz).

The device is capable of baseband output demodulation bandwidth of over 530MHz, which can support new generation wideband LTE multimode receivers’ and digital pre-distortion (DPD) receivers’ bandwidth requirements. The I/Q demodulator operates over a wide frequency range from 700MHz to 3GHz, covering virtually all cellular base station frequency bands.

Unique to this device are two built-in calibration features. One is advanced circuitry that enables the system designer to optimize the receiver’s IIP2 performance, increasing from a nominal 60dBm to an unprecedented 80dBm or higher. The other is on-chip circuitry to null out the DC offset voltages at the I and Q outputs. Both serve to enhance receiver performance. Moreover, the LTC5585 delivers excellent P1dB of 16dBm.

 To further enhance its use in direct conversion receiver applications, the LTC5585 offers very low I/Q amplitude and phase mismatch. The amplitude mismatch is typically 0.05dB, while the phase error is typically 0.7 degree, both measured at 1.95GHz. This combination produces a receiver image rejection capability of 43dB.

Because of its very wide bandwidth capability, the LTC5585 is especially well suited for multimode LTE, W-CDMA and TD-SCDMA base stations DPD receivers as well as for main receiver applications. Particularly for DPD, these latest generation base stations are pushing demodulation bandwidth of over 300MHz.

The LTC5585 can be easily configured to meet these bandwidth challenges. Beyond wireless infrastructure applications, the LTC5585 is ideal for applications in military receivers, broadband communications, point-to-point microwave data links, image reject receivers and long-range RFID readers.

The LTC5585 has an on-chip RF transformer to reduce external components, providing a highly compact solution with its 24-lead 4mm x 4mm QFN package. The device is specified for case operating temperature from -40°C to 105°C.

The LTC5585 is powered from a single 5V supply, drawing a total supply current of 200mA. The device provides a digital input to enable or disable the chip. When disabled, the IC draws typically 11μA of leakage current. The demodulator’s fast turn-on time of 200ns and turn-off time of 800ns enables it to be used in burst mode receivers. 

Production quantities are now immediately available.
www.linear.com/product/LTC5585.

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