Current Issue
June 2009
• Electro-Mechanical Broadband RF Switch.
• Single-Stage Driver Amplifier
• Quad-Band EDGE Radio Solution
• Modeling 3G / WCDMA / HSDPA
• Composite Filters
• Integration of Waveguide
• Coaxial Components
• Antennas Needed
• And More...
 
Dow-Key Microwave
 
  Search by TXTLINX Number:
 
   

New VCO
The CRO2781A-LF in S-band operates at 2780 MHz with a tuning voltage range of 0.5 to 4.5 Vdc. It features a typical phase noise of -115 dBc/Hz @ 10 KHz offset and a typical tuning sensitivity of 9 MHz/V. Its industry standard MINI-16 package is just 0.5 x 0.5 x 0.22".

Wideband PA Module
A new wideband power amplifier module for use in microwave radio, VSAT, military & space, fiber optic and broadband test equipment applications from 100 MHz to 20 GHz has been introduced. The HMC-C057 is a GaAs pHEMT MMIC PA in a miniature hermetic module.

Coaxial to Waveguide Adapters
Coaxial to Waveguide Adapters are offered in a variety of configurations. Option A, broadband adapters, have excellent electrical specs that are maintained over the entire adapter bandwidth. Option B offers enhanced performance over a specific band of the unit’s bandwidth.


Digital Communication Analyzer
The latest addition to the PXIT product family, the PXIT 10G Digital Communication Analyzer (DCA) with Passive Optical Network (PON) filter rate options and smart post processing for the PXIT N2100B DCA, helps optical transceiver test vendors reduce their cost of test.

LED Drivers
This new family of LED driver ICs significantly reduces the number and size of external components required by drive circuits. Operating at switching frequencies up to 600 kHz, AP880X Series step-down, DC-DC converters require only four smaller and lower cost inductors and/or capacitors.

RF Interface DAS Panel
Created to control the output power from PAs, the 15C2NB is designed to combine and attenuate RF signals in steps of 1 dB up to 70 dB of maximum attenuation. With the operating frequency covering 800 MHz to 3 GHz, this design is ready for field deployment for GSM, PCS, WiMAX and LTE network architectures.

Phase-Locked Crystal Oscillator
The PLXO-50 Phase-Locked Crystal Oscillator is used as the frequency reference in a surveillance RADAR application. The PLXO, which operates at 50 MHz, maximizes system performance with its exceptional phase noise (<-150 dBc/Hz @ 10 KHz) and other features.

Directional Antenna
A wide angle 2.4 GHz antenna, model HG2405P-135, is designed for compact installations and is ideal for Wi-Fi, PCS, DCS, and custom applications. It gives the system designer wide angle coverage of an area without multiple antennas or larger footprint antennas.

Band Reject Filters - Tunable
Band stop and cavity filters that can be re-adjusted by the customer to new center frequencies are now available. These filters are tunable over a +/-7.5% center frequency range with minimal change in bandwidth. Operating temperature range is -55 to +85ºC.

Fast Rise/Fall Time Logic
Four new logic devices which are optimized for systems requiring fast rise/fall times, low jitter, and low DC power consumption have been released. They provide operating clock and data rates of 13 GHz/13 Gbps, and are ideal for deployment in ATE, broadband T&M equipment, frequency synthesis and radar signal processing systems.
 
Ultra Low Phase Noise VCO
Model CRO1220A-LF in L-band operates at 1220 MHz with a tuning voltage range of 0 to 5 Vdc. This VCO features a typical phase noise of -118 dBc/Hz @ 10 KHz offset and a typical tuning sensitivity of 2 MHz/V. It is well suited for satellite communication and microwave radio applications.


Design Verification Test Systems
The GS-9000 Assisted GPS (A-GPS) Design Verification Test systems were designed around the 8960 wireless communications test set’s new A-GPS assistance data messaging test capabilities. The capabilities support A-GPS validation, Total Isotropic Sensitivity testing and A-GPS pre-conformance testing for mobile devices.

 

 

February 2006

Polar Modulation and Bipolar RF Power Devices
By Earl McCune, Chief Technology Officer and co-founder, Tropian Inc.

"Polar modulation technology is experiencing increased interest as the need grows to simultaneously provide high quality signal modulation of bandwidth efficient signals while achieving high DC to RF efficiency. Bipolar transistors have historically been problematic when applied to the most efficient architectures."

Polar modulation techniques have been used as early as 1915, to specifically address the problem of low transmitter power amplifier (PA) efficiency with envelope varying signals. While polar modulation techniques generally do increase overall PA efficiency, early implementations were rudimentary, with reduced output signal quality compared to contemporary good-practice linear circuit approaches. However, more recently modern wireless systems have adopted modulation schemes that provide high bandwidth efficiency, such as orthogonal frequency division modulation (OFDM), multi-level PSK and QAM. Use of these signals in battery powered mobile devices has made RF power amplifier efficiency a serious concern and polar techniques are experiencing a renaissance in addressing this problem.
Existing developments of polar modulated transmitters generally fall within three major categories: Polar Loop, Polar Lite (sometimes also called "Polar Modulator"), and Direct Polar.

Polar Loop
To address the traditional signal quality problem of a polar transmitter, feedback control can be used to correct the output signal. Advantages of such a polar loop includes improved PA efficiency gained from operating the power amplifier closer to saturation, a low wideband output noise floor and a reduction of circuit oscillation tendencies with varying output load impedance.

Disadvantages include the need for a precision receiver within the transmitter, control loop bandwidths which are much greater than the signal bandwidth, a restricted output power dynamic range, the stability of the non-linear feedback control loops, and a lack of circuit design techniques when operating with intentional circuit nonlinearity.

Polar Lite
One way to eliminate the nonlinear loop dynamics difficulties due to feedback around the PA in the polar loop is to restrict the polar operation to the signal modulator. In this type of transmitter, the output signal is amplified from the output of the modulator using conventional linear amplifier devices. This approach provides a lower modulator wideband noise floor, determined by the VCO phase noise, compared to what is available from a linear design using a quadrature modulator. Since the polar signals are at a low power level, it is possible to use digital sigma-delta techniques to realize the phase modulator allowing improved integration with digital baseband devices.

The obvious disadvantage of this approach is the use of a conventional linear amplifier with no improvement in transmitter efficiency.

Direct Polar
The direct polar approach eliminates the additional receiver associated with the PA feedback of the polar loop while still retaining the amplitude modulation on the power amplifier itself. Direct polar advantages include circuit simplicity and very high energy efficiency with greatly reduced heat losses. Furthermore, the circuit bandwidth is typically only slightly greater than the modulation signal bandwidth, the broadband noise floor is low with good modulation accuracy, and circuit oscillation problems are eliminated due to deeply compressed power amplifier operation.

Disadvantages include the need to fully characterize the power amplifier in compressed operation across the output dynamic range, time alignment of the AM/PM signal component paths, correction (generally through predistortion) of AM-AM and AM-PM distortion effects, and assurance of thermal and manufacturing stability.

Direct Polar RF Power
Just as digital power consumption dropped dramatically when ECL/TTL saturated-amplifier logic circuit structures were replaced by CMOS switches, direct polar RF power follows the same principle by replacing the linear circuitry with an RF switch. The power transistor's role is shifted from regulating the current through the load, to selecting when current will flow in the load. The amount of current which flows through the load impedance is set by the environment around the power transistor, and is independent of the power transistor itself. As long as the power transistor presents an ON resistance negligible to the load resistance on its output port, the power transistor itself becomes a "negligible" circuit element, no longer dominating the overall circuit signal-quality performance and efficiency.

The direct polar RF power "amplifier" is a 3-port structure (RFin, MagnitudeControlin, RFout). The switching transistor port impedances are not definable (because the circuit is not linear), so impedance matching is no longer a valid design technique. Successful design of such a power stage requires changes from conventional linear amplifier design techniques. Instead of setting a load line and biasing to avoid reaching the endpoints of the load line, the direct polar power "amplifier" must operate at these load-line endpoints and avoid the linear region.

The term "amplifier" in its conventional sense is a misnomer for this device as the output signal is not a proportional scaling of the RF input signal.

Practical Realization Issues
Despite the advantages, there are still barriers against widespread adoption of direct polar. These include a lack of appropriate engineering tools (due to strong circuit nonlinearities required), dealing with a different set of AM-AM and AM-PM effects, realization of system-specified dynamic range for all signals, and maintaining the required AM to PM time alignment. Finally, the manufacturing community must gain comfort with this technique, which has properties and characteristics in general opposition with the linear RF design.

Key to successful direct polar RF power realization is the accurate characterization of the power stage, primarily for output power, AM-AM nonlinearities, and AM-PM distortion. The direct polar RF power technique is "RF open loop" by definition; manufacturability is only assured if power stage characteristics are predictable and consistent. To first order, the fact that this stage operates as a switch leads to some confidence - as long as ON is ON and OFF is OFF, overall performance should be consistent. This premise has been extensively tested against production variations, temperature extremes, and accelerated aging. Results have demonstrated that this characterization is practical for GMSK/EDGE signals using pHEMT technology. Extensions to WCDMA and WLAN are also consistent with these results

Bipolar RF Power Challenges
Bipolar RF power devices are designed to be linear controlled current sources, and not operated as switches. For this application, full efficiency requires operation as a switch. Today there is a large bipolar technology manufacturing base (primarily GaAs HBT) so there is a strong incentive to apply this technology to polar modulation.

A number of issues tend to make a bipolar device less than ideal as a switch:

• VCE,SAT - offset voltage reduces the voltage available across the load resistor RL, reducing available output power to (VCC-VCE,SAT)2/RL. This lowers efficiency because IC*VCE,SAT is an additional power dissipation source. The existence of VCE,SAT redefines the practical zero-envelope voltage.

• RF leakage - When VCC is brought to zero volts, there is still RF power present at the PA output. This leakage power adds to the output signal distortion.

• Saturation recovery time - at 2 GHz there are just a few picoseconds available to shift from the saturated ON state to the OFF state. Accurate time domain models are needed to optimize these designs.

• Base current to hold ON state - To hold the bipolar power transistor in its ON (saturated) state, a continuous current must flow into the base terminal. This lowers overall efficiency. Higher beta ratios reduce this requirement.

• Ballast - When a large transistor is comprised of multiple connected smaller cells, some means of ballast is necessary to assure even distribution of current among the constituent cells. It is imperative that the means chosen does not affect the overall efficiency.

There are also characteristics that make bipolar devices attractive as switches, including:

• High transconductance - compared to FET technologies, smaller input voltages are required to alternate the transistor state between OFF and ON.

• High current density - the nature of bipolar structures allows for higher current densities than in most FET technologies.

• Small die size - with higher current density, bipolar technology allows smaller transistor sizes for a given collector current capability than for FET technologies.

• Low ON resistance - ignoring VCE,SAT and ballast effects, bipolar transistor ON state resistance is low.

Polar modulation technology is experiencing increased interest as the need grows to simultaneously provide high quality signal modulation of bandwidth efficient signals while achieving high DC to RF efficiency. Bipolar transistors have historically been problematic when applied to the most efficient architectures.

It is desirable to have the bipolar technology community develop devices which are intended to operate as saturated switches, minimizing the problematic effects discussed above while maximizing the desirable characteristics. Not all of these design goals are in opposition to objectives in linear amplifier design, but some are. The application circuitry is very different in a polar transmitter than in a linear one. As the acceptance of polar techniques for multimode transmitter architecture grows, bipolar RF power technology can maintain its dominant position by adapting to the different requirements of a polar transmitter.

TROPIAN INC.
www.tropian.com
TXTLINX.COM74
Email this article to a friend!
 


Copyright © 2007 Octagon Communication Inc. DBA MPDigest / MPDigest.com, All Rights Reserved.