QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1178 HIGH LINEARITY DIRECT QUADRATURE MODULATOR LT5568-2 DESCRIPTION Demonstration circuit 1178 is an I/Q modulator featuring (R) the LT 5568-2. The LT 5568-2 is a 700MHz to 1.05GHz direct I/Q modulator designed for high performance wireless applications, including wireless infrastructure. It is optimized for image rejection in the 850 to 960 MHz frequency range. It may also be configured as an image reject upconverting mixer, by applying 90 phase-shifted signals to the I and Q inputs. an on-chip RF transformer, which converts the differential mixer signals to a 50 single-ended output. The four balanced I and Q base-band input ports are intended for DC coupling from a source with a commonmode voltage level of about 0.5V. The differential input impedance of the baseband inputs is approximately 100, making them ideally suited for current-drive applications. A high-speed, internally matched LO amplifier drives two double-balanced mixer cores, allowing the use of a low power, single-ended LO source. It allows direct modulation of an RF signal using differential baseband I and Q signals. It supports GSM, EDGE, CDMA, CDMA2000, W-CDMA, 64-QAM, OFDM and other modulation formats. The LO path consists of an LO buffer with single-ended input, and precision quadrature generators which produce the LO drive for the mixers. The I/Q baseband inputs consist of voltage-to-current converters that in turn drive double-balanced mixers. The outputs of these mixers are summed and applied to Demonstration circuit 1178 is designed for an RF output frequency range from 700MHz to 1.05GHz. Design files for this circuit board are available. Call the LTC factory. LT is a registered trademark of Linear Technology Corporation. 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1178 HIGH LINEARITY DIRECT QUADRATURE MODULATOR Table 1. Typical Performance Summary (TA = 25C) PARAMETER CONDITION (fBB = 2MHz, fLO=850MHz) Supply Voltage VALUE 4.5V to 5.25V Supply Current VCC = 5V, EN = High 110 mA Maximum Shutdown Current VCC = 5V, EN = Low 245 A RF Frequency Range 700 to 1050 MHz Baseband Frequency Range DC to 380 MHz LO Input Return Loss Z0 = 50, PLO = 0dBm 15 dB RF Output Return Loss Z0 = 50 18 dB LO Input Power -10 to +5 dBm LO Frequency Range 700 to 1050 MHz PRF = -10dBm, PLO = 0dBm, defined as PRF/PBB -6.8 dB rd 2-Tone, PRF = -10dBm/Tone, f = 100KHz, PLO = 0dBm +22.9 dBm nd Output 2 Order Intercept 2-Tone, PRF = -10dBm/Tone, f = 100KHz, PLO = 0dBm +59 dBm Output 1dB Compression PLO = 0dBm +8.6 dBm LO leakage PLO = 0dBm -43 dBm Image Rejection PLO = 0dBm -52 dBc Conversion Gain Output 3 Order Intercept 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1178 HIGH LINEARITY DIRECT QUADRATURE MODULATOR APPLICATION NOTE RF FREQUENCY RANGE The LO leakage may be further reduced by the No tuning is required for operation over the RF frequency range of 700 MHz to 1.05 GHz. VBBQP - VBBQM) at the BASEBAND FREQUENCY RANGE The baseband frequency range extends from DC to 380 MHz (3dB bandwidth). LO TO RF LEAKAGE The LT(R)5568-2 offers excellent LO to RF leakage performance, typically -43 dBm. TEST EQUIPMENT AND SETUP Refer to Figure 1 for proper measurement equipment setup. Before performing measurements on the DUT, it is very important to evaluate the test system performance to ensure that: 1) distortion-free input signals are applied and 2) the spectrum analyzer internal distortion is minimized. Follow the guidelines below to do this. introduction of small differential DC offsets (VBBIP - VBBIM, baseband inputs, typically less than 10mV. These DC offsets may be introduced, for example, using the DC offset points shown in figure 1. SIDETONE TO RF LEAKAGE The LT(R)5568-2 also offers very good image rejection (sidetone suppression) at the RF port. The image rejection may be further enhanced by the introduction of small differential phase and amplitude offsets at the baseband inputs. Spectrum analyzers can produce significant internal distortion products if they are overdriven. Sufficient spectrum analyzer input attenuation should be used to avoid saturating the instrument. A typical input 3rd order intercept point for a spectrum analyzer is +40 dBm, with 20 dB input attenuation applied. This is more than 10 dB above the intercept point of the DUT, and should yield accurate 3rd order distortion results. Use high performance signal generators with low harmonic output (>75 dBc) for 2-tone measurements. The signal generators must provide 2 equal amplitude outputs in quadrature with one another. High quality combiners that provide broadband 50 ohm termination on all ports should be used. The combiners should have good portto-port isolation (>30 dB) to prevent the signal generators from modulating each other and generating intermodulation products. Attenuators on the outputs of these generators can also be used to increase the effective port-toport isolation. 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1178 HIGH LINEARITY DIRECT QUADRATURE MODULATOR QUICK START PROCEDURE Demonstration circuit 1178 is easy to set up to evaluate (R) the performance of the LT 5568-2. Refer to Figure 1 for proper measurement equipment setup. Follow the procedure below: 1. Connect all test equipment as shown in Figure 1. 2. Set the DC power supply's current limit to 150mA, and adjust output voltage to 5V. 3. Connect Vcc to the 5V DC supply, and then connect VCCEN to 5V; the modulator is enabled (on). 4. Set Signal Generator #1 to provide a 900MHz, 0dBm, CW signal to the demo board LO input port. 5. Set the Signal Generators #2 and #3 to provide two 10dBm CW signals to the combiner ports - one at 2MHz, and the other at 2.1MHz. For both generators, output B should lead output A by 90 degrees. These 2 signals must be in quadrature to drive the demo board properly. This is most conveniently accomplished by using signal generators with dual outputs with adjustable phase. An example is the HP3326A shown in Figure 1. 6. To measure 3rd order distortion and conversion gain, set the Spectrum Analyzer start and stop frequencies to 901.8MHz and 902.3MHz, respectively. Sufficient spectrum analyzer input attenuation should be used to avoid distortion in the instrument. 7. The 3rd order intercept point is equal to (P1 - P3) / 2 + P1, where P1 is the average power level of the two fundamental output tones at 902MHz and 902.1MHz; P3 is the average power level of the two 3rd order products at 901.9MHz and 902.2MHz. All units are in dBm. 8. To measure 2nd order distortion, set the Spectrum Analyzer start and stop frequencies to 904MHz and 906MHz, respectively. Sufficient spectrum analyzer input attenuation should be used to avoid distortion in the instrument. 9. nd The 2 order intercept point is equal to 2*P1 - P2, where P1 is the power level of the fundamental output tone at 902MHz, P2 is the 2nd order product at 904.1MHz. All units are in dBm. 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1178 HIGH LINEARITY DIRECT QUADRATURE MODULATOR Figure 1. Proper Measurement Equipment Setup 5 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1178 HIGH LINEARITY DIRECT QUADRATURE MODULATOR 6