QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 678A 40MHZ TO 900MHZ DIRECT CONVERSION QUADRATURE DEMODULATOR LT5517 DESCRIPTION Demonstration circuit 678A is a 40MHz to 900MHz Direct well-balanced divide-by-two circuit generates precision Conversion Quadrature Demodulator featuring the quadrature LO carriers to drive the I mixer and the Q LT5517. The LT 5517 is a direct conversion quadrature mixer. Consequently, the outputs of the I-channel and the demodulator optimized for high linearity receiver applica- Q-channel are well matched in amplitude, and their tions where high dynamic range is important. It is suit- phases are 90 apart. The LT5517 also provides excellent able for communications receivers where an RF signal is 50W impedance matching at the 2XLO port across its en- directly converted into I and Q baseband signals with a tire frequency range. bandwidth up to 130MHz. The LT5517 incorporates bal- Demonstration circuit 678A is designed for operations in anced I and Q mixers, LO buffer amplifiers and a preci- the frequency range between 40MHz and 900MHz. sion, broadband quadrature generator derived from an Design files for this circuit board are available. Call the on-chip divide-by-two circuit. LTC factory. The superior linearity and low noise performance of the LT5517 is achieved across its full frequency range. A Table 1. Typical Performance Summary (T = 25C) A PARAMETER CONDITION (f = 800MHz, f = 1602MHz) VALUE RF LO Supply Voltage 4.5V to 5.25V Supply Current V = 5V, EN = High 90mA CC Maximum Shutdown Current V = 5V, EN = Low 20A CC RF Frequency Range 40MHz to 900MHz 2XLO Frequency Range 80MHz to 1800MHz 2XLO Input Power -15dBm to 0dBm Conversion Gain Voltage Gain, Load Impedance = 1kW, P = -10dBm, P = -10dBm 3.3dB RF 2XLO Noise Figure P = -10dBm 12.4dB 2XLO rd Input 3 Order Intercept 2-Tone, -10dBm/Tone, Df = 200kHz, P = -10dBm 21dBm 2XLO nd Input 2 Order Intercept 2-Tone, -10dBm/Tone, Df = 200kHz, P = -10dBm 58dBm 2XLO Input 1dB Compression P = -10dBm 10dBm 2XLO Baseband Bandwidth 130MHz I/Q Gain Mismatch P = -10dBm, P = -10dBm, output frequency = 1MHz 0.03dB RF 2XLO I/Q Phase Mismatch P = -10dBm, P = -10dBm, output frequency = 1MHz 0.7 RF 2XLO Output Impedance Differential 120W 2XLO to RF leakage P = -10dBm -69dBm 2XLO LO to RF leakage P = -10dBm -80dBm 2XLO RF to 2XLO Isolation P = -10dBm 63dB RF 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 678A 40MHZ TO 900MHZ DIRECT CONVERSION QUADRATURE DEMODULATOR APPLICATION NOTE The 2XLO port is also internally matched to 50W im- FREQUENCY RANGE pedance. No external matching is required for its entire Demonstration circuit 678A is designed for the broad fre- operating frequency range. quency range between 40MHz and 900MHz. However, further improvements in performance at specific frequen- OUTPUT FILTERING cies may be achieved by employing appropriate external Proper filtering of unwanted high frequency mixing prod- RF input matching. ucts at the I- and Q-outputs is important to maintain su- perior linearity. The most convenient method is to termi- 2XLO INPUT nate each output with a shunt capacitor to ground. The The 2XLO frequency is required to be twice the desired capacitor value should be optimized depending upon the operating frequency in order for the chip to generate the operating frequency. However, the capacitors may reduce quadrature Local Oscillator (LO) signals for the demodu- baseband output bandwidth. In the standard demonstra- lator. The on-chip divide-by-two circuit delivers well- tion circuit 678A, each I- and Q-output is terminated with matched, quadrature LO carriers to the I mixer and the Q a 10pF shunt capacitor. mixer. QUICK START PROCEDURE Demonstration circuit 678A is easy to set up to evaluate 4. Set Signal Generator 1 to provide a 1602MHz, the performance of the LT5517. Refer to Figure 1 for -10dBm, CW signal to the demo board 2XLO input proper measurement equipment setup and follow the pro- port. cedure below: 5. Set the Signal Generators 2 and 3 to provide two NOTE: -10dBm CW signals to the demo board RF input port one at 799.9MHz, and the other at 800.1MHz. a. Use high performance signal generators with low harmonic output for 2-tone measurements. Other- 6. Set the Spectrum Analyzers start frequency to 100kHz nd wise, low-pass filters at the signal generator outputs and stop frequency to 1400kHz. Perform input 2 or- rd should be used to suppress higher-order harmonics. der and 3 order distortion measurement. Sufficient spectrum analyzer input attenuation should be used to b. High quality combiners that provide 50W termination avoid saturating the instrument. on all ports and have good port-to-port isolation should be used. Attenuators on the outputs of the 7. IIP2 = P1 P2 + Pin, IIP3 = (P1 P3) / 2 + Pin. Where signal generators are recommended to further im- P1 is the power level of the two fundamental output nd prove source isolation and to reduce reflection into tones at 900kHz and 1100kHz, P2 is the 2 order prod- rd the sources. uct at 200kHz, P3 is the 3 order product at 700kHz and 1300kHz, and Pin is the input power (in this case, - 1. Connect all test equipment as shown in Figure 1. 10dBm). All units are in dBm. 2. Set the DC power supplys current limit to 120mA, and 8. Voltage conversion gain can also be measured. But adjust output voltage to 5V. beware that the gain is reduced by combiner loss plus 3. Connect Vcc to the 5V DC supply, and then connect EN 6.85dB, because the load impedance to each output pin to 5V the demodulator is enabled (on). is 50W in this setup. Please refer to the LT5517 data sheet for more detailed explanation. 2