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Download AN-102A APPLICATION NOTE Interface Circuits for the QD Series
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AN-102A APPLICATION NOTE Interface Circuits for the QD Series Quadrature Demodulators by Lowell Hoover INTRODUCTION The two differential I/Q output ports, I / I The QD series quadrature demodulators provide Q / Q , each have 100 Ω differential source exceptional linearity and demodulation accuracy for impedance. Internally matched lowpass filters on the direct conversion and low-IF receiver applications. I/Q ports limit the demodulator’s noise bandwidth. This application note presents several circuit ideas for interfacing the differential I/Q outputs of the QD series demodulators to single-ended loads and modern differential A/D converters. and The I/Q outputs can be modeled as voltage sources with 100 Ω of differential source resistance. When a sinewave RF input signal at +3 dBm is demodulated, the resulting I and Q differential output signals are DEMODULATOR PORTS approximately 400 mV p-p centered at 0 V (ground). Figure 1 shows the block diagram of a QD series This is equivalent to four single-ended baseband demodulator. For proper operation, the demodulator outputs supplying –10 dBm each into a 50 Ω single- should be supplied with an LO drive signal having a ended load. power level within the demodulator’s specified LO To minimize amplitude imbalance and quadrature power range. The LO signal should have low 2nd harmonic content to achieve the rated LO-RF isolation. The LO and RF ports are single-ended and matched to 50 Ω. phase error, the baseband outputs should be terminated into a matched load impedance over the processed frequency bandwidth. The DC voltage offset present at each baseband output is typically within a few mV of ground and may be blocked with LO DC-blocking capacitors without degrading demodulator performance. 0° 90° INTERFACING TO SINGLE-ENDED LOADS RF A simple differential-to-single-ended amplifier circuit 1 based on the Analog Devices AD8130 is shown in 0° Figure 2. This circuit converts differential I and Q 0° outputs from a QD series demodulator into singleended I and Q signals capable of driving lowimpedance loads. The Vo input provides a means of level-shifting the I and Q signals to any convenient I I Q DC voltage level. This circuit has excellent amplitude Q and phase tracking from DC to 100 MHz. Figure 1. Quadrature Demodulator Block Diagram Tel: P o l yp h a s e M i c r o w a ve I n c . 1983 S Liberty Drive Bloomington, IN 47403 USA +1 (812) 323-8708 Fax: +1 (812) 323-8709 © 2008 Polyphase Microwave Inc. Web: www.polyphasemicrowave.com AN-102A The transformer interface of Figure 3 has excellent QDxxxxB Q amplitude and phase tracking from 100 kHz to over 49.9 Ω 300 MHz. It can drive 50 Ω single-ended loads with AD8130 Q CHANNEL OUTPUT less than 1 dB of insertion loss. 49.9 Ω Q DUAL A/D CONVERTER INTERFACE Vo In many direct conversion and low-IF receivers, the baseband I and Q outputs from the QD series I demodulators must be amplified and lowpass filtered 49.9 Ω before digitization. The QD series demodulator’s I AD8130 I CHANNEL OUTPUT 49.9 Ω I and Q differential outputs often must also be levelshifted to the A/D converter’s positive reference voltage. The differential amplifier circuit shown in Vo Figure 4 provides +14 dB gain and has a lowpass Figure 2. Active Interface to Single-Ended Loads The transformer circuit shown in Figure 3 provides an all-passive method of converting the differential I and Q outputs from a QD series demodulator into single-ended outputs. Since this method is ACcoupled, it cannot be used in applications that require I and Q DC response. The DC-blocking capacitors prevent self-modulation and degraded cutoff of 30 MHz. It also moves the common-mode voltage to the A/D converter’s reference voltage (Vref). The gain of the Analog Devices2 AD8132 differential amplifier is set by the ratio of the feedback resistor to the series input resistor. A 30 MHz differential lowpass filter is formed by the output resistors and capacitors. 51 Ω Q IIP2 performance. 499 Ω 100 pF 51 Ω 499 Ω 1 μF MINI-CIRCUITS Q CHANNEL OUTPUT Q CHANNEL A/D CONVERTER AD8132 51 Ω QDxxxxB Q Vref 2.49 kΩ QDxxxxB Q Vref 51 Ω 100 pF 2.49 kΩ Vref 2.49 kΩ 51 Ω I 499 Ω 100 pF 51 Ω Q 1 μF 499 Ω ADT2-1T+ I CHANNEL A/D CONVERTER AD8132 51 Ω I Vref 51 Ω 100 pF 2.49 kΩ I 1 μF MINI-CIRCUITS I CHANNEL OUTPUT Figure 4. A/D Converter Interface REFERENCES [1] Analog Devices, Inc., “AD8129/AD8130 I 1 μF Product Datasheet, Rev. 0”, 2001. ADT2-1T+ [2] Analog Devices, Inc., “AD8132 Product Figure 3. Transformer Circuit to Single-Ended Loads Datasheet, Rev. C”, 2003. Tel: P o l yp h a s e M i c r o w a ve I n c . 1983 S Liberty Drive Bloomington, IN 47403 USA +1 (812) 323-8708 Fax: +1 (812) 323-8709 © 2008 Polyphase Microwave Inc. Web: www.polyphasemicrowave.com