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ADL5390 Datasheet(PDF) 11 Page - Analog Devices |
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ADL5390 Datasheet(HTML) 11 Page - Analog Devices |
11 / 24 page ADL5390 Rev. 0 | Page 11 of 24 GENERAL STRUCTURE THEORY OF OPERATION The simplified block diagram given in Figure 26 shows a matched pair of variable gain channels whose outputs are summed and presented to the final output. The RF/IF signals propagate from the left to the right, while the baseband gain controls are placed above and below. The proprietary linear- responding variable attenuators offer excellent linearity, low noise, and greater immunity from mismatches than other commonly used methods. Since the two independent RF/IF inputs can be combined in arbitrary proportions, the overall function can be termed “vector multiplication” as expressed by VOUT = VIRF × (VIBB/VO) + VQRF × (VQBB/VO) where: VIRF and VQRF are the RF/IF input vectors. VIBB and VQBB are the baseband input scalars. VO is the built-in normalization factor, which is designed to be 0.285 V (1/3.5 V). The overall voltage gain, in linear terms, of the I and Q channels is proportional to its control voltage and scaled by the normali- zation factor, i.e., a full-scale gain of 1.75 (5 dB) for VI (Q)BB of 500 mV. A full-scale voltage gain of 1.75 defines a gain setpoint of 1.0. Due to its versatile functional form and wide signal dynamic range, the ADL5390 can form the core of a variety of useful functions such as quadrature modulators, gain and phase ad- justers, and multiplexers. At maximum gain on one channel, the output 1 dB compression point and noise floor referenced to 50 Ω are 11 dBm and −148 dBm/Hz, respectively. The broad frequency response of the RF/IF and gain control ports allows the ADL5390 to be used in a variety of applications at different frequencies. The bandwidth for the RF/IF signal path extends from approximately 20 MHz to beyond 2.4 GHz, while the gain controls signals allow for modulation rates greater than 200 MHz. Matching between the two gain channels is ensured by careful layout and design. Since they are monolithic and arranged symmetrically on the die, thermal and process gradients are minimized. Typical gain and phase mismatch at maximum gain are <0.5 dB and <0.5°. LINEAR ATTENUATOR LINEAR ATTENUATOR I-V VQBB Q CHANNEL BASEBAND INPUT VIRF, I CHANNEL SINGLE-ENDED OR DIFFERENTIAL VIBB OUTPUT DISABLE SINGLE-ENDED OR DIFFERENTIAL 50 Ω OUTPUT VQRF, Q CHANNEL SINGLE-ENDED OR DIFFERENTIAL I CHANNEL BASEBAND INPUT V-I V-I Figure 26. Simplified Architecture of the ADL5390 NOISE AND DISTORTION The signal path for a particular channel of the ADL5390 con- sists basically of a preamplifier followed by a variable attenuator and then an output driver. Each subblock contributes some level of noise and distortion to the desired signal. As the channel gain is varied, these relative contributions change. The overall effect is a dependence of output noise floor and output distortion levels on the gain setpoint. For the ADL5390, the distortion is always determined by the preamplifier. At the highest gain setpoint, the signal capacity, as described by the 1 dB compression point (P1dB) and the third- order intercept (OIP3), are at the highest levels. As the gain is reduced, the P1dB and OIP3 are reduced in exact proportion. At the higher gain setpoints, the output noise is dominated by the preamplifier as well. At lower gains, the contribution from the preamplifier is correspondingly reduced and eventually a noise floor, set by the output driver, is reached. As Figure 27 illustrates, the overall dynamic range defined as a ratio of OIP3 to output noise floor remains constant for the higher gain setpoints. At some gain level, the noise floor levels off and the dynamic range degrades commensurate with the gain reduction. 175 170 165 160 155 150 145 140 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 GAIN SETPOINT DYNAMIC RANGE = OIP3 – (OUTPUT NOISE FLOOR (NO CARRIER)) Figure 27. Dynamic Range Variation with Gain Setpoint |
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