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CX74017 数据表(PDF) 2 Page - Skyworks Solutions Inc. |
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CX74017 数据表(HTML) 2 Page - Skyworks Solutions Inc. |
2 / 10 page CX74017 On the Direct Conversion Receiver 2 Skyworks Solutions, Inc., Proprietary and Confidential 101735A Preliminary Data Subject to Change July 20, 2001 downconversion to IF, any unwanted activity at a frequency spaced at fIF offset from fLO, on the opposite side of fLO from the desired RF channel, produces a mixing product falling right into the downconverted channel, at fIF. In practice, a RF bandpass filter, usually a Surface Acoustic Wave (SAW) device, is utilized to perform band selection ahead of the Low Noise Amplifier (LNA), while a second filter follows the LNA to perform image rejection. If these filters are identical, then, in fact, they share the burden of the two functions. But some amount of image rejection must particularly follow the LNA, for without it, the LNA noise figure effectively doubles due to the mixing of amplified image noise into the IF channel. Instead of the RF SAW filter, other passive filtering technologies, such as dielectric or ceramic, can also be featured. It can be seen from Figure 2 that the higher the IF, the more relaxed the requirements on the image reject filter cut-off frequency. Once at the IF, the presence of an interfering signal in the vicinity of the channel mandates sharp filtering around the channel; this is performed after the first mixer by the channel select filter, which is also often an IF SAW filter. Essentially, the exercise is that of a carefully engineered balance among several variables: • Rejection provided by the various filters • Frequency planning • Linearity of the active stages Dual IFs provide additional room to maneuver with filter selectivity, but complicate the frequency planning somewhat. The selectivity required of the two aforementioned filters, in terms of fractional bandwidth, makes them unsuitable candidates in the foreseeable future for integration. This is because of low Qs of current silicon processes and the need to be implemented by bulky, off-chip components. The IF channel filter especially requires high-Q resonators for its implementation: the higher the IF, the lesser the filter’s fractional bandwidth, that is, its ratio of bandwidth to center frequency, necessitating ever-higher Q. This high-Q requirement is most commonly met by the use of a piezoelectric SAW and crystal filters. This introduces additional constraints, as those filters require often-inconvenient terminating impedances, and matching may impinge on such issues as noise, gain, linearity, and power dissipation of the adjoining active stages. The narrower the fractional bandwidth, the more likely that the filter’s passband shape will exhibit an extreme sensitivity to variations in matching element values. Additionally, the specificity of the IF filter to the signal bandwidth and hence the standard used, makes superheterodyne receivers unsuitable for multi-standard operation. Nonetheless, superheterodyne is praised for its high selectivity and sensitivity. Image-Reject Receivers Alternatively, by a smart use of trigonometric identities, the image can be removed without the need of any post-LNA image-reject filtering. This is the principle of image-reject receivers [8] and [10]. The first is the Hartley architecture, introduced in [11] in 1928, and shown in Figure 3. It uses two mixers with their local oscillators in a quadrature phase relationship. This separates the IF signal into in-phase (I) and quadrature (Q) components. It then shifts the Q component by 90 ° before recombining the two paths. This is where the desired signal, present in both paths with identical polarities, is reinforced, while the image, present in both paths with opposite polarities, is cancelled out. The dual of the Hartley architecture, known as the Weaver image-reject receiver [12], achieves the relative phase shift of one path by 90º by the use of a second LO enroute to another IF or to baseband, see Figure 4. The same result is achieved. However, the reliability of these receivers depends heavily on the accuracy of the I/Q paths, that is, the gain and phase imbalance between the two branches. 101735A 2_071901 f RF f inter f im f LO f IF f IF Interferer Channel Image Image- reject BPF f IF Interferer Channel Channel- select BPF Figure 2. Image-Rejection and Selectivity in a Superheterodyne Receiver (High-Side LO Injection) |
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类似说明 - CX74017 |
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