CX74017 SKYWORKS | Alldatasheet

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the renewed interest for DCRs. illustrated in the generic line-up of Figure 1. Figure 1. The Superheterodyne Receiver

1 Homo: Greek from “homos” - same; Hetero: Greek from

time; Dyne: Greek from “dunamis” – power.

  • 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 fRF finter fimfLO fIF fIF Interferer Channel Image Image- reject BPF fIF Interferer Channel Channel- select BPF

Figure 2. Image-Rejection and Selectivity in a Superheterodyne Receiver (High-Side LO Injection)

with twice the slope of the fundamental on a logarithmic scale. significant gain is allocated to baseband stages after the mixer. described, that accompany higher RF gain. baseband output, 1/f noise sees especially high conversion gain. transistor-size tradeoff is feasible at low frequencies. Figure 13. Second Order Intercept Point (IP2)

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