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74017 데이터시트(PDF) 5 Page - Skyworks Solutions Inc. |
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74017 데이터시트(HTML) 5 Page - Skyworks Solutions Inc. |
5 / 10 page On the Direct Conversion Receiver CX74017 101735A Skyworks Solutions, Inc., Proprietary and Confidential 5 July 20, 2001 Preliminary Data Subject to Change Conversely, a strong in-band interferer, once amplified by the LNA, may find a path to the LO input port of the mixer, see Figure 9, thus producing self-mixing, once again. 101735A 8_071801 LO leakage Figure 8. LO Leakage 101735A 9_071801 Strong Interferer Figure 9. Strong In-Band Interferer, Amplified by the LNA Some amount of LO power is conducted through the mixer and LNA, due to their non-ideal reverse isolation, to the antenna. The radiated power, appearing as an interferer to other receivers in the corresponding band, may violate emissions standards of the given system. It is important to note that since the LO frequency is inside the receive band, the front-end filters do nothing to suppress this LO emission. Additionally, the radiated LO signal can then be reflected by buildings or moving objects and re-captured by the antenna, as shown in Figure 10. This effect, however, is not of significant importance compared to the previously mentioned LO self-mixing and blocker self-mixing. LO or RF signal leakage to the opposite mixer port is not the only way in which unwanted DC can be produced. Any stage that exhibits even-order non-linearity also generates a DC output. This is covered in more detail later. 101735A 10_071801 Re r a d i a t e d Reflected, Refracted Figure 10. Emission, Reflection, and Refraction of LO Signal via Antenna Whether or not the DC product desensitizes the receiver depends on the system type. Obviously, it is preferable to alternate current (AC)-couple at the mixer output to eliminate the DC. Some modulation schemes used in paging applications, such as Frequency-Shift Keying (FSK), show little degradation if low frequency spectrum components are filtered out, see Figure 11. 101735A 11_071801 f 0 f c1 fc2 High pass Figure 11. High-Pass Filtering of the Modulation Spectrum However, other modulation schemes present a peak at DC, and capacitive AC-coupling will infer significant information loss, hence considerably degrading the BER. In TDMA systems such as GSM, although there is no significant low frequency spectral peak, it still becomes impossible to AC couple. This is because of the conflicting requirements on an AC-coupling capacitor in a Time-Division Multiple Access (TDMA) system: the capacitor must be large enough to avoid causing a wide notch at DC, but it must be small enough that all transients settle out upon receiver power-up (every frame) before data reception begins. |
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