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.

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LO leakage

Figure 8. LO Leakage

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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.

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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.

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f

0 f

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.