CX74017 データシートの表示（PDF） - Skyworks Solutions

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CX74017

On the Direct Conversion Receiver

Skyworks Solutions 

On the Direct Conversion Receiver

It can be seen from these equations and in Figure 13 that the

DC component due to the second order non-linearity is growing

with twice the slope of the fundamental on a logarithmic scale.

At the intercept point,

a2 A2

2

= a1 A ⇔

A=

2a1

a2

= IIP2

Due to the doubled slope of the second-order product,

IIP2 = Pin + ∆ with ∆ = Pout − IM2

Noise

Low frequency noise becomes a great concern in DCR [14], as

significant gain is allocated to baseband stages after the mixer.

Weak baseband signal levels of a few millivolts are still very

vulnerable to noise. This requires stronger RF stage gain to

alleviate the poor noise figure of baseband blocks, but of course

this must be traded against the linearity problems, just

described, that accompany higher RF gain.

Flicker noise, that is, 1/f noise, is the major baseband noise

contributor. Associated with DC flow, it has a spectral response

proportional to 1/f. In RF circuits, 1/f noise tends to be

modulated onto the RF signal. In the case of a mixer with

baseband output, 1/f noise sees especially high conversion gain.

In practice, flicker noise becomes an issue for Metal Oxide

Semiconductor (MOS) devices more than bipolar, and is

modeled as a voltage source in series with the gate. 1/f noise

CX74017

complicates the use of MOS transistors for RF circuits, since the

main method of reducing it in MOS is to increase the transistor’s

size, which increases the device capacitance. This adversely

affects RF gain. For this reason, it is preferable to use bipolar

transistors for DCR mixer designs. In the first baseband stages

after the mixer, it becomes possible to use MOS devices, as the

transistor-size tradeoff is feasible at low frequencies.

I/Q mismatches

Due to the high frequency of the LO, it is not possible to

implement the IQ demodulator digitally. An analog IQ

demodulator exhibits gain and phase imbalances between the

two branches, as well as the introduction of DC offsets. Such

imperfections distort the recovered constellation. Assuming

α ,ϕ being the amplitude and phase mismatch respectively

between the demodulator quadrature ports, and the complex

signal incident upon it having in-phase and quadrature

components I and Q:

Iout = (I cos(ωt) + Q sin(ωt))⋅ 2 cos(ωt)

Qout = (I cos(ωt) + Q sin(ωt))⋅ 2 (1+α ) sin(ωt + ϕ)

Filtering out the high frequency terms:

Iout = I

Qout = (1+α ) (− I sin(ϕ) + Q cos(ϕ))

Pout

IP3

Pout

OIP2

IP2

OIP2

IP2

fundamental

1

1

2

1

3

1

Pout

∆

fundamental

1

1

2

1

IIP2 Pin

Figure 13. Second Order Intercept Point (IP2)

Pin ∆ IIP2

Pin

+49dBm

101735A 13_071801

101735A

Skyworks Solutions, Inc., Proprietary and Confidential

7

July 20, 2001

Preliminary Data Subject to Change

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