RF2721 查看數據表（PDF） - RF Micro Devices

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RF2721

QUADRATURE DEMODULATOR

RF Micro Devices 

Preliminary

RF2721

Pin Function Description

Interface Schematic

1

IF IN+

Balanced IF input. An input level of 400mVPP gives full output swing; a

level of maximum 250mVPP is recommended for linear operation. This

pin has no internal DC block, and an external capacitor of 100nF is

IF IN+

IF IN-

needed if connected to a DC path.

1500 Ω

1500 Ω

350 Ω

350 Ω

2

GND

Ground connection. For best performance, keep traces physically short

and connect immediately to ground plane.

3

OSC B

Base of the LO transistor. This pin is not connected to anything else.

This transistor can be used for an oscillator. The collector of the transis-

tor is connected to VCC.

VCC

OSC B

OSC E

4

OSC E

Emitter of the LO transistor. This pin is not connected to anything else. See pin 3.

5

GND

Same as pin 2.

6

LO2-

Balanced input for the doubled LO. An internal divide-by-2 network

generates the actual LO for the mixers. The divided signal is not acces-

LO2+

200 Ω

sible. When the LO is driven single ended this pin should be connected

7

to a 100nF capacitor to ground. This pin has an internal pull-up resistor

200 Ω

to VCC and an external DC blocking capacitor of 100nF is recom-

LO2-

mended.

7

LO2+

Balanced input for the doubled LO. The input frequency should be

See pin 6.

twice (2x) the IF frequency. An internal divide-by-2 network generates

the actual LO for the mixers. This pin has an internal pull-up resistor to

VCC, and an external DC blocking capacitor of 100nF is recommended.

LO frequencies below 100kHz are acceptable, providing that the LO

signal is square wave. Above 100kHz a sine wave signal is acceptable.

The quadrature accuracy of the LO/2 frequency into the mixers is

affected by the duty cycle of the LO input signal. Square wave signals

with other than 50% duty cycle will degrade the quadrature accuracy of

the LO signal, thereby adversely affecting the I and Q quadrature accu-

racy. Since the LO input is AC coupled, asymmetric sine waves and

sine waves clipped on either the top or bottom half will not have 50%

duty cycles relative to the internal DC reference point. For this reason,

distorted LO sine wave signals will degrade performance in a fashion

similar to non-50% duty cycle square waves. A sine wave input with

even harmonics less than -15dBc is recommended. The internal limit-

ing buffer amplifier ensures the amplitude stability of the demodulator.

8

Q OUT

Demodulated baseband Q output. The reference DC level of this pin is

set by the voltage of pin 9 (REF IN), which is connected to this pin

through a 5kΩ resistor to the collectors of this push-pull output. This

results in an output impedance of 5kΩ if the REF IN pin is connected to

a low impedance source. The capacitance of the load determines the

maximum baseband frequency. A very low capacitive load may stretch

the 3dB bandwidth over 10MHz. Another way to increase bandwidth is

by connecting a shunt resistor. This will trade-off gain for bandwidth.

5 kΩ

REF IN

I/Q OUT

9

REF IN

Reference voltage input for the baseband outputs. This pin can be con- See pin 8.

nected to the reference voltage source of the Analog to Digital Con-

verter. It is connected to the I and Q outputs through 5kΩ resistors.

This pin should have an external decoupling capacitor large enough to

decouple the lowest baseband frequency.

Rev A2 010509

2-29

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