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RF3322

CABLE REVERSE PATH PROGRAMMABLE GAIN AMPLIFIER

RF Micro Devices 

RF3322

Preliminary

Evaluation Kit

General Description

The RF3322 PCBA is a fully assembled evaluation

board of the RF3322 reverse path high output power

programmable gain amplifier, useful for providing a

demonstration of the RF3322’s functionality. The

RF3322 PCBA is a digitally controlled variable gain

3

amplifier capable of driving a 75Ω source. The RF3322

is designed to send cable modem data with QPSK or

QAM modulated format at frequencies between 5MHz

and 65MHz. The gain is controlled by an 7-bit serial

data word which adjusts the output gain from -30dB to

+28 dB.

The kit includes a fully functional evaluation board

along with a serial data cable and software. The cable

connects directly to the parallel port of a standard PC.

The software is used to control the serially programma-

ble gain through a simple, easy to understand user

interface.

Input and output to the evaluation board is provided

through 50Ω SMA connectors. The input and output of

the evaluation board is matched to 50Ω and connected

through a balun for single-ended operation. This allows

easy connection to test equipment, but the evaluation

board can easily be converted to a 75Ω input and out-

put, or for differential input and output. The output cir-

cuit is matched using a 24Ω series resistor which is

used to bring the load impedance up to 75Ω when

using standard 50Ω test equipment. This will introduce

a loss which must be accounted for in all measure-

ments (see measurement section and evaluation board

schematic for more detail.)

PCBA Details

Input Circuit

The input to the RF3322 is differential and the imped-

ance is 75Ω; However, for ease of testing, the evalua-

tion board has been changed to single-ended and the

impedance has been matched to 50Ω. If a 75Ω input is

required, simply replace the 50Ω SMA connector with

a 75Ω F-style connector and remove R4 and R5.

Output Circuit

The output of the RF3322 is differential and the imped-

ance is 300Ω. In normal applications this is converted

into a single-ended 75Ω output using a 2:1 (voltage

ratio) transformer with a center-tap on the secondary

which supplies power to the output stage. The evalua-

tion board is configured for use with 50Ω test equip-

ment. This has been achieved with a 24Ω resistor in

series with the output to increase the load seen by the

3-36

device to 75Ω. This introduces a voltage loss of 3.5dB

which must be accounted for in all measurements.

Some spectrum analyzers have a setting to account for

this method of 75Ω testing (e.g., on a Rhode &

Schwartz spectrum analyzer the input can be set to

'”75Ω RAZ” and the loss is accounted for automati-

cally). A more accurate way of making this measure-

ment is to use a 75Ω spectrum analyzer, or use a

matching transformer or minimum loss pad. This

ensures that the source impedance seen by the equip-

ment is also 75Ω. If a 75Ω output is required, simply

replace the 50Ω SMA connector (J7) with a 75Ω F-

style connector and replace R5 with a 0Ω jumper. The

evaluation board is tested with a Coilcraft balun; how-

ever, additional baluns may be used as long as care is

taken in modifying the decoupling capacitors around

the balun. These capacitors can greatly affect the har-

monic suppression. Other baluns may be used but

should be tested for second and third order harmonic

suppression.

Transmit Enable

The transmit enable can be set to “continuous on” by

placing the TXEN jumper in the right-hand position

(right-hand position when viewing the top of the evalu-

ation board with the 25-pin connector closest to the

viewer) and placing the associated GND/VCC jumper

in the “VCC” position. The transmit enable can be set

to “continuous off” by placing the GND/VCC jumper in

the “GND” position. If a computer controlled signal is

used (J1), place the TXEN jumper in the left-hand posi-

tion.

TX EN

VCC

TX EN

VCC

GND

GND

Continuous ON

A

Continuous OFF

B

Figure 1. TX Enable Configuration

TX EN

VCC

GND

Software Controlled

C

Rev A2 010518

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