Preliminary
3-36
RF3322
Rev A2 010518
3
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
amplifier capable of driving a 75
Ω 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 50
Ω SMA connectors. The input and output of
the evaluation board is matched to 50
Ω 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 75
Ω input and out-
put, or for differential input and output. The output cir-
cuit is matched using a 24
Ω series resistor which is
used to bring the load impedance up to 75
Ω when
using standard 50
Ω 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 75
Ω; However, for ease of testing, the evalua-
tion board has been changed to single-ended and the
impedance has been matched to 50
Ω.If a 75Ω input is
required, simply replace the 50
Ω SMA connector with
a75
Ω F-style connector and remove R4 and R5.
Output Circuit
The output of the RF3322 is differential and the imped-
ance is 300
Ω. In normal applications this is converted
into a single-ended 75
Ω 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 50
Ω test equip-
ment. This has been achieved with a 24
Ω resistor in
series with the output to increase the load seen by the
device to 75
Ω. 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 75
Ω testing (e.g., on a Rhode &
Schwartz spectrum analyzer the input can be set to
'”75
Ω RAZ” and the loss is accounted for automati-
cally). A more accurate way of making this measure-
ment is to use a 75
Ω spectrum analyzer, or use a
matching transformer or minimum loss pad. This
ensures that the source impedance seen by the equip-
ment is also 75
Ω.If a 75Ω output is required, simply
replace the 50
Ω SMA connector (J7) with a 75Ω F-
style connector and replace R5 with a 0
Ω 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.
Continuous ON
A
TX EN
GND
VCC
Continuous OFF
B
TX EN
GND
VCC
Software Controlled
C
GND
VCC
TX EN
Figure 1. TX Enable Configuration
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