HFA3783 Datasheet, PDF (20/34 Pages) Intersil Corporation – I/Q Modulator/Demodulator and Synthesizer

HFA3783
steering mechanism for gain control. The amplifier main
function is controlling the power output of the transmit signal
and has very linear AGC characteristics as shown in Figure
35. The differential open collector outputs require V_CC
biasing as with any open collector application and exhibit
high isolation. The HFA3783 output impedance is constant
whether in the receive or transmit mode. Consequently, a
combination matching network with the use of a single SAW
filter can be used for both halves of the duplex operation.
Single ended operation is discouraged due to; TX and RX
return loss variation, loss of power output and lack of
cancellation of PLL induced spurious signals. Differential
summing match networks are strongly recommended when
using single end SAW devices. S parameters for the output
port are available in the S Parameter Tables section.
The AGC amplifier feature an output compression level of
1V_P-P, with a cascaded performance capable of generating
a typical CW power of -10dBm into 250Ω when differential
inputs of 250mV DC are applied to both “I” and “Q” inputs.
IF interface
Both modulator and demodulator of the HFA3783 AC
Cascaded Specifications in this document were
characterized in a 250Ω system. The high impedance of the
receive input and the open collector output structure of the
transmit channel permit the use of a combination match
network capable of interfacing with only one differential filter
device in duplex operation. In addition, the HFA3783 input
and output impedances have small variations when the
device changes its mode of operation from transmit to
receive. The system impedance (250Ω) is defined by the
filter input/output impedance including its own match
networks and this value has been chosen as a compromise
between current consumption, voltage swing and therefore
compression. A higher system Zo can compromise the
voltage swing capabilities due to the low voltage operation of
the HFA3783 and a low system Zo affects the power supply
current consumed by the application in general, for the same
RF power budget.
The output match network of the transmit output, includes a
differential “L” match network used to bias the differential
collectors which are of high impedance. This high
impedance is lowered to a value of around 2KΩ by a parallel
resistor placed across the collector terminals. This value
sets the output impedance of the two collectors and also
serves as a compromise value for the loaded “Q” of the
network for a desired system bandwidth. The other side of
the match network is set to match 250Ω (from a filter match
application) and is directly connected to the receive
differential terminals; therefore presenting a controlled
termination to the high input impedance port of the receive
AGC. The use of DC blocking capacitors is needed to avoid
a DC path between the HFA3783 receive terminals and is
maybe optional depending of the differential network used to
match an external filter to a 250Ω system.
FILTER MATCH
NETWORK
† AVOID GROUND RETURN
FILTER
FOR V_CC BYPASS
CLOSE TO PIN 5 GND.
250Ω
250Ω
V_CC
†
PIN 9
PIN 8
PIN 4 PIN 3
HFA3783
FIGURE 7. SIMPLIFIED IF INPUT/OUTPUT COMBINED MATCH
NETWORK
As with any differential network, symmetry is paramount.
The use of matched length lines and good differential
isolation, helps the structure reject common mode induced
signals from other parts of the system. Special attention to
the collector outputs is necessary to reject V_CC induced
spurious signals and to reject internally induced PLL
spurious tones. Although the network topology is simple
theoretically, its implementation is challenged by layout
routing and parasitics which have to be taken into
consideration.
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