ADL5350 Analog Devices, Inc., ADL5350 Datasheet - Page 17

• Manufacturer Part Number: ADL5350
• Description: Lf To 4 Ghz High Linearity Y-mixer
• Manufacturer: Analog Devices, Inc.

FUNCTIONAL DESCRIPTION
CIRCUIT DESCRIPTION
The ADL5350 is a GaAs pHEMT, single-ended, passive mixer
with an integrated LO buffer amplifier. The device relies on the
varying drain to source channel conductance of a FET junction
to modulate an RF signal. A simplified schematic is shown in
Figure 57.
The LO signal is applied to the gate contact of a FET-based
buffer amplifier. The buffer amplifier provides sufficient gain of
the LO signal to drive the resistive switch. Additionally,
feedback circuitry provides the necessary bias to the FET buffer
amplifier and RF/IF ports to achieve optimum modulation
efficiency for common cellular frequencies.
The mixing of RF and LO signals is achieved by switching the
channel conductance from the RF/IF port to ground at the rate
of the LO. The RF signal is passed through an external band-
pass network to help reject image bands and reduce the
broadband noise presented to the mixer. The band-limited RF
signal is presented to the time-varying load of the RF/IF port,
which causes the envelope of the RF signal to be amplitude
modulated at the rate of the LO. A filter network applied to the
IF port is necessary to reject the RF signal and pass the wanted
mixing product. In a downconversion application, the IF filter
network is designed to pass the difference frequency and
present an open circuit to the incident RF frequency. Similarly,
for an upconversion application, the filter is designed to pass
the sum frequency and reject the incident RF. As a result, the
frequency response of the mixer is determined by the response
characteristics of the external RF/IF filter networks.
Preliminary Technical Data
INPUT
LO
LO
VPOS
V
Figure 57. Simplified Schematic
S
GND1
OR OUTPUT
INPUT
RF
GND2
RF
IF
IF
OUTPUT
OR INPUT
Rev. PrE | Page 17 of 24
IMPLEMENTATION PROCEDURE
The ADL5350 is a simple single-ended mixer that relies on off-
chip circuitry to achieve effective RF dynamic performance.
The following steps should be followed to achieve optimum
performance (see Figure 58 for component designations):
1.
To start this procedure, it is necessary to provide an initial
guess. Table 7 can be used as a starting point. It is not necessary
to terminate or populate the RF and IF port networks to
complete this first step. The RF/IF pins can be left open while
tuning the LO buffer networks.
Table 7. Recommended LO Bias Inductor
Desired LO Frequency
380 MHz
750 MHz
1000 MHz
1750 MHz
2000 MHz
1
To test the supply current consumption, power up the device
and apply the desired LO signal. Next, attempt to increase and
decrease the LO frequency. If the current consumption
increases as the LO frequency decreases, increase the value of
L4. If the current consumption decreases as the LO frequency
also decreases, decrease the value of L4. After determining the
optimum inductor value, the current consumption should be
minimized at the desired LO frequency.
intended frequency of operation.
The bias inductor should have a self-resonant frequency greater than the
Tune LO buffer supply inductor for lowest supply current.
RF
IF
Figure 58. Reference Schematic
L1
L2
C1
RF/IF
RF/IF
8
1
Recommended LO Bias Inductor (L4)
68 nH
24 nH
18 nH
3.8 nH
2.1 nH
C6
C2
GND2
NC
ADL5350
7
2
VPOS
LOIN
LO
V
6
3
S
L4
L3
C3
C4
GND1
GND1
5
4
ADL5350
1