ADL5350-EVAL AD [Analog Devices], ADL5350-EVAL Datasheet - Page 18

• Manufacturer Part Number: ADL5350-EVAL
• Description: LF to 4 GHz High Linearity Y-Mixer
• Manufacturer: AD [Analog Devices]

ADL5350
FUNCTIONAL DESCRIPTION
CIRCUIT DESCRIPTION
The ADL5350 is a GaAs MESFET, 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 GC node is the “gate-contact”
of the RF/IF port resistive switch. The GC node enables external
control of the bias level of the switching FET, allowing the user to
override the internal bias generation circuitry, and allow further
optimization of the mixer’s dynamic performance at frequencies
outside of the 800 MHz to 2000 MHz band.
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 down-conversion 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 up-conversion 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.
INPUT
LO
LO
VPOS
Figure 57. Simplified Schematic
V
S
GND
GC
OR OUTPUT
INPUT
RF
GND
RF
IF
IF
OUTPUT
OR INPUT
Rev. PrC | Page 18 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 8 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 RFIF pins can be left open while
tuning the LO buffer networks.
Table 8. 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 is decreased, then increase the
value of L4. If the current consumption decreases as the LO
frequency also decreases, then decrease the value of L4. After
determining the optimum inductor value, the current
consumption should be minimized at the desired LO frequency.
The bias inductor should have a self-resonant frequency greater than the
intended frequency of operation.
Tune LO buffer supply inductor for lowest supply current.
RF
IF
Figure 58. Reference Schematic
L1
Preliminary Technical Data
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
GC
ADL5350
7
2
VPOS
LOIN
LO
V
6
3
S
L4
L3
C3
C4
GND1
GND1
5
4
1