11
LT5526
5526f
The purpose of L5 is to provide a DC return path for Pin 3.
(Another possible placement for L5 would be across Pins
2 and 3, thus using L1 as part of the DC return path.) The
inductance and resonant frequency of L5 should be large
enough that they don’t significantly affect the input imped-
ance and performance of the balun. Either multilayer or
wire-wound inductors may be used.
The impact of L5 on input matching can be reduced by
adding a capacitor in parallel with it. In this case, the
capacitor value should be the same as C7 and C9, while L5
should have the same value as L1 and L4.
Table 2. Component Values for Lumped Balun on RF Input
FREQUENCY BANDWIDTH
(MHz) L (nH) C (pF) L5 (nH) (MHz)
240 27 18 100 100
380 15 10 100 130
680 6.8 4.7 47 215
900 6.8 3.9 18 230
1100 3.9 2.7 15 230
LO Input Port
The LO buffer amplifier consists of high speed limiting
differential amplifiers designed to drive the mixer core for
high linearity. The LO
+
and LO
–
pins are designed for single-
ended drive, though differential drive can be used if de-
sired. The LO input is internally matched to 50Ω; however,
external DC blocking capacitors are required because the
LO pins are internally biased to approximately 1.7V DC. A
simplified schematic for the LO input is shown in Figure 7.
APPLICATIO S I FOR ATIO
WUUU
50Ω
C5
100pF
C6
100pF
LO
IN
50Ω
LT5526
LO
–
LO
+
5526 F07
V
CC
14
15
Figure 7. LO Input Schematic
Figure 8. Typical LO Input Return Loss
with 100pF DC Blocking Capacitors
External 100pF DC blocking capacitors provide a broad-
band match from about 110MHz to 2.7GHz, as shown in
the plot of return loss vs frequency in Figure 8. The LO
input match can be improved at lower frequencies by
increasing the values of C5 and C6.
Table 3. Single-Ended LO Input Impedance
FREQUENCY INPUT REFLECTION COEFFICIENT
(MHz) IMPEDANCE MAG ANGLE
400 63.4 – j12.0 0.158 –35.8
600 61.6 – j8.38 0.128 –31.5
800 61.8 – j6.86 0.122 –26.6
1000 62.4 – j7.09 0.127 –26.1
1200 62.8 – j8.32 0.135 –28.8
1400 62.6 – j10.3 0.144 –34.0
1600 61.9 – j12.6 0.154 –40.3
1800 60.5 – j14.4 0.160 –46.2
IF Output Port
A simplified schematic of the IF output circuit is shown in
Figure 9. The output pins, IF
+
and IF
–
, are internally
connected to the collectors of the mixer switching transis-
tors. Both pins must be biased at the supply voltage, which
can be applied through the center-tap of a transformer or
FREQUENCY (MHz)
0
–30
RETURN LOSS (dB)
–25
–20
–15
–10
–5
0
500 1000 1500 2000
5526 F08
2500
