hpmx-2003 ETC-unknow, hpmx-2003 Datasheet - Page 8

• Manufacturer Part Number: hpmx-2003
• Description: Silicon Bipolar Rfic Vector Modulator
• Manufacturer: ETC-unknow

HPMX-2003 Using Offsets
to Improve LO Leakage
It is possible to improve on the
excellent performance of the
HPMX-2003 for applications that
are particularly sensitive to LO
leakage. The nature of the
improvement is best understood
by examining figures 18 and 19,
below.
LO leakage results when normal
variations in the wafer fabrication
process cause small shifts in the
values of the modulator IC’s inter-
nal components. These random
variations create an effect equiva-
lent to slight DC imbalances at the
input of each (I and Q) mixer. The
DC imbalances at the mixer in-
puts are multiplied by 1 at the
LO frequency and show up at the
output of the IC as LO leakage.
Figure 18. LO Leakage vs. Frequency
Without DC Offsets (Upper Curve)
and LO Leakage vs. Frequency With
DC Offsets (Adjusted for Minimum LO
Leakage at 900 MHz). T
5 V, V
-20
-35
-50
-65
-80
Iref
-55
= V
-35
Qref
-15
TEMPERATURE ( C)
= 2.5 V, LO = -12 dBm.
5
A
25
= 25 C, V
45
65
CC
=
85
It is possible to externally apply
small DC signals to the I and Q in-
puts and exactly cancel the inter-
nally generated DC offsets. This
will result in sharply decreased
LO leakage at precisely the fre-
quency and temperature where
the offsets were applied (see fig-
ure 18).
This improvement is not very use-
ful if it doesn’t hold up over fre-
quency and temperature changes.
The lower curve in figure 18
shows how the offset-adjusted LO
leakage varies versus frequency.
Note that it remains below
-45 dBm over most of the fre-
quency range shown. In the
20 MHz range centered at
900 MHz, the level is closer to
-55 dBm.
Figure 19. LO Leakage With No DC
Offsets vs. Temperature (Upper
Curve) and LO Leakage With DC
Offsets (Adjusted for Minimum
Leakage at 25 C) vs. Temperature
(Lower Curve). Frequency = 900 MHz,
V
LO = -12 dBm.
CC
-20
-30
-40
-50
-60
= 5 V, V
650
Iref
750
= V
FREQUENCY (MHz)
7-45
Qref
850
= 2.5 V,
950
1050
1150
Figure 19 shows the performance
of the offset adjusted LO leakage
over temperature. Note that the
adjusted curve is at a level below
-50 dBm over most of the tem-
perature range.
The net result of using
externally applied offsets with
the HPMX-2003 is that an LO
leakage level below -40 dBm
can typically be achieved over
both frequency and
temperature.
The magnitude of the required ex-
ternal offset varies randomly from
part to part and between the I and
Q mixers on any given IC. Offsets
can range from -56 mV to +56 mV.
External offsets may be applied
either by varying the average level
of the I and Q modulating signals,
or by varying the voltages at the
I
ref
and Q
ref
pins of the modulator.