LT5516
10
5516fa
APPLICATIO S I FOR ATIO
WUU
U
I-Channel and Q-Channel Outputs
Each of the I-channel and Q-channel outputs is internally
connected to V
CC
though a 60Ω resistor. The output dc
bias voltage is V
CC
– 0.68V. The outputs can be DC coupled
or AC coupled to the external loads. The differential output
impedance of the demodulator is 120Ω in parallel with a
5pF internal capacitor, forming a lowpass filter with a
–3dB corner frequency at 265MHz. R
LOAD
(the single-
ended load resistance) should be larger than 600Ω to
assure full gain. The gain is reduced by 20 • log(1 + 120Ω/
R
LOAD
) in dB when the differential output is terminated by
R
LOAD
. For instance, the gain is reduced by 6.85dB when
each output pin is connected to a 50Ω load (100Ω differ-
ential load). The output should be taken differentially (or
by using differential-to-single-ended conversion) for best
RF performance, including NF and IM2.
The phase relationship between the I-channel output sig-
nal and Q-channel output signal is fixed. When the LO
input frequency is larger (or smaller) than the RF input
frequency, the Q-channel outputs (Q
OUT
+
, Q
OUT
–
) lead (or
lag) I-channel outputs (I
OUT
+
, I
OUT
–
) by 90°.
When AC output coupling is used, the resulting highpass
filter’s –3dB roll-off frequency is defined by the R-C
constant of the blocking capacitor and R
LOAD
, assuming
R
LOAD
> 600Ω.
Care should be taken when the demodulator’s outputs are
DC coupled to the external load, to make sure that the I/Q
mixers are biased properly. If the current drain from the
outputs exceeds 6mA, there can be significant degrada-
tion of the linearity performance. Each output can sink no
more than 13mA when the outputs are connected to an
external load with a DC voltage higher than V
CC
– 0.68V.
The I/Q output equivalent circuit is shown in Figure 7.
3
2
RF
J1
T1
LDB31900M20C-416
V
CC
RF
+
LT5516
RF
–
5516 F05
1k
1.54V
1.54V
L1
33nH
62
4
1
3
C1
1nF
NOTE: NO CONNECTION REQUIRED
ACCORDING TO BALUN TRANSFORMER
MANUFACTURER
Figure 5. RF Input Equivalent Circuit with External Matching
