CMX994 Datasheet, PDF (44/61 Pages) CML Microcircuits – Local Oscillator

Direct Conversion Receivers
CMX994/CMX994A/CMX994E
8.1.9
Rx Mixer Output (Broadband Operation)
The CMX994 Rx mixer can be used by coupling the signal from pins IFLT1N, IFLT1P (differential signal, I
channel) and QFLT1N, QFLT1P (differential signal, Q channel); see Figure 24. For optimum performance,
differential connection is recommended but a single ended connection will also work. The signal is dc coupled
with a bias voltage of typically 1.25V. The typical source impedance of each pin is 800 Ohms. When using this
configuration the following recommendations should be followed:
 Components C1, C2, C3, C4 and R1, as shown in Figure 9 are not required;
 If signals are to be coupled from the CMX994 mixer outputs then the baseband gain must be set to maximum
attenuation (-42dB, minimum gain) by setting all of b2, b1 and b0 of Rx Gain Register: C-BUS address $16 to a
value of ‘1’; this applies where the baseband sections are enabled or disabled;
 If the baseband sections are enabled (set b6 of Rx Control Register: C-BUS address $12, to ‘0’) dc offset
correction registers may be used to change the dc offset at mixer outputs;
 Capacitance, including stray capacitance in the PCB layout, must be minimised to achieve the maximum
available signal bandwidth.
External Baseband
Circuit, I
I
External Baseband
Circuit, Q
Q
No Connect
I+
_
/6 /4
/2
Q⁺
_
MIXERS
(C1)
(C3)
VGA
BASEBAND
(C2)
(C4)
RXIP
RXIN
RXQP
RXQN
DC Offset control blocks are present only
when the baseband section is enabled.
Always set VGA to -42dB (minimum gain), even
when the baseband section is disabled.
Figure 24 External Baseband Circuit Connections to Mixer
 2015 CML Microsystems Plc
Page 44 of 70
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