CMX994 Datasheet, PDF (9/61 Pages) CML Microcircuits – Local Oscillator

Direct Conversion Receivers
CMX994/CMX994A/CMX994E
5 External Components
5.1 Power Supply and Decoupling
The CMX994/CMX994A/CMX994E have separate supply pins for the analogue and digital circuitry; a 3.3V
nominal supply is recommended for all circuits but the data interface can run at a lower voltage than the rest
of the device by setting the VDD_IO supply to the required interface voltage.
R7
DV_DD
R6
R5
R4
AV_DD
R3
R2
AV_DD
R1
DVDD
VDDIO
VCCRF
VCCRXIF
VCCLNA
VCCLO
VCCSYNTH
GND
C7 C6 C5 C4 C3 C2 C1
GND Plane for: AGND
GND for: DGND
Figure 2 Recommended Power Supply Connections and Decoupling
C1
10nF
C2
10nF
C3 33pF//10nF
C4
10nF
C5
10nF
C6
10nF
C7
10nF
R1
10Ω
R2
3.3Ω
R3
3.3Ω
R4
3.3Ω
R5
3.3Ω
R6
10Ω
R7
10Ω
Table 2 Decoupling Components
Notes:
1. Maximum Tolerances: Resistors 5%, capacitors 20%.
2. It is expected that any low-frequency interference on the 3.3 Volt supply will be removed by active
regulation; a large capacitor is an alternative but may require more board space and so may not be
preferred. It is particularly important to ensure that there is no interference from the VDD_IO (which
supplies the digital I/O) or from any other circuit that may use the DV_DD supply (such as a
microprocessor), to sensitive analogue supplies (AV_DD). It is therefore advisable to use separate
power supplies for digital and analogue circuits.
3. The supply decoupling shown is intended for RF noise suppression. It is necessary to have a small
series impedance prior to the decoupling capacitor for the decoupling to work well. This may be
achieved cost effectively by using the resistor and capacitor values shown. The use of resistors
results in small dc voltage drops (up to approx 0.1V). Choosing resistor values approximately
inversely proportional to the dc current requirements of each supply ensures the dc voltage drop on
each supply is reasonably matched. In any case, the resultant dc voltage change is well within the
design tolerance of the device. If higher impedance resistors are used (not recommended) then
greater care will be needed to ensure the supply voltages are maintained within tolerance, even
when parts of the device are enabled or disabled.
4. It is advisable to have separate ground planes for analogue and digital circuits.
5. Separate regulators for local oscillator sections (VCCLO, VCCSYNTH) may be beneficial depending on
circuit noise and type of regulator and this is why two AV_DD connections are shown.
 2015 CML Microsystems Plc
Page 9 of 70
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