CMX994 Datasheet, PDF (15/61 Pages) CML Microcircuits – Local Oscillator

Direct Conversion Receivers
CMX994/CMX994A/CMX994E
5.3
5.3.1
Local Oscillator
Local Oscillator Input
LON and LOP signals form a differential signal pair however the LO input may be driven by a single-ended
source, in which case pin LOP should be connected to the LO signal and LON may be connected directly to
ground. The inputs have internal ac coupling, so external dc blocking capacitors are not required.
5.3.2
VCO and PLL
A typical configuration for using the internal VCO negative resistance amplifier at 440MHz is shown in Figure
10. For the other external components required to complete the PLL (the loop filter components) see Figure
11 which shows a third-order loop filter. Typical values for a 500Hz bandwidth are given in Table 12.
VCOP1 should be shorted directly to VCOP2 and similarly VCON1 to VCON2 in order to form the negative
resistance loop. It is recommended that the parallel LC tank (L1/C1) is situated as close to the package as
possible, with the L closest to the device pins. Also the shorting of VCOP1 to VCOP2 and of VCON1 to VCON2
occurs as close as possible to the tank circuit – this minimises the effects of series inductance on the oscillator
behaviour.
VCO
Negative
Resistance
(NR)
Amplifier
Enable
Enable
VCO
Output Buffer
Amplifier
L1 should have
a Q>30
L1
C1
C2
C3
CV1
CV2
R1 R2
Input from
Loop Filter
Figure 10 Example External Components – VCO External Tank Circuit
 2015 CML Microsystems Plc
Page 15 of 70
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