|ADF4112BRUZ-REEL||RF PLL Frequency Synthesizers|
|ADF4112BRUZ-REEL Datasheet PDF : 28 Pages |
LOCAL OSCILLATOR FOR GSM BASE STATION TRANSMITTER
Figure 33 shows the ADF4111/ADF4112/ADF4113 being used
with a VCO to produce the LO for a GSM base station
The reference input signal is applied to the circuit at FREFIN
and, in this case, is terminated in 50 Ω. A typical GSM system
would have a 13 MHz TCXO driving the reference input with-
out any 50 Ω termination. In order to have channel spacing of
200 kHz (GSM standard), the reference input must be divided
by 65, using the on-chip reference divider of the ADF4111/
The charge pump output of the ADF4111/ADF4112/ADF4113
(Pin 2) drives the loop filter. In calculating the loop filter
component values, a number of items need to be considered. In
this example, the loop filter was designed so that the overall
phase margin for the system would be 45 degrees. Other PLL
system specifications are
All of these specifications are needed and used to come up with
the loop filter component values shown in Figure 33.
The loop filter output drives the VCO, which in turn is fed back
to the RF input of the PLL synthesizer. It also drives the RF out-
put terminal. A T-circuit configuration provides 50 Ω matching
between the VCO output, the RF output, and the RFIN terminal
of the synthesizer.
In a PLL system, it is important to know when the system is in
lock. In Figure 33, this is accomplished by using the MUXOUT
signal from the synthesizer. The MUXOUT pin can be pro-
grammed to monitor various internal signals in the synthesizer.
One of these is the LD or lock-detect signal.
KD = 5 mA
KV = 12 MHz/V
Loop Bandwidth = 20 kHz
FREF = 200 kHz
N = 4500
Extra Reference Spur Attenuation = 10 dB
AVDD DVDD VP
CLK MUXOUT 14
1TO BE USED WHEN GENERATOR SOURCE IMPEDANCE IS 50Ω.
2OPTIONAL MATCHING RESISTOR DEPENDING ON RFOUT FREQUENCY.
DECOUPLING CAPACITORS ON AVDD, DVDD, AND VP OF THE ADF411x
AND ON THE POSITIVE SUPPLY OF THE VCO190-902T HAVE BEEN
OMITTED FROM THE DIAGRAM TO INCREASE CLARITY.
Figure 33. Local Oscillator for GSM Base Station
Rev. C | Page 22 of 28
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