Local Oscillator for GSM Base Station Transmitter
Figure 26 shows the ADF4117/ADF4118 being used with a
VCO to produce the LO for a GSM base station transmitter.
The reference input signal is applied to the circuit at FREFIN
and, in this case, is terminated in 50 Ω. Typical GSM system
would have a 13 MHz TCXO driving the Reference Input
without any 50 Ω termination. In order to have a channel
spacing of 200 kHz (the GSM standard), the reference input
must be divided by 65, using the on-chip reference divider of
The charge pump output of the ADF4117/ADF1118 (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 speciﬁca-
tions are given below:
KD = 1 mA
KV = 12 MHz/V
Loop Bandwidth = 20 kHz
FREF = 200 kHz
N = 4500
Extra Reference Spur Attenuation = 10 dB
All of these speciﬁcations are needed and used to come up with
the loop ﬁlter components values shown in Figure 27.
The loop ﬁlter output drives the VCO, which, in turn, is fed
back to the RF input of the PLL synthesizer and also drives
the RF Output 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 26, 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.
The attached circuit in Figure 27 shows how to shut down both
the ADF4116 family and the accompanying VCO. The ADG702
switch goes open circuit when a Logic 1 is applied to the IN
input. The low-cost switch is available in both SOT-23 and
micro SOIC packages.
DIRECT CONVERSION MODULATOR
In some applications a direct conversion architecture can be used
in base station transmitters. Figure 28 shows the combination
available from ADI to implement this solution.
The circuit diagram shows the AD9761 being used with the
AD8346. The use of dual integrated DACs such as the AD9761
with speciﬁed ± 0.02 dB and ± 0.004 dB gain and offset match-
ing characteristics ensures minimum error contribution (over
temperature) from this portion of the signal chain.
The Local Oscillator (LO) is implemented using the ADF4117/
ADF4118. In this case, the OSC 3B1-13M0 provides the
stable 13 MHz reference frequency. The system is designed
for a 200 kHz channel spacing and an output center frequency
of 1960 MHz. The target application is a WCDMA base sta-
tion transmitter. Typical phase noise performance from this LO
is –85 dBc/Hz at a 1 kHz offset. The LO port of the AD8346 is
driven in single-ended fashion. LOIN is ac-coupled to ground
with the 100 pF capacitor and LOIP is driven through the ac-
coupling capacitor from a 50 Ω source. An LO drive level of
between –6 dBm and –12 dBm is required. The circuit of Figure
28 gives a typical level of –8 dBm.
The RF output is designed to drive a 50 Ω load but must be
ac-coupled as shown in Figure 28. If the I and Q inputs are driven
in quadrature by 2 V p-p signals, the resulting output power will
be around –10 dBm.
7 15 16
AVDD DVDD VP
RFINA 6 100pF
DECOUPLING CAPACITORS (10F/10pF) ON AVDD, DVDD, VP OF THE
ADF4117/ADF4118 AND ON VCC OF THE VCO190-902T HAVE BEEN
OMITTED FROM THE DIAGRAM TO AID CLARITY.
Figure 26. Local Oscillator for GSM Base Station