ACE9020
Vcc
6k8
100p
10
5,12,21,25,28
19
20
11
100n
18p
VCO Control
BB545
TXOsc
+
-
Vcc
27n
1p
6k8
100p
6
Rxvcoin
Mod Cntrl
Ratio Sel
PD1
PD2
ACE9020
23
7
14
15
16
17
1 2
4
8
3,9,13,
18,22
+
TXPA
-
27p
18n
27n
1p
Tx
Output
Div Out
+
-
22k
18k
faux = 90 MHz
Figure 3 - ACE9020 Test circuit
Description
The ACE9020 is designed for use in a transceiver such as
an analog cellular phone, which uses an offset modulation
transmit architecture. The circuit consists of a VHF voltage
controlled oscillator to generate the offset frequency, an
upconverter to transmit frequency and also a prescaler for the
main UHF phase locked loop. The Rxvcoin signal to the
ACE9020 is normally the UHF local oscillator used for
downconversion.
A basic block diagram is shown in fig. 2, further
information on external connections is provided in the test
circuit (fig. 3) and the applications diagram (fig. 4).
VHF Oscillator
This oscillator is a differential design which uses an
external tank circuit as shown in fig. 3 and fig. 4. The
components shown in fig. 3 give a VCO frequency of 90MHz.
A varactor diode is coupled capacitively to the tank circuit; the
anode is referenced to ground via a resistor. The VCO control
from a synthesiser (eg ACE9030) charge pump output is
applied to the cathode of the varactor also through a resistor.
These resistors should be the same value to keep the
differential circuit balanced. The VCO gain with the
components shown will be typically 2 MHz/V. Modulation is
applied to the anode via a resistive divider as shown in fig. 4;
the actual signal applied to the varactor will be small as the
frequency deviation will typically be a maximum of 12kHz in
many applications. Differential buffered outputs from the
oscillator (TXOSC) interface directly to the ACE9030 auxiliary
synthesier inputs.
Upconverter
An image reject mixer is used for the upconversion. This
provides typically 20dB rejection of the unwanted upper
sideband. The quadrature networks for the mixer are all
provided on chip; this is optimised for UHF local oscillator and
VHF offset oscillator frequencies typically used for analog
cellular phones on the AMPS and TACS systems. Further
filtering of the TXPA output will be required to provide further
suppression of the unwanted upper sideband, local oscillator
signal and harmonics to meet cellular telephone
specifications. SAW filters are available for the various
transmit frequency bands.
The upconverter outputs (TXPA + and -) are differential
current outputs. The use of differential outputs minimises
current switching within the device and thus minimise cross-
talk to other circuit blocks. The TXPA outputs must be
matched to the external filter, normally 50Ω and single-ended.
The network shown in fig. 3 provides a transformation from
400Ω differential to 50Ω single-ended and also provides dc
bias from the Vcc supply to the open collector TXPA outputs.
This network provides plus and minus 90° phase shift in each
output which are then summed. Alternatively a Balun
transformer could be used, it will again be necessary to
provide dc bias to the TXPA outputs. The load to the current
outputs should be maximised to obtain the maximum power
output; 400Ω is an optimum figure as higher values require
impractical component values for matching.
Prescaler
The two modulus prescaler is part of the UHF phase
locked loop. It will typically be operating with ACE9030 radio
interface and synthesiser. There is also a choice of divider
ratio, set by the ratio select input as shown in table. 1, below.
Ratio Sel
= LOW
÷129
÷128
Table 1
The differential divider outputs can be directly coupled to
the ACE9030 main synthesiser inputs.
Ratio Sel
= HIGH
÷65
÷64
Mod_Cntrl = LOW
Mod_Cntrl = HIGH
4
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