4

ACE9020

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 50

Ωand single-ended.

The network shown in fig. 3 provides a transformation from

400

Ω 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; 400

Ω 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.

The differential divider outputs can be directly coupled to

the ACE9030 main synthesiser inputs.

ACE9020

Rxvcoin

22k

18k

Vcc

Vcc

VCO Control

TXOsc

Div Out

+

-

+

-

+

TXPA

-

27n

27n

18n

1p

27p

1p

Tx

Output

100p

100p

18p

100n

BB545

6k8

6k8

5,12,21,25,28

10

11

23

48

3,9,13,

18,22

16

15

19

20

6

7

Mod Cntrl

Ratio Sel

14

17

PD1

12

PD2

faux = 90 MHz

Ratio Sel

Ratio Sel

= LOW

= HIGH

Mod_Cntrl = LOW

÷129

÷65

Mod_Cntrl = HIGH

÷128

÷64

Table 1