ACE9040 datasheet(34/39 Pages) MITEL

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ACE9030

APPLICATIONS HINTS

DD & VSS Supply Pins

All V

DD pins must be well decoupled to ground. All VSS pins

must be connected through very low impedance lines to the

ground point.

Serial Bus

Edge speeds on the serial bus should not be too fast in

order to avoid ringing which then can cause significant modu-

lation of the synthesisers, including CLK8.

Loop Filters

Both synthesisers use passive loop filters and typical

circuits can be either of these two configurations; the need for

the extra roll-off in the right hand circuit is only for the more

critical applications.

The loop filter needed is partly set by the application

specification and partly by the architectural design of the

cellphone. The main synthesiser will need to hop channels at

a rate set by the hand-off times of the network and so is well

defined. The auxiliary synthesiser is always on the same

Fig. 29 Typical Synthesiser Loop Filters

frequency and so could be a very slow loop to lock but in many

systems it will be powered down for as much time as possible

to economise on battery use and will then need to power-up

and lock quickly, leading to a more complex filter.

The values of the components in these filters may be

calculated with the help of appendix AB43 in the Personal

Communications Handbook or for a more complete analysis

the application note AN94, available from Mitel Semiconduc-

tor Marketing Department, may be used. This note was written

specifically for the NJ88C33 synthesiser but the mathematics

apply equally well to the ACE9030.

AFC Circuit

Fig. 30 Simplified Receiver Architecture

In order to fine trim the crystal oscillator frequency to the

correct value the ACE9030 includes a sampling circuit to

convert the final intermediate frequency signal (input on

AFCIN) to a logic signal and then to mix it down to a low

frequency output (on AFCOUT) for counting in the

microcontroller. The operation of this system can be explained

with the use of a simplified block diagram of the receiver

architecture as in figure 30.

Most receivers run the first mixer with a high-side local

oscillator controlled by the Main synthesiser, so a positive

crystal frequency error will give an increased First I.F. This is

then mixed down further by a low-side second oscillator, LO2

in the ACE9030, derived by multiplying the same crystal as

used for the Main synthesiser. A positive crystal frequency

error would now give a reduced second I.F. if the error in the

first I.F. is ignored, but the overall effect is an increase by an

amount slightly smaller than the increase in the first I.F.

The second I.F. signal AFCIN at around 450 kHz drives

the F.M. Discriminator to recover the modulation and also

feeds a third mixer where high-side injection is used to give a

very low output frequency, around 54 kHz, and is output on

AFCOUT for counting. This third mixer is driven by a clock

derived from the crystal but at a much reduced frequency so

the effect of the high-side mixing dominates to give an output

which drops in frequency when the crystal has a positive

frequency error. As a result of the chain of mixing stages the

error in the first local oscillator due to the crystal frequency will

give a similar frequency shift at the output of the third mixer

which is then a large percentage change in the frequency of

AFCOUT so it is possible to measure AFCOUT against the

crystal to determine the trim needed.

To illustrate the sensitivity of the AFC loop a numerical

example can be used, and in the calculations that follow the

selection of parameters for the synthesisers are also included

to show how some choices are made.

Assume the required receiver frequency is AMPS chan-

nel 1, that is 870·030 MHz and that the cellular terminal is built

with a 45 MHz first I.F., a 450 kHz second I.F., and a

14·85 MHz crystal.

To receive 870·030 MHz with a 45 MHz first I.F. needs

the first local oscillator, the Main synthesiser, to run at a

frequency of 870·030 + 45·000 = 915·030 MHz when using

high-side injection. For AMPS the most convenient compari-

son frequency is the channel spacing of 30 kHz so the total

division from VCO to phase comparator (N

TOT as used else-

where) will be 30501 for this channel and the reference

PDA

VCO

PDP

PDI

VCO

FIRST

I.F

SECOND

I.F

LO2

MAIN

SYNTH

RX BAND

RECEIVED

SIGNAL FROM

BASESTATION

AFCIN

AFCOUT

AFC SAMPLE

AT 504 kHz

ACE9030

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