ACE9030
The reference divider needs to produce the main com-
the negative numbers need more processing to avoid nega-
tive N2 values. The simplest answer is to add an offset to the
channel number and then subtract an equivalent value from
the N1 equation. As the channels are in blocks of 32 for the
calculationsitishelpfultochooseamultipleof32fortheoffset,
and the most negative channel is –719 so the lowest suitable
offset value is 736 (that is 23 x 32). This gives the following
steps for all TACS/ETACS channels:
parison frequency of 12·5 kHz from the crystal at 14·85 MHz,
a ratio of 1188, which can be formed by a NR of 1188 followed
by a SM select giving ÷1, or 594 followed by ÷2, or by 297 and
÷4. The auxiliary divider must divide 90 MHz down to the
auxiliary comparison frequency, which is related to the main
comparison frequency but can usefully be larger. Using
12·5 kHz needs a ratio of 7200 which is too large a value for
NA, 25 kHz needs 3600 and could be used, but 50 kHz and a
ratio of 1800 helps to minimise loop filter size. With this choice
the values to be set are:
CNOFF = Channel Number + 736
CB32 = int((CNOFF – 1) ÷ 32)
CN2 = CNOFF – (32 x CB32)
N2 = (2 x CN2) – 1
NR = 297, to give 50 kHz into SA, SM selector.
SA = 00, to give ÷1, and leave 50 kHz for auxiliary
comparison frequency.
N1 = 1202 – N2 + CB32
SM = 01, to give ÷4, and hence 12·5 kHz for main
These operations are given in easy to understand stages
but in a real system it could be more efficient to combine or
rearrange some steps. If a high level language is used then
integer and remainder functions might be available and could
save a little programming time, whereas if low level or assem-
bler language is used an integer function will need to be built,
in this case by a 5 bit right shift to both divide by 32 and to lose
the fraction.
It might be noticed that avoiding N2 = 0 was very easy as
allvaluesofN2inthissystemareoddnumbers,duetothe12·5
kHz offset from band edges. Other systems do not have this
offset so a little care is needed in choosing constants in the
corresponding equations.
compari
son frequency.
NA = 1800, to give 90 MHz.
In this example Fractional-N operation is not chosen, but if it
is required then SM should be set to 00 to give 50 kHz
comparison frequency in both synthesisers and then fractions
of 1/4 or 3/4 used by setting NF = 2 or 6, with FMOD set to 1 to
1
give /8’s. The values of N1 and N2 can then be found by
following a procedure similar to the following.
For the main synthesiser, starting at channel 1, to divide
980·0125 MHz down to 12·5 kHz is a total ratio of 78401 and
this will be split between the prescaler and the ACE9030
programmable divider. A ÷64/65 prescaler is most common,
and will be in ÷64 mode as its normal state, so the 78401 can
besplitintoa÷64followedby÷1225witharemainderof1. The
remainder is achieved by setting the prescaler to ÷65 for 1
cycle, sousingR1=64, andR2=65theprogrammablevalues
can be:
DETAILED OPERATION OF FRACTIONAL-N MODE
Without using the Fractional-N mode the loop will lock the
VCO frequency, fVCO to the comparison frequency, fCOMP at a
multiple set by the total division ratio NTOT, where:
NTOT = (N1 + N2) x R1 + N2
N2 = 1, and (N1 + N2) = 1225, thus N1 = 1224
giving:
fVCO = fCOMP x NTOT
These values are suitable for use but are not the only
possible set - if desired N2 can be increased to 65 if N1 is
reduced to 1160. The actual choice in practice is set by
whichever gives the more convenient mathematics in the
system controller, the only limits are the basic equation:
NTOT =(N1+N2) x R1+N2, whichmustbemetforallchannels
andthefactthatasetvalueof0forN2willactuallygiveacount
of 256 so for easy calculations N2 0 (in practice for a TACS
system not using Fractional-N all N2 values are odd numbers
so 0 is never needed).
From these equations it can be seen that if NTOT is an
integer the minimum frequency step is fCOMP. It is not possible
to make a non-integer divider but by alternating the ratio
between NTOT and NTOT + 1 in a suitable pattern the effect of a
fractional increase in NTOT can be achieved. This is called
Fractional-N operation.
The control of the pattern of NTOT and NTOT + 1 cycles is by
an accumulator set to count with a modulus equal to the
fractional denominator and which adds the numerator of the
fraction every comparison cycle. When the accumulator over-
flows by its value exceeding the value of the denominator the
total division ratio is increased for one cycle. In ACE9030 the
choice of denominator is 5 or 8 and is set by the FMOD bit in
Word D, the numerator is set by the three NF bits in Word A or
A2 and the increase in total division from NTOT to NTOT + 1 is
done by changing the modulus control signal to the prescaler
so that an R1 cycle becomes an R1 + 1 cycle.
Other channels can easily be added, without forgetting
that each channel is two comparison steps (2 x 12·5 kHz)
above the next lower, so for channels 1 to 32,
N2 = 2 x Channel Number – 1, and N1 = 1225 – N2
and for channels 33 to 64,
N2 = 2 ∞ ( Channel Number – 32 ) – 1, and N1 = 1226 – N2
As an example of the operation of the accumulator
consider FMOD set HIGH to give modulo-8 counting and NF
set to 011 to give a 3/8 fraction and the accumulator starting at
any arbitrary value as shown in table 7.
From this table it can be seen that the pattern repeats every
8cyclesandthattheratioisincrementedfor3ofeach8, giving
the desired NTOT + 3/8. It can be shown that the pattern always
repeats every 8 cycles, or whatever modulus is chosen for all
fractions and that the number of NTOT + 1 cycles is always the
fractional numerator.
Rather than having several sets of separate equations for
each group of channels it is possible to combine them all into
one set by adding two variables to split the channel number
into a modulo-32 and a remainder number. Let C32 =
int((Channel Number – 1) ÷ 32), where “int(x)” means the
integerpartof(x),andletCN2=ChannelNumber–(32xC32),
then:
N2 = (2 x CN2) –1, and N1 = 1225 – N2 + C32
By spreading the (NTOT + 1) counts throughout the pattern
rather than having them as a continuous block the loop is less
These are clearly valid for channels 1 to 600 (the original
TACS channels) but to cover the extra channels for ETACS
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