AD5930 Datasheet, PDF (19/28 Pages) Analog Devices – Programmable Frequency Sweep and Output Burst Waveform Generator

AD5930
SETTING UP THE FREQUENCY SWEEP
As stated previously in The Frequency Profile section, the
AD5930 requires certain registers to be programmed to enable a
frequency sweep. The following sections discuss these registers
in more detail.
Start Frequency (F_START)
To start a frequency sweep, the user needs to tell the AD5930
what frequency to start sweeping from. This frequency is stored
in a 24-bit register called FSTART. If the user wishes to alter the
entire contents of the FSTART register, two consecutive writes
must be preformed, one to the LSBs and the other to the MSBs.
Note that for an entire write to this register, the Control Bit B24
(D11) should be set to 1 with the LSBs programmed first.
In some applications, the user does not need to alter all 24 bits
of the FSTART register. By setting the Control Bit B24 (D11) to 0,
the 24-bit register operates as two 12-bit registers, one
containing the 12 MSBs and the other containing the 12 LSBs.
This means that the 12 MSBs of the FSTART word can be altered
independently of the 12 LSBs, and vice versa. The addresses of
both the LSBs and the MSBs of this register is given in Table 8.
Table 8. FSTART Register Bits
D15 D14 D13 D12
1
1
0
0
1
1
0
1
Frequency Increments (∆f)
D11 to D0
12 LSBs of FSTART <11…0>
12 MSBs of FSTART <23…12>
The value in the Δf register sets the increment frequency for the
sweep and is added incrementally to the current output frequency.
Note that the increment frequency can be positive or negative,
thereby giving an increasing or decreasing frequency sweep.
At the start of a sweep, the frequency contained in the FSTART
register is output. Next, the frequency (FSTART + Δf ) is output.
This is followed by (FSTART + Δf + Δf) and so on. Multiplying the
Δf value by the number of increments (NINCR), and adding it to
the start frequency (FSTART), gives the final frequency in the
sweep. Mathematically this final frequency/stop frequency is
represented by
FSTART + (NINCR × Δf).
The Δf register is a 23-bit register, and requires two 16-bit
writes to be programmed. Table 9 gives the addresses associated
with both the MSB and LSB registers of the Δf word.
Table 9. ∆f Register Bits
D15 D14 D13 D12
0010
0011
0011
D11 D10 to D0
12 LSBs of ∆f
<11…0>
0 11 MSBs of
Δf <22…12>
1 11 MSBs of
Δf <22…12>
Sweep
Direction
N/A
Positive Δf
(FSTART + Δf )
Negative ∆f
(FSTART − Δf )
Number of Increments (N_INCR)
An end frequency, or a maximum/minimum frequency before
the sweep changes direction is not required on the AD5930.
Instead, this end frequency is calculated by multiplying the
frequency increment value (Δf) by the number of frequency
steps (NINCR), and adding it to/subtracting it from the start
frequency (FSTART), that is, FSTART + NINCR × Δ f. The NINCR register
is a 12-bit register, with the address shown in Table 10.
Table 10. NINCR Register Bits
D15 D14 D13 D12
0
0
0
1
D11 to D0
12 bits of NINCR <11…0>
The number of increments is programmed in binary fashion,
with 000000000010 representing the minimum number of
frequency increments (2 increments), and 111111111111
representing the maximum number of increments (4095).
Table 11. NINCR Data Bits
D11
D0 Number of Increments
0000 0000 0010 2 frequency increments. This is the
minimum number of frequency
increments.
0000 0000 0011 3 frequency increments.
0000 0000 0100 4 frequency increments.
…………
1111 1111 1110 4094 frequency increments.
1111 1111 1111 4095 frequency increments.
Increment Interval (t_INT)
The increment interval dictates the duration of the DAC output
signal for each individual frequency of the frequency sweep.
The AD5930 offers the user two choices:
• The duration is a multiple of cycles of the output frequency.
• The duration is a multiple of MCLK periods.
This is selected by Bit D13 in the tINT register as shown in Table 12.
Table 12. tINT Register Bits
D15 D14 D13 D12 D11
010xx
011xx
D10 to D0
11 bits <10…0>
Fixed number of output
waveform cycles.
11 bits <10…0>
Fixed number of clock
periods.
Programming of this register is in binary form with the
minimum number being decimal 2. Note in Table 12 that 11
bits, Bit D10 to Bit D0, of the register are available to program
the time interval. As an example, if MCLK = 50 MHz, then each
clock period/base interval is (1/50 MHz) = 20 ns. If each
frequency needs to be output for 100 ns, then <00000000101>
or decimal 5 needs to be programmed to this register. Note that
the AD5930 can output each frequency for a maximum
duration of 211 −1 (or 2047) times the increment interval.
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