Analog Baseband Signal Generation - Analog Devices AN-826 Application Note

AN-826

ANALOG BASEBAND SIGNAL GENERATION

The Tx DAC is one of the most critical components in this
signal chain because it needs to provide the closest to ideal
analog signal to be upconverted and amplified. The DAC
signal-to-noise ratio (SNR) and sampling rate define the
spectral purity and signal quality of the modulated signal
driving the IQ modulator.
SNR and SFDR
The chosen Tx DACs for this application are part of the
AD9860/AD9862 mixed signal front-end family (MxFE®) and
are 12 bits and 14 bits, respectively. They have a maximum
sampling rate of 128 MHz. The output of the Tx DACs is a
current source, with a programmable peak current between
2 mA and 20 mA. Programming the full-scale current through
register writes provides the flexibility of adjusting the peak-to-
peak input voltage to the IQ modulator while maintaining the
12-bit/14-bit resolution.
Recommended SNR for this application should be at least
60 dB to be able to meet the spectral mask at maximum power
levels and EVM at the lowest power levels (for instance, an SNR
of 31.4 dB plus margin is required for a 64 QAM three-quarter
OFDM even at the minimum output power at the antenna). Both
the 12-bit AD9860 and the 14-bit AD9862 provide better than
70 dB SNR. In some BTS applications, higher SNR is required
to meet stringent spectral masks. It is then recommended to use
a 16-bit DAC like the AD9779.
In addition, the spurious-free dynamic range (SFDR) within
the first Nyquist zone stays constant at −76 dBc whether the
signal frequency is at 1 MHz or 6 MHz. This is important, for
instance, when dealing with large signal bandwidths like an
OFDM 10 MHz WiMAX signal centered at dc.
Sampling Frequency
Depending on the maximum modulation bandwidth, the sampling
frequency can be appropriately chosen. For example, a WiBro
complex OFDM signal with an 8.75 MHz bandwidth would require
a DAC sampling rate , f
SAMPLING
Nyquist minimum (OFDM modulation sampling frequency = n ×
bandwidth = 8/7 × 8.75e6 = 10 MHz). But all sampling alias would
be at n × 20 MHz, which would fall in band for RF frequency band-
widths higher than 20 MHz. These images can only be filtered by
the off-chip reconstruction filter at the DAC output.
Good filtering of the DAC images at multiples of the sampling
frequency then requires higher order filters. This can be
, of at least 2 × 10 MHz or twice the
avoided using the interpolation filters, available within the
DAC. While the input data rate to the DAC remains the same,
the interpolation filters increase the sampling frequency of the
DACs. As a result, the images appear further away from the
main input signal. Figure 3 and Figure 4 show the effect of
enabling the 4× interpolation filters on the AD9860/AD9862.
0
–20
–40
–60
–80
–100
–120
0
Figure 3. AD9860/AD9862 Tx DAC Generating an OFDM Signal with
1× Interpolation (f
0
–20
–40
–60
–80
–100
–120
0
Figure 4. AD9860/AD9862 TxDAC Generating an OFDM Signal with
4× Interpolation (f
Therefore, while the Tx digital data are updated at a rate of
20 MHz only, the 4× interpolation filter effectively increases the
overall sampling rate to 80 MHz. This allows a simple third-
order Bessel LPF at the DAC output (see the IQ Modulator
section).
Rev. B | Page 4 of 16
20 40 60 80 100
FREQUENCY (MHz)
= 32 MSPS, 1× Interpolation)
SAMPLE
20 40 60 80 100
FREQUENCY (MHz)
= 32 MSPS, 4× Interpolation)
SAMPLE
120 140
120 140