Page 268 - Complete Wireless Design

Frequency Synthesizer Design

Frequency Synthesizer Design 267

cant problem, and will decrease the signal-to-noise level of the synthesizer’s
output. This digitally produced noise refers to the errors made when the DDS
signal is converted from digital to analog, since the analog synthesized fre-
quency, with its infinite number of potential amplitudes, is really only being
fabricated from a large, but finite, number of discrete digital levels. This
makes absolute accuracy during conversion to analog impossible, resulting in
a noise output along with the synthesizer’s desired CW signal.
Nonetheless, one method to accomplish a practical high-frequency DDS/PLL
hybrid frequency synthesis system is as shown in Fig. 5.12. When the radio’s
on-board microprocessor outputs control words into the DDS/PLL synthesizer
for a change in frequency, the actual DDS section will be able to reproduce a
low, stable synthetic analog frequency (with the assistance of its DDS refer-
ence, the DAC, and the LPF), controlled by fine step words from the micro-
processor. The LPF’s output, a relatively clean but low-frequency sine wave, is
then fed into a mixer for premixing. This MIXER1 sums the low frequency
made by the DDS to the high-frequency reference of the LO in order to output
an increased frequency through the BPF and into MIXER2. The PLL, with its
own crystal reference or one shared from a common clock, functions as MIX-
ER2’s LO, and is controlled by the coarse frequency digital control words from
the microprocessor. The sum of the PLL and the output of MIXER2 is sent
through the wide bandwidth BPF to obtain a very wide-ranging adjustable out-
put frequency, with decent resolution, fast lock time, and low spurious output.
Another method to increase the operating frequency of a pure DDS synthe-
sizer, one that allows operation of a DDS device at VHF and above, is standard
premixing (Fig. 5.13). A low-phase-noise, fixed-frequency oscillator is inserted
into one mixer port, while the other mixer port accepts the DDS synthesizer.
The output of the mixer is filtered, resulting in a frequency synthesizer. This
keeps DDS phase noise and spur generation to relatively low levels, while out-
putting a very high, but tunable, frequency selection. The premixing technique
is limited only by the ability of the output bandpass filter to filter the fre-
quencies of the LO feedthrough, the undesired difference frequency, and the
mixer spurs. Nonetheless, DDS will usually have a high spurious output—as
well as high DC current requirements—making DDS inappropriate for many
RF applications.

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