DDS and spurs

Andre' Kesteloot
Mon, 10 Jul 2000 10:21:04 -0400

Johan Bodin wrote:

> Hello All,
> At first sight, the DDS may look as an ideal frequency source
> for LF. This may be true for transmitters but not for receivers!
> In-band spurious signals are generated due to the limited
> DAC resolution. These spur's are predictable, to some extent,
> but still unavoidable. Depending on the phase increment, the
> frequency setting, the spur's may show up very close to the
> wanted frequency.
> Popular DDS chips, such as AD9832 and AD9850, have 10-bit
> DACs and about 60 dB SFDR (spurious free dynamic range).
> LO spur's at -60 dBc may seem harmless but imagine such a
> spur mixing with, for example, DCF39 producing a signal right
> on top of the weak signal you are trying to receive...
> If strong unwanted signals are present at the mixer input, a
> receiver is never better than it's LO.
> The DDS signal can be spectrally "purified" by putting a
> "clean-up PLL", with a good VCO, after the DDS and LPF.
> The PLL acts as a flywheel. Spur's further away from Fc
> than the loop filter BW will be rejected.
> Another option is to make the receiver almost totally digital.
> The only analog parts would be a preselector filter and a
> preamplifier. A suitable ADC for LF is the AD9260. It is a
> 16-bit oversampling ADC (20 MHz) with Fs/8 decimation
> (decimation filters are on-chip). Input BW is 1 MHz and
> output Fs=2.5 MHz. The output from the ADC can be fed into
> an Intersil (formerly Harris) HSP50016 downconverter chip.
> The HSP50016 has a quadrature DDS LO (the actual tuning),
> digital I/Q mixers, programmable decimation, main I/Q filters
> (FIR) and a serial data output. The output can be fed to a DAC,
> for direct audio output, or a complex signal can be fed to a DSP
> for further processing. The RX BW depends on the selected
> decimation ratio and the filter skirts are STEEP!
> The HSP50016 has a lot of different output formats, both
> complex (I/Q) and real (Weaver method).
> 73 de Johan, SM6LKM