[hpsdr] Interesting email on SDR transmit noise

[Dan Mills]

The noise sidebands actually look to me to be the noise shaped dither
applied as part of the delta sigma conversion process in the audio
DAC, the noise is deliberately placed above the audio baseband so that
it is not audible in the normal use case for that chip, this noise
will be present even under conditions of no modulation input).

Obviously (But obviously not obvious enough!) when using a delta sigma
converter to drive a modulator it requires a suitable post dac lowpass
filter.

This particular source of noise is a non issue in a DUC transmitter,
but dither should still be applied prior to any word length reductions
in the transmit chain to make the system linear. Fortunately once at
the high speed end of the CIC chain, the dither can be shaped to place
the energy above the lowpass filter cutoff.

73, M0HCN.

 1382183601.0