Using a wideband ADC (the 16 bit, 130 MSPS/sec, LTC2208 for example)
> seems like a poor choice for a narrow band radio compared to the
> simpler, cheaper and less power hungry Tayloe mixer approach.
>
For an effective number of 16 bits, this receiver will have a dynamic range
of about 100 dB. Assuming that the receiver will have a MDS of -140 dBm,
this means that it goes in saturation at about -40 dBm or 10µW, which is 22
mV over 50 ohm. Maybe this result in some for problem for the few local
medium and nearby shortwave stations, preselection is required to attenuate
these signals. But there is some good news too. Mixing the sampled signal
digitally, does not introduce additive spurious components.
Which DDS has the best spur performance: AD9952 (14 bit DAC) or
> the AD9958 (10 bit DAC)?
Even in a 14-bit DDS the narrow-band spurious is about -80 dBc (optimistic
view: not taking the wide-band spurs of -60dBc into account). Using a DDS in
a Tayloe setup will raise the noisefloor of the receiver which would worsen
the MDS. Let assume a MDS of -140dBm again, this means that weak signal can
no longer be received due to the raising noisefloor, when there is somewhere
in the band a signal that is stronger than -60dBm! The weak signal reception
of this receiver setup is more vulnerable compared to the 16 bit ADC
receiver.
Guido
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