Re: what adc to use for digital am receiver
- From: Tom Bruhns <k7itm@xxxxxxx>
- Date: 21 Apr 2007 22:39:17 -0700
On Apr 21, 11:00 am, Bhargav <bhargavall...@xxxxxxxxx> wrote:
Hi All,
I am thinking of designing a digital am receiver, I dont know what
typical am voltage levels would be and what a/d convertor( i mean what
should be the resolution) to use for it. Can anybody help?
Thanks.
If you are not too close to a local station, and if your local
stations don't run a lot of power, you should be OK with about 0dBm
full scale. If you have one station that is very strong (e.g., 5kW a
kilometer away) I would recommend a passive notch filter to attenuate
it. If you set full scale at about 0dBm, you should do fine if your
ADC has a noise level of about -120dBfs/Hz. You'll want an ADC with
low intermodulation distortion and good linearity to low levels. You
only need a sample rate a little over twice the highest frequency of
interest, but beware of the alias protection requirements. We
generally use about 11th order elliptical filters. Nice thing is that
the AM broadcast signals are generally pretty strong, so unless there
are local hams or other users of the HF band, the potential alias
signals shouldn't be terribly strong compared with the AM broadcast
stuff. But because local conditions can vary drastically from place
to place and even time to time, be prepared to either characterize
your spectrum beforehand, or adjust things after you build the
receiver. The actual number of bits in the ADC is less relevant than
you might think, provided it has low enough noise and good enough
linearity and distortion performance. Also, you can trade off sample
rate for number of bits, to some extent: decimating a faster sample
rate will get you more bits (slowly).
If it were my project, I'd look to digitize at 5Ms/s; the antialias
filter would cut off above 2MHz and would be at least 100dB down at
3MHz and above. (11th order elliptical should get you close to 120dB
if you're careful in its construction and use good coils.) I think
there are a few good ADCs that will do 5Ms/s and 16 bits...but since I
generally work at much higher sample rates, I'm not sure what the
current state of 5Ms/s parts is.
Cheers,
Tom
.
- References:
- what adc to use for digital am receiver
- From: Bhargav
- what adc to use for digital am receiver
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