[Discuss-gnuradio] IF ADC, and the radio itself

[Brian_Whitaker]

All,

Akshay mentioned Maxim ADCs for IF sampling -- here's a link to the
front-end of Maxim's parametric search page for high-speed ADCs. You can
either 'show all products' or drill-down to filter out only the ones with
full-power BW, ENOB, etc that you want. 

http://para.maxim-ic.com:80/ss.asp?Fam=Fast_ADC&Tree=ADConverters&HP=ADCDACR
ef.cfm&ln=

Of particular interest are the MAX104/106/108 family... probably over-kill
for any real IF sampling, but they offer 600 to 1500 Msps (yes,
1.5giga-sample per second) with the analog front end to match this
performance (2.2GHz FullPower BW).

Have any comm buffs out there laid down minimum required specs for things
like Spurious-Free-Dynamic-Range (SFDR), Signal-to-Noise-ratio+distortion
(SINAD), or effective-number-of-bits (ENOB) ?  The IF itself will tell us
what the input BW needs to be, and Nyquist  will tell us what the sample
rate needs to be.

Just to clarify one point -- hardware folks here are expecting to
under-sample the IF, right, not try to grab _everything_ from DC to 500+
MHz?  There was a comment a while back that sounded like people were talking
about needing to sample a 300MHz IF at more than 600Msps -- remember that
Nyquist under-sampling will effectively mix down for us, so the sample rate
only needs to be 2x the actual BW used at IF, since that's the only info
we're interested in.  ...and this is assuming we're going to provide
ourselve's the opportunity to grab every channel in the band. Sorry if I'm
just stating the obvious.

In this light, the MAX 1180 and 1190-series (there are about a dozen in
these families) all have input track/hold stages worth about 400MHz, with
sample rates anywhere from 10Msps to about 100Msps, all 10-bit res... see
that parametric search page for more details.  Most cellular handsets use
IFs in the range of 100MHz to 300MHz, and the widest receive band is only
60MHz (UMTS, US PCS), so just in these respects, these ADCs should be a good
match. Frankly, the target application for most of Maxim's high-speed ADCs
are as basestation IF sampling, so they ought to do well for cellular
applications.  

For HDTV, the specs might be similar, since we're only grabbing a single
channel... can someone post what they know about HDTV's modulation scheme?

A quick question about the radio itself -- most of the conversation here is
about the software implementation of the SDR, and very little about the
radio front-end (how we get from RF to IF).  What are people's plans here
for desiging the radio?  I'm a junior RF guy (only 5 years out of school),
but I'd like to contribute to this part of the effort if I can. 

Brian Whitaker
Maxim RF Applications