I have read the app note at the Harris site:
http://www.intersil.com/data/AN/AN9/AN9509/an9509.pdf
It uses an IF. The example it uses has an IF of 45 MHz and an ADC of 4
MHz to get an IF of 1 MHz. The limiting factor in going to higher
frequencies is the aperture error of the ADC. The formula is:
ta = 1/(2^n*pi*fmax)
where:
n = number of bits
pi = 3.14...
fmax = IF maximum
ta= aperture error in pico-seconds
For a 1 GHz signal an aperture error of no more than 1.24 ps is
required. Currently, I have found no ADC with these characteristics.
But, Analog Devices is developing an ADC with 1.0 ps error and 10 bits
at 200 MHz. Then direct RF at 1 GHz will be possible.
But, as for 'software radios'. I have read the vanu.com site. A
simpler,more flexible and faster solution is a digital radio consisting
of an RF I/Q modulator, DDS (direct digital synthesizer), and CPU/DSP.
It eliminates the need for a multi-band front end. The latest Analog
Devices DDS does the frequency selection/hopping (at 100 Million
frequencies per second),PLL, ADC, DAC, and flexible modulation (PSK,
QPSK,FSK,...). The DDS has a 100 MHz serial port/parallel port for
direct connection to a CPU/DSP. The DDS integrates many of the hardware
functions that occur before the ADC in the 'software radio'. It also
hardware accelerates these functions. With the high speed frequency
hopping of a DDS you can still sample the whole band of 10 MHz. Check
Analog.com for a DDS.
Paul Koning wrote:
>
> >>>>> "Kulwinder" == Kulwinder Atwal <[EMAIL PROTECTED]> writes:
>
> Kulwinder> How many channels do you need? I have done sampling of RF
> Kulwinder> at 900 MHz. I sampled I and Q of the signal after it was
> Kulwinder> demodulated from the carrier and sent through a mixer to
> Kulwinder> produce separate I and Q channels.
>
> Kulwinder> Do you need two channels? One for I and one for Q?
>
> Kulwinder> Also, since your carrier is 1 GHz and your IF 10 MHz there
> Kulwinder> is a mod/demod stage between your 1 GHz RF and your 10 MHz
> Kulwinder> IF. It is not all done at a 'digital level'. The
> Kulwinder> mod/demod of your IF is all digital.
>
> Kulwinder> Your answer to the I and Q question will determine the
> Kulwinder> type of ADC you need. - Kal Atwal.
> >> Again ...our application is not the traditional analog ones as u
> >> think....we do digitize the whole band and do the mod/demod at the
> >> digital level....
>
> >> 1. Physical parameter...?? Say a wideband signal centred at
> >> 1Gc.....(10Mhz bandwidth...)...to be more specific let it be a
> >> wideband CDMA signal.... 2. If I say 20Msps at 10 bits....
>
> If your bandwidth is 10 MHz, then the min sampling rate you need is 20
> Ms/s. Since your carrier is at 1 GHz, the sample/hold and analog
> input stages of the A/D have to have sufficient bandwidth for that,
> but it doesn't mean you have to sample at rates anywhere near there.
>
> If you can do this -- and with modern A/Ds I believe you can, since
> they are aimed at this exact application -- then you can do direct RF
> sampling, no analog mixing to a lower IF is needed.
>
> "Undersampling" is the term that describes this process. I remember
> some nice apps notes on the subject from Harris, related to their
> HSP50214 digital receiver ASIC.
>
> paul
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