Understanding modern TxDAC's?

Hi all,

Let's say I want to synthesize a 40 MHz wide transmit band centered at 70
MHz, from a baseband sampled at 100 MHz. I played with an AD9772 DAC and
found that I can do it in the Direct IF mode if I use the first image
(Fs-Fin), but the problem is that the second image (Fs+Fin) is too close for
me to be able to filter it out efficiently by traditional means . One way of
dealing with this is explained in e.g. the AD9775 datasheet, where it is
shown how to reject this image by using a complex analog signal
representation and an external analog quadrature modualtor (which is an old
well known method of creating an SSB signal). What I want to understand is
whether it is possible to achieve the same result with a single AD9786 by
using its Hilbert transform block?

Any ideas will be appreciated.

/Mikhail

MM wrote:
> Hi all,
> 
> Let's say I want to synthesize a 40 MHz wide transmit band centered at 70
> MHz, from a baseband sampled at 100 MHz. I played with an AD9772 DAC and
> found that I can do it in the Direct IF mode if I use the first image
> (Fs-Fin), but the problem is that the second image (Fs+Fin) is too close for
> me to be able to filter it out efficiently by traditional means . One way of
> dealing with this is explained in e.g. the AD9775 datasheet, where it is
> shown how to reject this image by using a complex analog signal
> representation and an external analog quadrature modualtor (which is an old
> well known method of creating an SSB signal). What I want to understand is
> whether it is possible to achieve the same result with a single AD9786 by
> using its Hilbert transform block?
> 
> Any ideas will be appreciated.
> 
> /Mikhail
> 
> 

Hello Mikhail,

How can you do it in the direct IF mode (I'm assuming you configure
the interpolation filter here in lowpass mode)? At 160 MHz, your usable
bandwidth only extends to 80 MHz, and a 40 MHz-wide signal (+/- 20 MHz)
centered at 70 MHz would extend to 90 MHz.
-- 
%  Randy Yates                  % "...the answer lies within your soul
%% Fuquay-Varina, NC            %       'cause no one knows which side
%%% 919-577-9882                %                   the coin will fall."
%%%% <yates@ieee.org>           %  'Big Wheels', *Out of the Blue*, ELO
http://home.earthlink.net/~yatescr

Hello Randy,

> How can you do it in the direct IF mode (I'm assuming you configure
> the interpolation filter here in lowpass mode)? At 160 MHz, your usable
> bandwidth only extends to 80 MHz, and a 40 MHz-wide signal (+/- 20 MHz)
> centered at 70 MHz would extend to 90 MHz.

The input data rate is only 100 MHz, not 160. Thus, my fundamental cannot go
higher than 50MHz (realistically less than that of course). Besides, if I
understand correctly the modes of this DAC, the direct IF mode means the
interpolation filter is in highpass... It is lowpass in the baseband mode
and I can't use the baseband mode because of the above limitation on the
fundamental frequency.

/Mikhail

MM wrote:

> Hello Randy,
> 
> 
>>How can you do it in the direct IF mode (I'm assuming you configure
>>the interpolation filter here in lowpass mode)? At 160 MHz, your usable
>>bandwidth only extends to 80 MHz, and a 40 MHz-wide signal (+/- 20 MHz)
>>centered at 70 MHz would extend to 90 MHz.
> 
> 
> The input data rate is only 100 MHz, not 160. Thus, my fundamental cannot go
> higher than 50MHz (realistically less than that of course). Besides, if I
> understand correctly the modes of this DAC, the direct IF mode means the
> interpolation filter is in highpass... It is lowpass in the baseband mode
> and I can't use the baseband mode because of the above limitation on the
> fundamental frequency.
> 
> /Mikhail
> 
> 

Hello Again, Mihail,

If you're going to use the AD9786, then why not plain old baseband
operation? It has an input sample rate to 200 MHz, thus you could
get your signal at 70 +/- 20 MHz tucked in without a problem. Then
you'd have to 400 - 90 = 310 MHz to go before your first image.

-- 
%  Randy Yates                  % "...the answer lies within your soul
%% Fuquay-Varina, NC            %       'cause no one knows which side
%%% 919-577-9882                %                   the coin will fall."
%%%% <yates@ieee.org>           %  'Big Wheels', *Out of the Blue*, ELO
http://home.earthlink.net/~yatescr

Hi Randy,

> If you're going to use the AD9786, then why not plain old baseband
> operation? It has an input sample rate to 200 MHz, thus you could
> get your signal at 70 +/- 20 MHz tucked in without a problem. Then
> you'd have to 400 - 90 = 310 MHz to go before your first image.

The problem is that the baseband is really a result of digital upconversion
and there are no upconverters on the market that could go higher than 100 or
105 MSPS. That means I would have to insert an additional  interpolator
between the upconverter and the DAC. I am considering this solution as well
but would prefer to avoid it.

/Mikhail

> That means I would have to insert an additional  interpolator between the
upconverter and the DAC.

I was meaning to say another upconversion stage, most likely a home-made
one, in an FPGA...

/Mikhail