RadioBanter - View Single Post

Vladimir Vassilevsky · July 1st 07, 06:27 PM posted to rec.radio.amateur.homebrew,comp.dsp

wrote:

I'm trying to get an idea of some of the tradeoffs involved in
determining
where to perform the analog to digital conversion for a SDR design
which
uses 200 MHz as the first IF (pick due to SAW filter availability and
it's
high enough to simplify the front end filter bank ... the receiver is
a
homebrew type intended to cover .5 - 600 MHz with a maximum signal
bandwidth of 200 kHz).

Each frequency conversion inevitably reduces the IP3 and the KP of the
receiver. The other problem with the multiple conversions is the
feedthrough of the harmonics and intermods of the heterodynes to the
input of the receiver; this creates the "deaf" points in the frequency
range. So, the intention is avoiding multiple conversions unless it is
really necessary.

The multiple conversion could be necessary if the high attenuation of
the adjacent channel is required and if the desired channel bandwidth is
too narrow. It is difficult to make a good narrowband filter for the
high IF.

Some options:
1) Sample at the first IF using subsampling.

Advantages: Eliminates the need for an additional LO and mixer.

Disadvatages: Brad Evans points out in a comp.dsp article entitled
"Undersampling (was: RF/IF digital receiver)" that using a high IF
means that it will be more sensitive jitter in the ADC clock.

The clock jitter can be taken care off. The suppression of the aliases
is determined by the 1st IF filter. You will need a good filter,
especially if subsampling.

[...]

What are peoples thoughts, opinions, and experiences regarding these
types of
tradeoffs?

It depends.

What are the final parameters of the receiver that you are planning to
achieve?

What benefits do you expect from doing the processing in the software?

How much of time and money do you have?

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com