[linrad] RE: SDR Hardware

[Leif Asbrink <leif.asbrink@xxxxxxxxxxxxxxxxxxxx]

Hi all,

Gian, I7SWX wrote:
> The problem with direct conversion receivers is that
> all the gain is at audio frequencies.  
> Why not to try a single conversion receiver where the
> gain is equally distributed between IF and audio?

The problem comes when you want a large bandwidth.
(There are several good reasons for that)

The WSE converters make a quadruple conversion receiver
in analog hardware. There is no gain at any of the IF
frequencies, there is 5 dB gain in the RF stage and all
the remaining gain is at audio.

From inside Linrad the audio signal ia an incoming IF
frequency and Linrad is a superheterodyne in digital
hardware with a number of conversions that depends on
how you choose the parameters.

Direct conversion gives an image spur. It can be made
low, but as far as I know it is not possible to suppress
it completely. The advantage is that one does not need
a sophisticated filter.

If you use a filter, and a single audio channel, it will 
be very hard to get a good utilization of the digital 
bandwidth if you require a good suppression of the mirror
image. There are limitations on how good shape factors
one can get with realistic numbers of crystals.

A good idea is to use both methods. That means make a 
direct conversion receiver with say 70 dB image suppression.
Place a "simple" filter in front of it to select one
sideband only. The bandwidth will be reduced to something
like 40% of the direct conversion bandwidth but you easily
get an image suppression of 120 dB and if you use sampling
at 48 kHz, the useful bandwidth could easily be as high as 
20 kHz. 

There are many ways.......

73

Leif / SM5BSZ




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