WiNRADiO G65DDC User’s Guide
112
Another issue is 'image response'. As already mentioned, traditional oscillator
based spurious responses are eliminated in a direct sampling receiver,
however, there is still a potential for a spurious signal similar to the traditional
image response.
The ADC sampling frequency of the
Excalibur Sigma
is 210 MHz. According to
the
Nyquist theorem
, this would allow sampling of an RF spectrum to close to
half of that frequency, i.e. close to 105 MHz.
To make things simpler, let’s pretend for a while that the
Excalibur Sigma
features only one lower frequency range from 1 kHz to 88 MHz. With a signal
input of less than 105 MHz, all is well. But above 105 MHz, an image or alias
signal will appear in the spectrum below 105 MHz. For example, if a signal at
195 MHz is present at the input, this would appear on 15 MHz due to the
aliasing effect and potentially mask a wanted signal there.
In the
Excalibur Sigma
, this effect is significantly reduced by the use of a high
quality low-pass filter preceding the ADC and effectively removing the 195
MHz signal before it is presented to its input. This filter is known as the
anti-
aliasing filter
. However, this anti-aliasing filter cannot be made perfect; its
skirt is always somewhat sloping rather than being an ideal vertical line, and its
stop-band attenuation cannot be made infinite. As a result, the receiver’s lower
bandwidth is smaller than 105 MHz, and there is also an “image response” of
signals higher than 105 MHz. However, in the
Excalibur Sigma,
the image
response is still excellent compared to any conventional receiver.
Now, let’s get back to second frequency range of the
Excalibur Sigma.
The
second frequency range from 118 to 190 MHz features a separate antenna
input and dedicated signal chain. Here, the aliasing effect is taken advantage
of and grants the
Excalibur Sigma
the ability to process signals of frequencies,
which according to the Nyquist theorem are higher than half of the sampling
rate of the ADC, i.e. 105 MHz. However now the
anti-aliasing filter
is still
needed and has to be even more complex: on the lower side it has to filter out
the frequency components below 105 MHz, which are now considered as
spurious and on the higher side it has to filter out frequency components
higher than 210 MHz. So, for the second frequency range from 118 to 190
MHz the anti-aliasing filter consists of two high quality separate filters – the
high pass at 118 MHz and the low pass filter at 190 MHz.
