WiNRADiO G35DDC HF Receiver
112
overloaded, resulting in distortion (much like overloading a speaker with too
much audio).
As the ADC is effectively ‘wide open’ to any input frequency, even if a user is
tuned to 10 MHz, a strong signal in the Medium Wave band can still overload
the receiver. This effect is easily avoided by two methods: Using the
Auto
setting for the
Attenuator
, the input level is continuously monitored, and
should the level exceed the threshold, an appropriate level of attenuation is
automatically inserted.
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 Ultra
is 100 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 50 MHz.
With a signal input of less than 50 MHz, all is well. But above 50 MHz, an
image or alias signal will appear in the spectrum below 50 MHz. For example,
if a signal at 85 MHz is present, due to the aliasing effect this would appear on
15 MHz, and potentially mask a wanted signal there. In the
Excalibur Ultra
, this
effect is significantly reduced by the use of a high quality low-pass filter
preceding the ADC and effectively removing the 85 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 skirting 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 sensitivity decreases above
approximately 45 MHz, and there is also an “image response” of signals higher
than 50 MHz. However, in the
Excalibur Ultra,
the image response is excellent
compared to any conventional receiver: On 98 MHz (the center of the
88-108 MHz FM broadcast band and potentially the strongest source of
interference), these images are typically suppressed by 100 dB.
Содержание WR-G35DDC EXCALIBUR ULTRA
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