User's Manual

WiNRADiO G39DDC User’s Guide
23
The Preselector of the VHF/UHF/SHF section contains a bank of fixed filters
that improve the image response of the receiver and reduce the possibility of
overloading and intermodulation distortion. The signal then passes through a
low-noise and low-distortion two-stage amplifier whose first stage can be
enabled or disabled by the user. When enabled, this adds approximately
10 dB of amplification, making it possible to achieve higher sensitivity (at the
expense of strong signal handling capability).
The HF section includes an input Attenuator. The attenuator is adjustable
from zero to 18 dB in 6 dB increments and serves to reduce signal levels in the
proximity of strong stations and can be operated either manually or left to the
receiver to determine its optimum level automatically to avoid overloading.
Following the last intermediate frequency (70 MHz) amplifier is an anti-
aliasing filter (not shown) necessary for the digitization, and then the 16 MHz
wide signal is fed into a 16-bit analog-to-digital converter which samples the
signal at 100 MSPS (megasamples per second), producing a steady stream of
digital data for the digital down-conversion, performed in the receivers FPGA
(Field Programmable Gate Array).
There are two digital down-conversion paths, corresponding to two separate
“virtual receiver” channels. These channels can be tuned anywhere within the
16 MHz intermediate frequency bandwidth and can operate entirely
independently, just like two separate receivers.
Each channel includes two digital down-converters, the first of which is able to
output a digitized signal of up to 4 MHz bandwidth, while the second one has a
maximum bandwidth of 320 kHz. The digital data then proceeds via the USB
or PCIe interface to the computer which provides all the additional processing,
filtering, demodulation, signal strength measurement, squelch, noise blanking,
notch, audio filtering and other functions.
The DDC may be a new concept for newcomers to software-defined radio, so
it perhaps deserves a little more explanation:
In conventional superheterodyne receivers, the incoming signal is first
converted to a fixed intermediate frequency (IF) by mixing it with a variable
frequency signal produced by an internal local oscillator, and then further
processed by hardware amplification, filtration and demodulation.