Following a new path
The main focus of the hardware design
was to digitize an anologue shortwave signal as accurately as possible without
quality loss. Further signal processing and demodulation are done in software.
The radio is a dual conversion superheterodyne
receiver, wherein the RF signal is converted first to a high IF frequeny of 45
MHz by using a high-IP actice mixer in conjunction with a very low-noise VFO.
An ultra steep special discrete 10-pole
crystal roofing filter with 16KHz bandwidth is used for 45MHz IF filtering. Next
conversion is done to the very low second IF of 12KHz. This working principle is
also state-of-the-art used for professional communication receivers.
Then a novel method is used for optimized
The second intermediate frequency
is splitted into two different channels (RX & DX channel), whereby the DX channel
is amplifed by an amount of 27dB to increase the resolution for weak signals just
above the noise threshold. Then each channel is converted with 16dB ADC resolution
and transferred to the PC via USB.
IGC replaces AGC
By splitting into two channels, a
conventional automatic gain control (AGC) is superflous. A standard AGC may cause
signal degradation problems:
Within the passband of the roofing
filter, the strongest signal determines the setting of the AGC. If a strong and
a weak signal are close to each other, the weaker signal will also be attenuated,
because of the reduced gain and therefore its signal-to-noise ratio (SNR) will
be also reduced.
Instead, a software-based gain control
(IGC- Intelligent Gain Control) is used, whereby both channels are monitored and
adjusted simultaniously to obtain always the maximum achievable SNR.
RX channel (red) and DX channel
(white) spectrum signals simultaneously displayed by an AM transmitter on longwave.
The lowered noise floor of the DX channel is clearly visible.
Taking all control options, level
differences of 137dB could be processed.