5-3. Receiver
The receiver functions as an SSB demodulator for the HF band. It is based on a triple conversion
scheme, which converts a specific 2.7 kHz band of the received spectrum to the input of an ADC
(Analog to Digital Converter), centered at 20 kHz.
The conversion chain is based on analog circuitry that was designed to withstand out-of-band
interference. The frequency resolution of both injections is limited to 1.25 kHz, which is insufficient
to provide the specified 10 Hz resolution. Therefore, the analog section of the conversion chain
provide a coarse tuning only.
After being digitized by the ADC, the received signal is being frequency shifted to the exact frequency
by a digital algorithm in the DSP. This fine tuning process complements the coarse tuning performed
by the analog section of the receiver. After being fine tuned, the signal is SSB demodulated and
filtered according to the radio specifications.
The distribution of the overall frequency resolution between the DSP stage (fine tuning) and the
analog stages (coarse tuning) is common to the receive and the transmit chains.
The RGC (Receiver automatic Gain Control) is an important segment of the receiver. It is based on
analog attenuation stages, distributed along the analog part of the receiver chain, and a digital
algorithm within the DSP. The analog attenuation stages serves as a step-attenuator. Each stage
provides 0 dB attenuation or, when activated, a predefined attenuation. Therefore, the analog section
of the receiver can provide only a coarse gain control.
The DSP controls the activation of these step-attenuator stages, while adding a smoothly varying gain
control by scaling the signal after fine tuning and filtering. The overall performance of the RGC
provides a 100 dB range, starting from -90 dBm.
The DSP processed signal is an audio signal which is amplified to levels up to 5W.
Figure 5-4. Signal Flow through the Receiver
Cat. No. 6886859J01 5-7
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