 » [JRC] NRD-545 fuori produzione
 The quality of radio reception, especially in the shortwave bands, is greatly affected by the performance of the receiver, because of the high density of the shortwave radio spectrum and its wide dynamic range (of small to large signals). The receiver's sensitivity (capacity of receiving small signals) is constricted by the noise from the space or the atmosphere, and by the internal noise generated by the receiver itself, while the capacity of handling large signals is affected by intermodulation between strong radio signals.
If it is required to reject intermodulation distortion over a wide dynamic range, then digital signal processing (DSP) achieves a remarkable advantage.
Signal quantization (conversion of analog signals to digital quantities) is performed in a completely linear domain over the entire range, thus never generating any intermodulation distortion.
The NRD-545 DSP Receiver is loaded with a 40-bit extended floating point DSP IC (Digital Signal Processor IC), a 18-bit over-sampling A/D converter, and a 16-bit D/A converter. The DSP IC performs signal processing in all the circuits after the intermediate frequency (IF) stage. The functions of 13 types of circuits after the IF stage, which have previously been configured as analog circuits, are now handled by the DSP IC.
The digital IF filter, one of the DSP features, provides a sharper attenuation gradient and frequency characteristic than a crystal filter, thus allowing continuous bandwidth adjustment. All traditional crystal filters and mechanical filters that have previously determined the selectivity characteristics of receivers are eliminated from the new DSP receiver.
In addition to JRC's recognized receiver front end, complete digital signal processing in all the circuits after the IF stage has been realized. Thus, the new-generation NRD-545 receiver has come into the world.
Digital Signal Processing by One-Chip DSP
Digital signal processing uses an algebraic operation method for signal detection, generation, inference, processing, or transmission. Compared to analog signal processing circuits, digital signal processors are more advantageous: They only require less precise components; they are robust against drift and interference; they are easier to integrate with high precision; and they offer greater flexibility with software processing.
The DSP IC consists of a multiplier, an adder-subtractor, and a memory (shift register). The digital signal processing algorithm is based on repetition of operations such as data readout from the memory, multiplication, addition and subtraction.
All-Mode Detection
The DSP IC performs signal detection in all the modes including LSB, USB, CW, RTTY, FM, AM and ECSS (Exalted Carrier Selectable Sideband). Digital IF Filter
Superior selectivity performance is achieved by a simultaneous Chebyshev type digital IF filter with an IIR (infinite impulse response) configuration. Pass bandwidth is continuously adjustable in a range of 1 kHz to 9.99kHz in 1OHz steps (BWC). Default values can also be set for each mode in NARROW, INTERmediate, and WIDE positions (except for AMS, FM and WFM modes).
Pass-Band Shift (PBS)
Radio interference is pushed out of the band by shifting the center frequency of the digital IF filter upward or downward in the variable range of within +-2.3kHz (in 50Hz steps).
Noise Reduction (NR)
Received signals become audible, emerging from mixed noise, because the spectrum of the periodic signals is enhanced while random signals such as noise are attenuated. Signal processing is adjustable in 256 steps, allowing the most audible conditions to be set. The NR feature is also very effective for such noise which the previous noise blanker has not been able to handle.
Beat Canceller (BC)
Completely periodic signals such as beat sounds are exalted and cancelled. However, voice signals are not processed as periodic signals in this BC, but processed to be free from distortion. Signal processing can be set in 256 steps to achieve the most effective condition. The BC is also effective for multiple beats.
Noise Blanker (NB)
Like the previous noise blanker, this NB detects noise amplitudes and removes noise depending on the amplitude level on the time axis. NB1 (narrow blanking width) and NB2 (wide blanking width) can be switched over, and the NB detection level is adjustable.
Notch Filter (NOTCH)
Beat sounds are attenuated (at 40dB or more) using the steep attenuation characteristic of the IIR notch filter. Despite its steep characteristic, beat sounds are digitally processed and stably attenuated when once the NOTCH is preset. It is adjustable within +-2.5kHz in 1OHz steps. With the notch tracking set to ON, the notch filter follows in the range of +/-10kHz even when the tuning dial is rotated
AGC
Digital AGC is applied to all modes except in the WFM mode. An AGC loop is not only configured inside the DSP, but AGC is also applied to the primary IF amplifier via a D/A converter. Although the discharge time constant is fixed in the FM, WFM, AM and AMS modes, AGC is continuously adjustable between 0.04sec. and 5.1sec. in 20 msec. steps in the other modes.
BFO
The beat frequency oscillator (BFO) generates the BFO signals to demodulate the received signals in the LSB, USB, CW and RTTY modes.
RF Gain
The information transferred to the RF GAIN control via the A/D converter is entered into the DSP to adjust the gain inside the DSP and the gain of the primary IF amplifier.
Squelch (SQ)
The squelch function is operated in all the modes. If an input signal is small, noise squelch is activated, while signal level squelch is operated as the input signal becomes larger.
Tone Control (TONE)
The sound quality of the audio output can be continuously adjusted in a range of the low bandwidth of 1 kHz or less to the high bandwidth of 1 0kHz (except in the RTTY and WFM modes).
S-Meter
The signal level is determined in reference to the input signal to the DSP, and the level is converted into an antenna input level, which activates the S-meter.
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