YAESU HF/50 mHz Transceiver FT-450 User’s Manual
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3.4
Extracting the Signal from the Noise
Every stage of the FT-450 Transceiver contributes to the effort to improve the signal to noise
ratio. In order to deal with the wide dynamic range of input signals, the operator may optionally
insert 20 db of attenuation or disable the radio frequency (RF) amplifier in what is called
intercept point optimization (IPO). The RF amplifier gain is controlled by the automatic gain
control (AGC) circuitry.
The signal is then mixed with the first local oscillator to produce an intermediate frequency of
67.899 mHz, passed through a 10 kHz wide roofing filter and then passed through the first
intermediate frequency (IF) amplifier. The gain of the IF amplifier is also controlled by the AGC
circuitry.
The amplified IF signal is mixed to produce a 24 kHz second IF. The second IF signal is then
converted through an analog to digital converter (ADC). Further filtering is then performed by
the digital signal processor (DSP). The DSP then detects (DET) the signal and provides the AGC
control. The detected signal then passes through a digital to analog converter (DAC) to drive the
speaker circuitry
The operator has direct control of ATT, IPO, noise blanker (NB), several DSP filter properties,
AGC speed, IF gain (or squelch), and audio frequency (AF) gain.
3.4.1 RF Attenuation and Intercept Point Optimization (IPO)
Filtering is done first by the “ATT/IPO” button. To reduce
strong signals, apply these filters. Each time the
“ATT/IPO” button is pressed it advances through the
sequence:
ATT off, IPO off
no attenuation, input preprocessor active
ATT on, IPO off
20 db attenuation, input preprocessor active
ATT off, IPO on
no attenuation, input processor off
ATT on, IPO on
20 db attenuation, input processor off
3.4.2 Noise Blanking
Interference rejection within the bass band follows the IF filtering. This is used to filter noise from
automobiles or any other electrical sparks. It is toggled on and off by the “NB” button and the display will
show the status of the NB on the second figure top right on the display.
3.4.3 Digital Signal Processor
Pressing the “DSP” button will cause a “>” sign to
appear in front of the CONTOUR signal processing
