Receiver Operation ( Front End Block Diagram ) - Yaesu FT-991A Operating Manual

Convenience Features

Receiver Operation ( Front End Block Diagram )

The FT-991A includes a wide range of special features to suppress the many types of interference that may be encoun-
tered on the HF bands. However, real world interference conditions are constantly changing, so optimum setting of the
controls is somewhat of an art, requiring familiarity with the types of interference and the subtle effects of some of the
controls. Therefore, the following information is provided as a general guideline for typical situations, and a starting
point for your own experimentation.
The FT-991A interference-fighting circuitry begins in its "RF" stages, and continues throughout the entire receiver sec-
tion. FT-991A allows configuration of the features described below.
CONTOUR Filter
The DSP Contour filter has the unique ability to pro-
vide either a null or a peak in tunable segments of the
receiver passband. You may suppress interference and
excessive frequency components of an incoming signal,
or you may peak those tunable frequency segments. The
level of the null or peak, and the bandwidth, over which
it is applied, are adjustable via the Menu.
IF SHIFT
The passband center frequency of the IF DSP filter may
be moved up or down by adjusting this control.
IF WIDTH
The width of the IF DSP filtering may be adjusted using
this control.
IF NOTCH
The IF Notch filter is a high-Q notch filter that can elim-
inate, or significantly reduce an interfering carrier.
DNF ( Digital Notch filter )
When multiple interfering carriers are encountered dur-
ing reception, the Digital Notch Filter can significantly
reduce the level of these signals.
VHF/ HF/
UHF 50MHz
ANT ANT
Page 46
RF
HF/50MHz
RF AMP/
ATT BPF
IPO
1st Local
144MHz
LPF BPF RF AMP
430MHz
LPF RF AMP BPF
Front End Block Diagram
DNR ( Digital Noise Reduction )
The DSP's Digital Noise Reduction ( DNR ) feature uti-
lizes 15 different mathematical algorithms to analyze
and suppress different noise profiles encountered on the
HF/50 MHz bands. Choose the selection that provides
the best noise suppression, and allows the signal to rise
up out of the noise.
AGC
The AGC system automatically adapts to changing sig-
nal and fading characteristics, making reception possible
under the most difficult conditions.
The FT-991A uses the 15 kHz roofing filter on
AM/FM/C4FM modes. All the other modes use
the 3 KHz filter automatically. There is no manual
selection possible.
1st IF 2nd IF
69.450 MHz 9 MHz
ROOFING 1st
MCF
FILTER IF AMP
3 kHz
2nd Local
15 kHz
2nd IF
450 kHz
1st Local
FM DET
FM
2nd Local
FT- 991 Operating Manual
3rd IF
24 kHz
3rd DSP
IF AMP UNIT
3rd Local
C4FM
DSP UNIT
C4FM