Yaesu FTDX9000D Operation Manual page 67

Advanced Interference-Suppression Features: RF Front End
µ-Tune
Inspection of the illustrations to the right will demonstrate
the profound advantage of the µ-Tune circuit. In illustration
[ ]
A , the gray area represents the passband of a typical fixed
bandpass filter covering the 1.8 ~ 3 MHz range; this is typi-
cal of the kind of bandpass filter found in many high-quality
HF receivers today. Note also the hypothetical distribution
of signals across the 160-meter band.
[ ]
In illustration B , note the narrow white segment within the
gray passband of the fixed BPF. These narrow segments repre-
sents the typical bandwidth of the µ-Tune filter, and one can see
that the passband has been reduced from about 750 kHz 9in the
case of the fixed BPF) to a few dozen kHz when µ-Tune is en-
gaged. The vast majority of the incoming signals are outside the
passband of the high-Q µ-Tune filter, and they will not impinge
on any of the RF/IF amplifiers, the mixers, or the DSP. Very
strong out-of-band signals like this can cause Intermodulation,
blicking, and an elevated noise floor for a receiver.
VRF
In this example, illustration [a] depicts a typical fixed bandpass
filter covering 14.5 to 22 MHz, and once again the gray shaded
area depicts the fixed bandpass filter's frequency coverage.
The vertical lines in the illustration, once again, represent
hypothetical signals throughout this frequency range.
Figure [b] shows the same fixed BPF, with the white area
representing the typical passband of the VRF filter operating
in the same frequency range. Although the selectivity of the
VRF is not as tight as that of the µ-Tune filter, the RF selec-
tivity of the VRF preselector is still magnitudes better than
that of the usual fixed bandpass filter, affording significant
protection against the ingress of high signal voltage from
strong out-of-band signals.
Advice
With µ-Tuning, the center frequency of the filter is continuously adjust-
able throughout its operating range, and the quality L/C components
ensure a tight passband due to the high Q of the circuit. The RF
preselection design task involves not only the selection of quality L/C
components, but the crafting of a tuning mechanism and tuning concept
that preserves system Q (thus assuring a tight bandwidth) while provid-
ing a wide operating frequency range and consistent, automated tuning.
The smooth tuning is achieved by varying the inductance over a wide
range; this is accomplished by motor-driving a large 1.1" (28 mm) fer-
rite core stack through a 2" high (50 mm) coil structure. Three µ-Tune
modules provide coverage of the 1.8, 80/40, and 30/20 meter bands on
the FT 9000D, and the Q of this circuit, being over 300, yields
DX
unmatched RF selectivity for outstanding rejection of undesired sig-
nals.
FT 9000D O
DX PERATION
U
SING THE
µ-Tune and VRF: Comparisons to Fixed Bandpass Filters
M
ANUAL
µ-T F
UNE EATURE
A
1.8MHz
B
1.8MHz
a
14.5MHz
b
14.5MHz
ANTENNA
VFO-A
1
2
3
4
VFO-B
2.5MHz
FRONT-END BAND WIDTH
TUNE BAND WIDTH 2.5MHz
μ
22MHz
FRONT-END BAND WIDTH
VRF BAND WIDTH 22MHz
BPF
50-60 MHz
22-30 MHz
14.5-22 MHz
13.5-14.5 MHz
10-11 MHz
6.5-7.5 MHz
VRF
3.5-4 MHz
1.8-2.5 MHz
µ-T
50-60 MHz
VRF
22-30 MHz
14.5-22 MHz
13.5-14.5 MHz
10-11 MHz
6.5-7.5 MHz
3.5-4 MHz
1.8-2.5 MHz
Page 65