Yaesu FTDX-9000MP Operation Manual page 153

RF µ-T
UNING
µ-Tune and VRF: Comparisons to Fixed Bandpass Filters
µ-Tune
Inspection of the illustrations to the right will demon-
strate 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 typical of the kind of bandpass fil-
ter 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 seg-
ments represents the typical bandwidth of the µ-Tune
filter, and one can see that the passband has been re-
duced from about 750 kHz in the case of the fixed BPF)
to a few dozen kHz when µ-Tune is engaged. The vast
majority of the incoming signals are outside the pass-
band of the high-Q µ-Tune filter, and they will not im-
pinge on any of the RF/IF amplifiers, the mixers, or
the DSP. Very strong out-of-band signals like this can
cause Intermodulation, blocking, 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 fil-
ter 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 selectivity 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
adjustable throughout its operating range, and the quality L/C com-
ponents 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 mecha-
nism and tuning concept that preserves system Q (thus assuring a
tight bandwidth) while providing 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) ferrite core stack through
a 2" high (50 mm) coil structure. Three µ-Tune modules provide
coverage of the 160, 80/40, and 30/20 meter bands on the FT
9000MP, and the Q of this circuit, being over 300, yields un-
matched RF selectivity for outstanding rejection of undesired sig-
nals.
FT 9000MP O
U
NITS MTU-160, MTU-80/40, MTU-30/20
M
A
1.8MHz
FRONT-END BAND WIDTH
B
1.8MHz TUNE BAND WIDTH
μ
a
14.5MHz
FRONT-END BAND WIDTH
b
14.5MHz VRF BAND WIDTH
ANTENNA
VFO-A
VRF
1
2
µ-T
3
4
VFO-B
VRF
2.5MHz
2.5MHz
22MHz
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
3.5-4 MHz
1.8-2.5 MHz
50-60 MHz
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
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