Icom IC-7851 Quick Start Manual page 5

B a s i c F u n c t i o n s
Premium components yield
premium performance
+40dBm IP3
(3rd Order Intercept Point)
The IC-7851 continues the +40dBm, 3rd order
intercept point and 110dB receiver dynamic
range benchmark set by the IC-7800. To
achieve this superb receiver performance, the
entire analog circuitry and components have
been re-engineered to match the DSP units. A
newly designed LO amplifier generates high
output while keeping flat frequency characteris-
tics over a 60MHz wide range.
Dynamic Range Characteristics
Intercept Point
At 14 MHz band
+40dBm IP3
Noise Floor
level
Dynamic range 110dB
Antenna Terminal input
Dynamic Range (typical)
Dynamic Range (dB)
Roofing
Spacing
Filter
Bandwidth
1kHz 2kHz 5kHz
15kHz
100 101 104
IC-7851
1.2kHz
99 105
BPF Switching:
Mechanical Relays
T h e I C - 7 8 5 1 i n c o r p o r a t e s h i g h - g r a d e ,
long-term reliable mechanical relays rather
than PIN diodes for switching the Band Pass
Filters (BPF). This deployment of relays re-
moves secondary
distortion products
from the primary
s t a g e o f s i g n a l
processing.
BPF unit
Four 1st IF "Roofing" Filters
Three high-spec 1st IF "Roofing" filters plus
t h e n e w 1 . 2 k H z O p t i m u m R o o f i n g F i l t e r
allow only signals within the filter passband
to the 1st IF amplifier stage. You can select
the filter width from 15kHz, 6kHz, 3kHz and
1.2kHz, depending on your operating mode.
(FM mode is fixed at 15kHz)
1.2kHz
Optimum Roofing Filter
3, 6, 15kHz
High-Spec 1st IF filters
Digi-Sel Preselector
T h e p r e s e l e c t o r
w o r k s b e t w e e n
1.5MHz and 30MHz
and rejects distortion
components derived
f r o m o u t o f b a n d
i n t e r f e r e n c e s u c h
as multi-multi operation or strong broadcast sta-
tions. It automatically tracks the intended signal
keeping the preselector's bandwidth centered on
the operating frequency. The center frequency of
the preselector is manually adjustable from the
DIGI-SEL tuning knob on the front panel.
Automatic Preselector bandwidth characteristics
REF 0.0 dBm
10 dB/ *A_Write Norm
MARKER
1 4 . 16 MHz
0dB
START 0 Hz
RBW 300 kHz VBW 300 kHz
20 kHz
114
111 113
Triple DSP Power
Three separate 24-bit AD/DA converters and
dedicated DSP processors are at the heart of the
IC-7851. With one AD/DA and DSP dedicated to
the spectrum scope and two 24-bit AD/DA and
DSP chips for the receivers and transmit circuits,
there is plenty of DSP power for the most
demanding RF environment.
Dual AGC Loops
The IC-7851 has multiple AGC loops. The AGC voltages detected in front of and behind the digital IF filter
in the DSP unit. The first AGC loop prevents saturation of the 1st IF amplifier. The other AGC loop detects
the AGC voltage at the digital IF filter output which has only passed the intended signal and draws the full
potential from the digital IF filter.
Co m b i n i n g t h e d i g i ta l I F fi l te r,
manual notch, and the 1st IF stage,
these are all controlled by the DSP
unit. 110dB of ultra wide dynamic
range in the receiver means the IF
amplifier is distor tion free from
strong signals.
Automatic Preselector
DL −30.0 dBm MKR 14.16 MHz
B_Blank Norm −8.73dBm
STOP 30.00 MHz
SWP 200ms ATT 10 dB
Transmit/
Receiver DSP (left) and
Receiver DSP (right)
ADSP-21489
by Analog Devices
32-bit floating point DSP
2400 MFLOPS
393 MHz clock speed
2nd Mixer (Image Rejection Mixer)
IF IF IF
AMP AMP AMP
IF IF IF
AMP AMP AMP
AMP AMP AMP
90 degrees 90 degrees 90 degrees
Phase shifter Phase shifter Phase shifter
Image Rejection Mixer
Simple receiver system configuration is the
best way to eliminate sources of the spurious
signals and distortion from transceivers. The
IC-7851 uses a double conversion system com-
posed of a D-MOS FET 1st mixer and image
rejection mixer for the 2nd stage. The 1st mixer
stage is driven with a signal from the high-drive
Local Oscillator with excellent C/N. The image
rejection mixer for the 2nd stage reduces signal
distortion through IF processing and provides a
high-fidelity signal to the DSP unit. This system
provides wide dynamic range, reducing the dis-
tortion from strong signals and lower intermodu-
lation.
High Performance OCXO Unit
The IC-7851 uses the
OCXO (Oven Control
Crystal Oscillator) unit
w h i c h i s s t a b l e t o
within ±0.05ppm at
0 ° C t o 5 0 ° C . T h i s
specification means
OCXO unit
that even on the 50MHz band, frequency error
is less than 2.5Hz! In addition, a 10MHz refer-
ence frequency can be input and output for
accurate tuning.
200W Output Power
at Full Duty Cycle
The push-pull power
a m p l i f i e r s u s i n g
p o w e r M O S - F E T s
w o r k o n 4 8 V D C .
They provide a pow-
e r f u l 2 0 0 W o u t p u t
p o w e r a t f u l l d u t y
PA unit and heat sink
cycle and low transmit intermodulation. An
effective cooling system maintains internal
t e m p e r a t u r e s w i t h i n a s a f e r a n g e a n d
prevents thermal runaway.
Solid Aluminum Main Dial Knob
The main dial tuning
knob is pure functional
b e a u t y . M a c h i n e d
from a solid aluminum
block, the main dial
tuning knob gives you
a solid operational feel. The diamond engraved
accents are cosmetic perfection – expected
only from a radio of this caliber.
Receiver DSP Unit
A/D
IF IF IF
AGC AGC AGC
DET DET DET
D/A
AGC Loop line
AMP AMP AMP
BPF
AGC AGC AGC
DET DET DET
4