Solid design basics and HF expertise raising the bar

Contesters and DXers are always looking for that competitive edge to magically pull out the weak signal that is either the rare country or multiplier they need to climb up the list. Larger antennas, higher gain pre-amps and other devices in line are great. However, what happens inside the radio with all those signals coming down your feedline can defeat all your efforts.

With the design of the IC-7851, Icom's engineers focused on a new Local Oscillator (LO) that drastically reduces the phase noise. As a result of this design, the purity of the LO achieves a Reciprocal Mixing Dynamic Range (RMDR) of 110dB. In addition to the incredibly clean LO allowing you to hear the weak signals, the new spectrum scope design enables you to see the weak ones! Faster processors higher input gain, higher display resolution and a cleaner signal from the receiver's LO will give you a new window into the RF world. Adding this performance and functionality for both receivers give you a dual scope portal.

Competitive advantage: reciprocal mixing dynamic range

RMDR: 110dB raising the bar

Design advances developed by the Icom HF engineers for the Local Oscillator (LO) enable the IC-7851 to set a new benchmark for amateur radio receivers. The goal was to dramatically reduce the phase noise that degrades the target signal due to the sum of the entire signal present. The result was a RMDR of 110dB*. Below is a comparison of the improvement over the IC-7800.

*At a 1kHz offset frequency
Receiving frequency: 14.2 MHz Mode: CW, IF BW: 500 Hz
Roofing Filter IC-7800 = 3 kHz, IC-7851 = 1.2 kHz  

RMDR comparison

RMDR (Reciprocal Mixing Dynamic Range) is the relative level of an undesired signal, offset “n” kHz from the RX passband, which will raise noise floor by 3 dB. The local oscillator phase noise will mix with strong unwanted signals and unavoidably generate noise which masks a wanted signal.

1.2 kHz Optimum roofing filter

Optimum roofing filter characteristic diagram

Crystal clear LO (local oscillator) design

Breaking the boundaries of traditional designs, the IC-7851 employs a Direct Digital Synthesizer (DDS) along with a Phase Locked Oscillator for the LO. The C/N ratio excels beyond the IC-7800 and other similar class HF transceivers. This design significantly reduces noise components in both receive and transmit signals.

LO C/N characteristics comparisons

Receiving Frequency: 14.2 MHz Mode: CW 1st LO frequency: 78.655 MHz SPAN = 20 kHz, RBW = 30 Hz, VBW = 10 Hz

Improved phase noise characteristics

Phase noise is coherent in radio circuit design and the new LO design introduced in the IC-7851 makes some major breakthroughs while utilizing the 64MHz, up-conversion receiver design introduced in the IC-7800. An impressive 20dB improvement is seen with the IC-7851's 10 kHz measurement and more than 30dB improvement at a 1 kHz measurement in comparison to the IC-7800.

LO C/N characteristics comparisons

Receiving Frequency: 14.2 MHz Mode: CW 1st LO frequency: 78.655 MHz

IC-7851

IC-7800

Twice the speed, sensitivity, resolution and more control

 

Improved spectrum scope

Following the design linage of the IC-7800, the IC-7851 uses a dedicated DSP unit for the Fast Fourier Transform (FFT) spectrum. The 2250 MFLOPS DSP processor enables a new dual scope function and significantly faster sweep speeds and better accuracy than in the IC-7800.

Scope DSP
TMS320C6745 by Texas Instruments
32-bit floating point
2250 MFLOPS
370 MHz clock speed

Scope comparison

*1 Number of dots shown at the 60 dB level, when receiving a signal.
*2 SPAN = More than 20 kHz, SPEED = Slow
*3 SPAN = Less than 20 kHz, SPEED = Fast
*4 SPAN = 500 kHz, SPEED = Slow  

Audio scope function

The audio scope simultaneously shows an oscilloscope and FFT for receive and transmit audio. Adjust your transmit audio by watching your compressor level, equalization and mic gain settings to give you the audio you want for SSB. The oscilloscope shows the CW waveform. On receive, you can see the power of your filtering by watching filtering adjustments take out interfering signals including filter width and notch filter placement. The processing power in the IC-7851 allows for dual mini band scopes as well as the audio scope.

Mini spectrum scope and audio scope

Mini dual spectrum scope and audio scope

Specifications for the audio scope

• Attenuator: 0 dB, –10 dB, –20 dB and –30 dB
• FFT scope with waterfall and FFT scope without waterfall
• Waveform color and drawing (outline or fill) settings for the FFT scope

Specifications for the oscilloscope

• Level: 0dB, –10 dB, –20 dB and –30 dB
• Sample rate: 1ms/Div, 3ms/Div, 10ms/Div, 30ms/Div 100ms/Div and 300ms/Div, 5 Div width
• Waveform color setting

Dual scope function

While you can watch both receivers on the scope of the IC-7800, within the limits of the scope bandwidth, the IC-7851 introduces the new dual scope – the ability of watching both receivers in separate spectrum scopes. The dual scope function is vital for watching for multipliers or band openings in contests, or working all bands/modes on a DXpedition.

Dual scope example (Vertically aligned)

Dual scope example (Horizontally aligned)

High resolution waterfall display

The waterfall display captures signal strengths over time. This allows you to see signals that may not be apparent on a normal scope. Additionally, the combination of the scope attenuator and the wide screen mode gives you a better view of weaker signals as band conditions change. For the ultimate scope enhancement, the IC-7851 has a digital video interface (DVI-I) for a larger display.

Spectrum scope with waterfall 
(wide screen setting)

Click control

By connecting a human interface device (such as a USB mouse, trackball or touchpad) to the USB port on the rear panel, you gain control over the spectrum display pointer for “Click-and-Listen” receiver control. Fix/Center mode, sweep speed and other settings are controllable.

For example

• Left click to change operating frequency
• Click a button (either left or right) and move right or left side on the screen to increase or decrease the operating frequency (similar to rotating the main dial)
• Right click to temporarily change the receive frequency. Release the mouse button to return.

General
Frequency coverage*1 U.S.A. Version RX TX	0.030– 60.000 MHz*2 1.800– 1.999 MHz 3.500 – 3.999 MHz 5.255 – 5.405 MHz*2 7.000 – 7.300 MHz 10.100–10.150 MHz 14.000– 14.350 MHz 18.068–18.168 MHz 21.000– 21.450 MHz 24.890–24.990 MHz 28.000– 29.700 MHz 50.000–54.000 MHz   *1 Frequency ranges vary depending on version. *2 Some frequency ranges are not guaranteed.
Mode	USB, LSB, CW, RTTY, PSK31/63, AM, FM
Number of channels	101 (99 regular, 2 scan edges)
Antenna connector	SO-239×4 and BNC×2 (50Ω unbalanced (Tuner off))
Power supply requirement:	85–265V AC
Temperature range	0°C to +50°C; +32°F to +122°F
Frequency stability	Less than ±0.05ppm (0°C to +50°C @ 54MHz, after warm up)
Frequency resolution	1Hz (minimum)
Power supply requirement	85–265V AC
Power supply requirement	Tx Max. power: 800VA Rx Stand-by: 200VA (typ.) Max. audio: 210VA (typ.)
Dimensions	425×149×435 mm; 16.73×5.87×17.13 in
Weight	23.5 kg; 51.8 lb

Transmitter
Output power SSB, CW, RTTY, PSK31, FM AM Transverter Connector, CW	(continuously adjustable) 5–200W 5–50W More than –20dBm
Modulation system SSB AM FM	Digital P.S.N. modulation Digital low power modulation Digital phase modulation
Spurious emission	More than 60dB (HF bands) More than 70dB (50 MHz band)
Carrier suppression	More than 63dB
Unwanted sidebandr suppression	More than 70dB
δTX variable range	±9.999 kHz
Microphone impedance	600Ω (8-pin connector)

Receiver
Receive system	Double conversion super-heterodyne system
Intermediate frequencies Main receiver:   Sub receiver:	64.455MHz/36kHz (1st/2nd) 64.555MHz/36kHz (1st/2nd)
Sensitivity (typical) SSB, CW, RTTY, PSK31, FM 0.1–1.799 MHz 1.8–29.999 MHz 50.0–54.0 MHz AM 0.1–1.799 MHz 1.8–29.999 MHz 50.0–54.0 MHz FM 28–29.999 MHz 50.0–54.0 MHz	(BW: 2.4 kHz at 10dB S/N) 0.5μV 0.16μV 0.13μV (BW: 6 kHz at 10dB S/N) 6.3μV 2μV 1μV (BW: 15 kHz at 12dB SINAD) 0.5μV 0.32μV
Squelch sensitivity (Pre-amp: OFF) SSB, CW, RTTY, PSK FM	Less than 5.6μV Less than 1μV
Selectivity (representative value) SSB (BW: 2.4 kHz, sharp)   CW, RTTY, PSK (BW: 500 Hz, sharp)   AM (BW=6KHz)   FM (BW: 15 kHz)	More than 2.4 kHz/–3dB Less than 3.6 kHz/–60dB More than 500 Hz/–3dB Less than 700 Hz/–60dB More than 6.0 kHz/–3dB Less than 15.0 kHz/–60dB More than 12.0 kHz/–6dB Less than 20.0 kHz/–60dB
Spurious and image rejection ratio	More than 70dB
Audio output power	More than 2.6W at 10% distortion with an 8Ω load
RIT variable range	±9.999kHz

Antenna Tuner
Matching impedance range	HF bands: 16.7Ω to 150Ω unbalanced (VSWR better than 3:1) 50MHz: 20Ω to 125Ω unbalanced (VSWR better than 2.5:1)
Minimum operating power	HF bands: 8W 50MHz band: 15W
Tuning accuracy	VSWR 1.5:1 or less
Insertion loss	Less than 1.0 dB (after tuning)

All stated specifications are subject to change without notice or obligation.

The LCD display may have cosmetic imperfections that appear as small or dark spots. This is not a malfunction or defect, but a normal characteristic of LCD displays.

Supplied Accessories

• Rack mount handles
• SD card
• AC power cable
• Spare fuses
• Plugs