Icom IC-756PROII User Manual 2

TECHNICAL REPORTTECHNICAL REPORT

2

CONTENTS

11. Profile. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

12. Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

13. Front and rear panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3-1 Front panel

3-2 Rear panel

14. What is DSP in radio Communication? . . . . . . . . . . . . . . . . 10

15. Circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5-1 Digital IF filter

5-2 Digital functions

5-3 PSN modulation

5-4 Manual notch

5-5 Speech compressor

5-6 Microphone equalizer

5-7 RTTY demodulator

5-8 Receiver

5-9 Transmitter

5-10 Dual-watch function

5-11 Real-time spectrum scope

5-12 Voice record/playback function

5-13 PLL circuit

16. Connection to option/peripheral units . . . . . . . . . . . . . . . . . 25

6-1 ACC Sockets

6-2 HF/50MHz, 1kW linear amplifier

6-3 Interface for digital mode

6-4 External control unit for voice memory keyer

6-5 Installation of UT-102 optional Voice Synthesizer Unit

17. CI-V control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

7-1 Remote jack

7-2 Data format of CI-V

7-3 List of commands

18. Inside Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

19. Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

10. Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

10-1 General

10-2 Transmitter

10-3 Receiver

10-4 Antenna tuner

11. Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . foldout

3

The IC-756PROII is the high performance HF transceiver of choice for today’s discriminating amateur

radio operator. Icom’s engineers took cutting-edge digital technology and paired it with Icom’s

extensive experience with analog technology. The result is a major advancement of Icom’s original

digital IF filter which, in the earlier IC-756PRO model, enjoyed a great reputation around the world.

The IC-756PROII uses the same 32-bit floating point DSP and a 24-bit A/D-D/A converter as the IC-

756PRO. It is now possible to execute the digital IF filter, noise reduction and the digital IF filter in the

AGC loop processing, and to select the soft/sharp filter shapes. The IC-756PROII employs exclusive

DSP/analog circuit matching to further improve receiver performance.

Icom’s engineers analyzed the influence of the AGC loop upon the received audio, matching it to an

analog circuit suitable for the dynamic range of the A/D converter and the other parts used, and also

re-examined the core stage of the receiver (ranging from RF top to mixer circuit), to distribute the

mixer levels properly. As a result, the matching of digital and analog technology has attained a level

never before achieved.

This technical report does not explain in depth all the digital engineering with its many calculations and

formulas. Instead, it focuses on the DSP engineering in an easy-to-understand manner. This report

also explains why the 32-bit floating point DSP and 24-bit A/D-D/A converter are included. The

dynamic range of the 32-bit floating point DSP and the 24-bit A/D-D/A converter may seem to be an

over specification for amateur radio. But this is not the case. This technical report helps clarify these

points.

It is Icom’s hope that in providing you with this report you will discover the IC-756PROII’s many digital

advantages. Enjoy!

4

32-bit floating point DSP and 24-bit A/D-D/A
converter

The adoption of a 32-bit floating point DSP and 24-bit A/D-D/A
converter in the IF stage (36kHz) was originally developed by
Icom. It enables various digital functions which amateur radio
operator’s desire.

■ 51 types of digital IF filtering

The digital IF filter has superior filtering performance and a
distinguished shaping factor that demonstrates the power of 32-bit
floating point DSP. The digital filter is completely free from
deterioration due to deviations in band characteristics, temperature
change, or mechanical vibration, all of which have been observed in
analog filters. It also provides excellent ripple characteristics that
have never been available with analog filters. The passband
(50Hz–3.6kHz) of the digital IF filters used for IC-756PROII come in
51 types. This function allows 3 of these 51 types to be pre-set for
each mode and to be changed instantaneously by using the filter
button, depending on the situation.

■ 2 types of filter shape (Sharp/Soft)

Changing the IF filter shape is a feature that is not available with
analog radios. Select the filter shape from two types, Sharp and Soft,
depending on the purpose, operating band, band conditions, etc. It is
possible to set CW and SSB filter characteristics independently and
also select the filter shape while actually receiving a signal.

■ Digital IF filter in AGC loop

The digital IF filter, manual notch filter, etc. are located in the AGC
loop, using DSP, which completely eliminates interference from
adjacent strong signals. This allows the AGC to be operated only on
the selected frequency. It is also possible to pre-set the operation of
AGC in each mode in accordance with 13 types of time constants.

■ Digital twin PBT

The IC-756PROII is equipped with DSP based twin PBT filtering. It
provides excellent performance on a completely different level than
analog filters. Set the frequency, and then adjust the passband width
of the received signal in steps of 50Hz using the dedicated two­position knob. The passband width and direction of shift may be
graphically displayed on the LCD, if the operator chooses.

■ High-accuracy digital modulation and demodulation in all
modes

The DSP unit allows you to increase transmit/receive audio levels,
modulation, and demodulation — even while decoding RTTY. This
makes it possible to set the passband width of the IF filter for SSB
transmit to 3 different stages. The DSP unit also provides a
demodulation level suitable for high-grade HF performance and high­fidelity sound.

■ Manual notch with superior attenuation level

The IC-756PROII’s manual notch filter has extremely sharp
characteristics for processing in the DSP and provides tremendous
performance for attenuation levels >70dB. Analog notch circuits are
susceptible to fluctuations in attenuation or changes in temperature.
The DSP-based manual notch provides stable performance and is
not susceptible to such changes. Also, the DSP signal processing
executed within the AGC loop completely shuts off undesirable
signals, even with the AGC set to high speed. An automatic notch is
included to further enhance receiver performance.

■ Demodulator/decoder for RTTY

This transceiver is equipped with a demodulator and a decoder for
BAUDOT RTTY as a standard feature. On-air station calls may be
recognized instantaneously by reading the received RTTY message
directly on the IC-756PROII’s LCD – no personal computer or
external components are required. The transceiver is fitted with an
on-screen tuning indicator that allows the RTTY to be fine tuned with
ease. A DSP based twin-peak audio
filter further improves the S/N ratio.
This filter will reduce interference that
appears between each tone (mark
and space), which cannot be removed
by conventional analog filters. This
twin-peak audio filter works to capture
noise-level signals accurately and to
significantly reduce the generation of
noise distortion.

■ New-generation speech compressor

The DSP based speech compressor enhances the readability of your
transmitted signal at a receiving station without any distortion, even
when the compression is set to a high level. The gradation of voice
processing is extremely close to the original sound. This assures
superior sound quality at all compression levels.

■ Microphone equalizer (enables 121 different settings)

The IC-756PROII is equipped with a microphone equalizer that makes it
possible to set the frequency characteristics of the transmitted signal in
11 different stages for both the high-tone range and the low-tone range.
Considering all permutations, this provides for a total of 121 different
settings. With this flexibility of DSP based waveform shaping, it is
possible to adjust transmit audio quality depending on the application.
For example, it is possible to set the dynamic sound quality for “Pileup”
or to set pleasant sound for “Ragchewing”.

■ Variable level type noise reduction

The 32-bit floating point DSP has excellent calculation performance,
which processes complex and sophisticated algorithms. This allows
the DSP to attenuate noise without delay and extracts noise-level
signals. It is possible to vary the suppression level in 16 stages.

■ CW keying waveform shaping function

DSP controls the rise and fall of the CW transmit waveform. The
result is a proper CW waveform. The rise/fall timing is selectable to 4
stages of 2ms, 4ms, 6ms and 8ms. This makes it possible to set a
“Soft” or “Hard” CW signal, depending on your preference.

455
kHz36kHz

3Lo

Manual

NOTCHIFFilter

DET

AGC

AGC

DSP

DAC

DAC

ADC

AF

100W

2.5ms/Div2.5ms/Div

100W

Speech compressor on Speech compressor off

RTTY reception screen

2. Features

5

Enhanced functions

■ Advanced receive functions

The RF stage’s front-end receive mixer is designed in a 4-element
configuration. In the IC-756PROII, this configuration is used in the
BPF stage at the RF top. Each element is examined to tune the
circuit after RF stage to mixer, which makes it possible to enhance
the receive performance. This significantly reduces 3rd and other
order distortion and provides a wide dynamic range. This means the
IC-756PROII will accurately capture weak signals that analog type
radios cannot hear, even in low bands with high noise levels.

■ Real-time spectrum scope

A real-time spectrum scope is recognized as indispensable for DX
hunting. The IC-756PROII’s spectrum scope uses two colors to
display all RF signal activity within a user-selectable bandwidth. One
color indicates real-time RF signal activity, while the other color
provides peak hold indication. The spectrum scope may be used for
sophisticated applications such as identifying the band conditions,
quick discovery of stations, and confirmation of interval or call-back
frequency. Additionally, you may monitor normal band conditions,
while you display sub readout or transmit markers. In case of high­band noise, the IC-756PROII is equipped with an attenuator
(10/20/30dB) dedicated to the spectrum scope, which allows a
reduction of total signal level at the band scope without affecting the
received signal.

■ Exceptionally clear SSB transmit signal

Using Icom’s advanced digital PSN modulation, the IC-756PROII
emits high-quality signals, which makes its transmitter suitable for
use as the exciter of a linear amplifier. Unwanted sidebands and
carrier leaks are almost completely eliminated. Further, the
transmitter employs a wide band power amplifier that incorporates
highly reliable bipolar transistors (2SC5125 × 2). The linearity and
IMD characteristics achieve superior signal quality never before seen
in any amateur redio transceiver. This makes it possible to transmit
RF signals with significantly reduced distortion.

■ Built-in high-stability reference crystall oscillator

The IC-756PROII’s transceiver
exhibits excellent frequency
stability of < 0.5ppm. This
assures stable communication
even for RTTY and SSTV
modes for which particularly
high frequency stability is
required.

The best in operating convenience and features

■ Dual-watch

Dual-watch enables simultaneous two-frequency receive in the same
band, providing identical band and filter configurations in both receive
systems. This makes it possible to receive two signals simultaneously
as if two separate receivers are being used. This greatly enhances
split frequency operation; enjoy enhanced DX-operation by searching
for pickup frequencies while watching the transmit frequency of a DX
station experiencing pile-up. Or have a QSO while simultaneously
monitoring a DX net.

■ Triple band stacking register

With the push of a band button, get quick memory recall of three
preferred operating settings (including antenna port) per band. Band
or mode hopping has never been easier. It’s the ultimate in multi­mode flexibility.

■ Digital Voice Recorder (DVR)

The DVR feature is an indispensable function for DX hunting and
contests. The IC-756PROII is equipped with a DVR with 4 channels
for transmit and 4 channels for receive, for a total of 8 channels. High
quality digital mapping of the transmitted or received analog signal
provides high quality audio reproduction, resulting in a natural
sounding voice without any noticeable degradation. It is also possible
to use these 4 communication channels by allotting them freely with a
total recording time of 90 seconds. Each of the 4 channels for receive
has a recording time of 15 seconds, or 60 seconds total. Press the
key once in any TFT display mode and it becomes possible to not
only record or reproduce voice but also to record for up to 30 minutes
continuously. The receive audio may
be reproduced for the most recent 15
seconds back to an interruption in
recording. By constructing the
simplified control unit (page 26) and
connecting it to the microphone
connector, digital voice recorder
function may also be operated.

■ Full-scale electronic keyer

Plug a CW, iambic paddle into the electronic keyer jack on the front
panel. Especially handy during long hours of operation, it is possible
to set the CW speed between 7 and 56WPM. The discriminating
operator may also set the dot/dash keying ratio (2.8:1 to 4.5:1) and
polarity, depending on preference. The keyer may also be set for
either right or left hand use. For the CW operator who prefers not to
use the IC-756PROII’s built-in electronic keyer, an ordinary key jack
is available on the rear panel, for bug or straight key and is fully
compatible with external keyers or PC keying.

■ Multi-function memory keyer

Enhance your contest operations. The IC-756PROII is fully equipped
with a convenient memory keyer, offering features such as memory
content editing function, auto-repeat function, serial contest number
automatic count-up function, contest number abbreviating function, and
more. These features will reduce effort when repeating a formatted
contents for calling CQ, continuous transmission of call sign, or contests.
Since it is possible to confirm the contents of memory on the display,
transmission mistakes are eliminated. Construct the simplified control
unit (page 26) and connect it to the microphone connector to enhance
operation of these memory keyer functions.

Real-time spectrum scope screen

DVR key

High-stability reference crystal oscillation unit

6

■ Quick split function

When the split button is pressed and held, the frequency of the sub­VFO is adjusted to the frequency in the main VFO. Using the split
function, it is also possible to control the following:

1. Vary the transmit frequency via the main dial.

2. Direct entry of the designated frequency.

3. Direct entry of the shift frequency.

You are now ready to “bag the DX” while other operators are still
tuning up.

■ Preamplifier and attenuator

The IC-756PROII incorporates two types of receive preamplifiers:
Preamplifier 1 (10dB) emphasizes modulation across all bands, and
preamplifier 2 (16dB) emphasizes sensitivity especially for high
bands. The attenuator is selectable in three stages, 6, 12 and 18dB.
When there is a strong signal from a local commercial station it
becomes possible to control the generation of distortion at the RF
stage of the receiver. It is also possible to retain the preamplifier and
attenuator settings for each band.

■ Variable noise blanker

The transceiver uses a new noise blanker design that provides
significant reduction of pulse-type noise. The noise blanker also
greatly enhances weak signal copy, allowing the operator to change
the sensitivity in 100 stages in accordance with the noise level
without distorting the target signal.

■ Frequency shift function for change from/to SSB to/from CW

A frequency shift function automatically adjusts the CW carrier point
when selecting from SSB mode, or vice versa. You may select “Shift
function off” whereby the frequency remains the same (by moving the
carrier point), or “Shift function on” in which the frequency is shifted
without moving the carrier point. Using CW-R mode it is possible to
set the carrier point to USB.

Enhanced TFT color display

■ High visibility

A high visibility 5-inch TFT color display has been integrated into the
IC-756PROII to provide ease of use and clear indication of the radio’s
many features. Various function settings such as frequency, memory
frequency, comment, filter setting status, RTTY tuning indicator, and
more are displayed in the upper portion of the display, The lower
portion of the display gives voice memory, characters of received
RTTY, and the real-time spectrum scope information. The display
color may be selected from 8 types, from vivid color to muted grays. 7
different font types may also be selected. These settings may be
made in any combination – customize your display to best suit your
personality or favored operating set-up.

■ Memory channel/memory list

The transceiver is equipped with 99 regular memory channels and 2
scan edges, totaling 101 channels. It is possible to enter text of up to
10 characters in each memory channel. It is also possible to display a
list of up to 13 memories at a time.

■ Simplified set mode

The IC-756PROII has a list display that allows the status of each set
mode item to be seen at a glance. Each function is divided into 4
setting groups and multiple items are listed or displayed to allow
quick access to the desired item. This allows the many functions of
the radio to be used with ease.

■

Digital meter simultaneously displays 4 transmit level indicators

With the digital meter (including peak-hold), it is possible to confirm
the output power, ALC, SWR, and COMP, all at the same time while
transmitting. The signal strength is also displayed while receiving.

Enhanced functions

■ Antenna system

· High-speed built-in auto antenna tuner covering up to the 50MHz
band

· 2-piece antenna terminal (incorporated with auto antenna selector)

· Dedicated receive antenna connector

■ Receive system

· General coverage receive (30kHz to 60MHz)

· Control of RF gain and squelch with one knob

■ Transmit system

· IF Monitor function allows the transmissions of your station to be
listened to locally

· Built-in 50 frequencies of tone encoder/decoder

· VOX function allows the automatic selection of transmit and receive
for “hands free” operation

· All-mode power control function

■ CW system

· CW pitch control function allows the CW receive tone to be set to a
desired frequency (300 to 900Hz continuously)

· Double key jack allows 2 types of keys to be connected
simultaneously

· Full break-in function allows receive during a break while keying

■ Operation system

· 5-channel memo pad saves frequency and mode
(It is also possible to change the 5-channel memo pad to a 10­channel type.)

· RIT and TX variable up to ± 9.999kHz

· 1Hz pitch tuning

· Optional frequency speech allows the S-meter level to be

announced

· High visibility needle type white-tone analog meter

· Various scanning functions (program, memory, select memory, F)

· Auto Tuning Step

· Dial-lock

· Split frequency lock

· Torque adjustment mechanism for main dial

· Band edge beep function

· CI-V terminal allows control from a personal computer

· Clock/timer function

· AH-4 control circuit

7

3. Front and rear panel

3-1 Front panel

A

A

D E

B C F

G

B

C

POWER SWITCH [POWER TIMER]

S/RF METER

TRANSMIT SWITCH [TRANSMIT]

HEADPHONE JACK [PHONES]

ANTENNA TUNER SWITCH [TUNER]

MONITOR SWITCH [MONITOR]

NOISE BLANKER SWITCH [NB]

NOISE REDUCTION SWITCH [NR]

ELECTRONIC KEYER JACK [ELEC-KEY]

AF CONTROL [AF]

RF GAIN CONTROL/SQUELCH CONTROL

[RF/SQL]

BALANCE CONTROL [BAL]

NOISE REDUCTION LEVEL CONTROL [NR]

MICROPHONE CONNECTOR [MIC]

MIC GAIN CONTROL [MIC GAIN]

RF POWER CONTROL [RF POWER]

COMPRESSION LEVEL CONTROL [COMP]

SEMI BREAK-IN DELAY CONTROL [BK-IN DELAY]

ELECTRONIC CW KEYER SPEED CONTROL

[KEY SPEED]

LCD FUNCTION SWITCHES [F1]-[F5]

MODE SWITCHES

FILTER SWITCH [FILTER]

EXIT/SET SWITCH [EXIT/SET]

8

D

E

F

MULTI-FUNCTION SWITCH GUIDE

LCD FUNCTION DISPLAY

KEYPAD

MEMORY UP/DOWN SWITCHES [ ][ ]

MEMORY WRITE SWITCH [MW]

MEMORY CLEAR SWITCH [M-CL]

QUICK TUNING SWITCH [TS]

TRANSMIT FREQUENCY CHECK SWITCH [XFC]

MEMO PAD-READ SWITCH [MP-R]

MEMO PAD-WRITE SWITCH [MP-W]

MAIN/SUB CHANGE SWITCH [CHANGE]

VFO/MEMORY SWITCH [VFO/MEMO]

MAIN/SUB CHANGE SWITCH [CHANGE]

DUALWATCH SWITCH [DUALWATCH]

SPLIT SWITCH [SPLIT]

RECEIVE INDICATOR [RX]

TRANSMIT INDICATOR [TX]

REC/PLAY SWITCH [REC/PLAY]

LOCK/SPEECH SWITCH [LOCK/SPEECH]

TUNING DIAL

LOCK INDICATOR [LOCK]

9

G

PASSBAND TUNING CONTROLS [TWIN PBT]

PBT CLEAR SWITCH [PBT CLR]

NOTCH SWITCH [NOTCH]

MANUAL NOTCH FILTER CONTROL [NOTCH]

CW PITCH CONTROL [CW PITCH]

TX SWITCH [ TX]

RIT SWITCH [RIT]

RIT/ TX CONTROL [RIT/ TX]

CLEAR SWITCH [CLEAR]

3-2 Rear panel

CI-V REMOTE CONTROL JACK [REMOTE]

STRAIGHT KEY JACK [KEY]

ACCESSORY SOCKET 1 [ACC (1)]/

ACCESSORY SOCKET 2 [ACC (2)]

TUNER CONTROL SOCKET [TUNER]

RECEIVE ANTENNA CONNECTOR [RX ANT]

TRANSVERTER JACK [XVERT]

GROUND TERMINAL

ANTENNA CONNECTOR 1 [ANT 1]/

ANTENNA CONNECTOR 2 [ANT 2]

DC POWER SOCKET [DC 13.8V]

SEND CONTROL JACK [SEND]

ALC INPUT JACK [ALC]

EXTERNAL SPEAKER JACK [EXT SP]

ANT1ANT2

AC C (2)AC C (1)

10

4. What is DSP in radio communication?

The term DSP stands for “digital signal processor”. When DSP is
used in a communication unit, the electrical signal processes
(amplification, filtering mixer, modulation, demodulation, etc.) are
handled by the DSP. Such signal processing, using numerical
calculations, is called “digital signal processing”.
Digital signal processing assures the same results every time
providing for the characteristics defined in the design phase. When
digital signal processing is utilized, it is not necessary to take the
adjustment deviations of the conventional analog circuit into
consideration. These deviations are caused by variations in
component characteristics, temperature change, or deterioration over
time. It is also possible to perform complicated processing tasks such
as Fourier transformation, adaptive control, special function
processing (*1), and more. Such complicated processing tasks are
very difficult and costly for a conventional analog circuit.

*1Special function processing: Trigonometric function, inverse function of trigonometric

function, square root, logarithmic function, exponential function, etc.

Digital signal processing is also widely used in fields other than radio
communication units, such as:

· Modems for telephone circuits

· Surround-sound effects (stereo sets, stereo components)

· Echo canceller (telephone)

· Voice compression/coding (cellular phones)

It is possible for a computer CPU to execute digital signal processing.
However, a DSP differs from a CPU in that it has the dedicated
hardware construction required for the effective execution of digital
signal processing. Basically the unit has a multiplication/addition
circuit widely used for DSP to execute the combination of
multiplications and additions in one clock, and with an internal data
bus of more than two circuits, to fetch two data items required for
calculation at the same time. It also has a loop processing function to
execute repeated calculations with high efficiency and a data address
creation function to transmit signal data effectively, which are
assigned to consecutive addresses. These functions are incorporated
as dedicated hardware.

Because their performance has developed quickly, the CPUs
currently used for computers can execute digital signal processing. A
CPU with a high clock frequency may be superior in calculation
performance to a low-end DSP chip. When it is compared with a CPU
of equivalent performance, a DSP with hardware specialized to digital
signal processing has the following advantages:

· Low clock frequency

· Low integrity (reduced logic scale)

· Low power consumption (Low

heat energy generated)

· Low cost
When various judgment functions
are required, or when different
calculations are repeated each time,
a DSP is not suitable. The CPU is
then more suitable for such
processing.

Background to development of the first-generation
IC-756PRO

From the early stages of research into DSP transceivers Icom has
been on the forefront of shifting IF filter design from analog to digital
type filters. To put the digital IF filter to practical use it was necessary
to incorporate the digital IF filter into the AGC loop. It was also
necessary to provide AGC processing using the DSP. To achieve this
there remained a lot of technical problems to be resolved.

In the initial stage of research, it was not possible to complete the
DSP, A/D, and D/A devices in a radio unit at a practical cost, as
shortage in device capacity was a significant factor. Icom conducted
research into digital PSN modulation, noise reduction, automatic
notch, and audio peak filter, while also proceeding with research into
digital IF filter processing and digital AGC processing. This research
includes the ultra-narrow filter for CW that allows the advantages of
DSP to be fully utilized for commercialization of a DSP transceiver.
As the first devices were developed with improved capacity, Icom
started full-scale research into integrating the digital IF filter
processing (*2) and digital AGC processing in practical applications.

*2Advantages of digital IF filter processing:

Since a digital IF filter is free from deterioration due to passband width deviations,
temperature changes, change in mechanical strength, etc., the changes seen in an
analog filter will not occur. It will not deteriorate through years of use and will provide
excellent ripple characteristics that are not possible with analog filters.

When the DSP filter is processed at the AF stage, the demodulated
AF signal is filtered after this. This filter type will function effectively
when the level of the interfering signal is equal to or less than that of
the desired signal. However, when the level of the interfering signal
increases, the AGC activates reducing the level of the desired signal
causing it not to be heard (AGC blocking phenomenon). This
phenomenon is caused by filtering taking place outside the AGC
loop. Even if filtering is executed at the IF stage before demodulation,
it is not possible to avoid this blocking phenomenon when the digital
filter is not incorporated in the AGC loop. Therefore, it is necessary to
execute both IF filter processing and AGC processing using the DSP
to prevent the AGC blocking phenomenon.
To realize a digital AGC, it is necessary to obtain the adjustment
range for AGC gain internally in the DSP (*3), and to input both the
desired signal and the interference signal into the A/D converter
without them distorting (*4). For these reasons, Icom decided on a
dynamic range for the A/D converter of at least 110dB, and
approximately 120dB when the margin is taken into consideration.

*3To control the AGC attack response properly, it is necessary to adjust the gain even after

the completion of IF filter processing. If the adjustment range of gain within the DSP is set
to 60dB, it is necessary to obtain a wider dynamic range, as the noise floor is raised 60dB
under full-gain conditions where AGC is not applied.

*

4

If the signal is distorted before entering the A/D converter, a distortion component may be
mixed in the band. If it is mixed in the band, it is extremely difficult to remove it by post
processing.

The DSP in the IC-756PRO/756PROII employs a 24-bit A/D
converter. The logical value of the dynamic range of a 24-bit A/D
converter is 144dB, however the actual value of the analog
performance is smaller than this and performance may differ
considerably, depending on the type of A/D converter used.

DSP chip

11

The A/D converter used for the IC-756PROII is a super-high
performance A/D converter that is also used in digital mixers for
recording studios and provides an actual analog performance value
of 120dB. To bring this performance to an optimum level it is
necessary to execute calibration for 10 seconds after powering on.
The wait time when IC-756PROII is started is allotted to the
calibration operation.

To execute the processing of data sampled by the 24-bit A/D
converter it is necessary to obtain 24-bit calculation accuracy. Since
the dynamic range is decreased substantially due to the scaling
operation (*5) for the accumulation of calculation errors or digital filter
processing, Icom felt the 24-bit fixed decimal point DSP would
provide insufficient calculation accuracy.

*5Scaling

For digital filter processing, a frequency which causes the gain to increase may exist at
the intermediate stage of processing even if the filter used provides a passband gain of
0dB. For a fixed decimal point, DSP the calculation is executed with the gain decreased
in advance so as not to allow an overflow to occur due to a signal of that frequency.
This gain adjustment operation is called “scaling”.

Since the level of scaling required is also increased to provide an IF
filter with a sharp shape factor the calculation accuracy is liable to be
decreased, even if double-precision (32-bit fixed decimal point)
calculation is executed when using a high-speed 16-bit DSP. To
provide both the digital IF filter processing and digital AGC
processing using DSP, Icom determined it was necessary to use a
32-bit floating point DSP.

For a 32-bit floating point DSP, the numerical data within the DSP is
adjusted automatically according to the size of the numerical value.
Consequently, errors generated due to calculation are extremely
limited and the influence of calculation errors is almost negligible.
Because it is not necessary to consider the overflow during
calculation, the dynamic range will not be decreased due to the
scaling operation.

The 32-bit floating point DSP and 24-bit A/D-D/A converter use a
signal processing algorithm (newly developed to demonstrate its
performance) in combination for the reasons above, which make it
possible to provide highly accurate digital IF filter processing and
digital AGC processing. These new functions (FM demodulation, AM
modulation/demodulation, RTTY modulator, etc.) were incorporated
in the IC-756PRO to make it an IF DSP radio.

Two Dynamic ranges

Dynamic range as RF performance

“Dynamic range” from the viewpoint of RF performance indicates to
what extent the distortion component (generated due to the
frequency of a signal) can be heard at the receive frequency when a
frequency component different from two receive frequencies is input.
“Dynamic range” generally means the value by 3rd order distortion
component.

If the receive frequency is substituted for “fRX”, the input frequency for
“f1” and the input frequency for “f2” respectively, the following
relationship is established for 3rd order distortion component.

f1 × 2 ± f2 = fRX, or
f1 ±f2 × 2 = fRX

If there are inputs of 14.2MHz and 14.3MHz while 14.1MHz is being
received, the distortion component is heard at 14.1MHz. The relative
value of the input level when the signal can be heard at 14.1MHz and
the level of the signal received at the essential receive frequency is
called the “dynamic range”.

Figure 1 shows an example in which the following are plotted on the
same axis.

· Input/output characteristics at receive frequency, or the
characteristic data (a) for a case when the receive frequency
component input from the ANT is detected and output as a low
frequency signal

· Input level of frequency component (generating 3rd order distortion
from the receive frequency) and level (b) at which the distortion can
be heard at the receive frequency.
The difference in level at which (a) and (b) above can be heard is
the dynamic range.
The level at intersecting point between (a) and (b) above is called
IP3 (3rd order intercept point).

If these numerical values are large, it can be said that signal
processing is executed without distortion. When the numerical values
are small, a frequency component that does not exist in the essential
receive frequency is heard and distortion will be generated.

Dynamic range for A/D converter

Consider the dynamic range (used as an index for the performance of
an A/D converter) as the ratio between maximum value and minimum
value to be treated by the A/D converter. If the maximum resolution
for one bit is “Vmax” in the case of a 16-bit A/D converter, the
following is given:

Vmax ÷ 216= Vmax ÷ 65536

In other words, the change in level for one bit is 1/65536 of Vmax.
This value seems to be an extremely small value, in decibels it will be
as follows:

20log (1/65536) = –96.33dB

This means that an S/N ratio of over 96dB is never allowed for
transmit. The minimum resolution of signal the A/D converter can
treat is affected by its specifications, which are 24 bits and 144dB
logical value. Some may say that a transceiver is not a high-grade
audio system and therefore does not require a specification of 144dB,
or that a specification of 96dB is sufficient; however this value is not
an over specification. If there is no AGC in the DSP and the input
level of the A/D converter is properly controlled by the analog circuit
AGC, the specification of 96dB will be sufficient (the IC-775DSP uses
this system). When the A/D converter is in the AGC loop, the input
level of the A/D converter may fluctuate significantly. For this reason,
the gain control by AGC within DSP requires at least the dynamic
range of the A/D converter.

Fig. 1

–140

–130

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

0

20

40

60

80

100

120

140

160

180

200

010203040

50

[dBm]

[dB]

Output

3rdIMD

Dynamic IP3

(a)

(b)

12

5. Circuit description

5-1 Digital IF filter

For IC-756PROII the transmit/receive passband width in all modes is
determined by the digital IF filter using DSP. A filter of this type
provides an ideal shape factor that cannot be achieved by an analog
filter. If an attempt is made to increase the shape factor and band
ripple characteristics of an analog filter, it is necessary to increase the
number of crystal components (or ceramic elements), which may
result in physical restrictions. A digital IF filter using DSP assures the
desired characteristics by overlaying multiple filters. This is governed
only by the processing volume of the software and it is possible to
overlay such filters with any number of stages.

The diagram shows a graph of receive selectivity when the IC­756PROII is set to the SSB BW mode of 2.4kHz as well as the
selectivity characteristic of each Collins 10-pole mechanical filter. The
digital IF filter of the IC-756PROII is of a design equivalent to a 14­pole filter. The filter serves to cut the undesired adjacent signals
sharply under any circumstances using the superior shape factor
(sharp/soft) and 51 types of variable passband width provided by IF
stage processing using the DSP. When viewing a received CW signal
the difference between the cut-off performance of this filter and that
of an analog IF filter is evident.
In a transceiver equipped with a conventional analog IF filter the beat
frequency of an adjacent signal is present when the CW signal is
received resulting in interference. The beat frequency is contained in
the skirt of the filter even if it is out of the set band range. (Fig. 1.1)

When using a digital IF filter the beat frequency of an unwanted
adjacent signal moves out of the filter passband width, which will not
cause interference. (Fig. 1.2) This is the greatest difference between
an analog IF filter and a digital IF filter. During “pile-ups”, such as
those that occur in DX’peditions, contests, etc., it is possible to make
a proper selection suitable to the application by selecting the broad
filter shape (SOFT).

5-1-1 CW sharp filter

The digital IF filter offers an ideal shape factor which has never been
available with conventional analog filters. It enables a greater ability
to receive weak stations that may lie behind radio interference. This
is the filter shape that Icom would suggest to the DX hunter due to its
superior cut-off performance. The cut-off performance is of a level to
actually extend the CW band as explained above.

5-1-2 CW soft filter

The skirt characteristics of the soft filter are broadened so that the
listening level of the filtered signal is the same level as that of a
conventional analog filter. When using the radio for DX’pedition the
filter is recommended for “pile-up” operation and is most suitable for
the CW DX’peditioner and CW contestant.

Response

–70

–2000

–1500

–1000

–500

0

500

1000

1500

2000

–60

–50

–40

–30

–20

–10

–0

10

[dB]

Input frequency

[Hz]

756PROII SSB COLINS10

Fig. 1.1

Conventional
analog filter

The signal out of setting
band is also heard as
CW reception sound.

Beat frequency

0

10

20

30

–10
–20
–30
–40
–50
–60
–70
–80
–90

–100

–600–800 –400 –200

500Hz setting

0 200 400 600 800

[dB]

[Hz]

30

Digital filter of
IC-756PROII

The signal out of setting
band is not heard as
CW reception sound.

Beat frequency

10

20

–10
–20
–30
–40
–50
–60
–70
–80
–90

–100

–600–800 –400 –200

500Hz setting

0 200 400 600 800

[dB]

[Hz]

Fig. 1.2

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

0

10

[dB]

500450400350300250200150100500

[Hz]

CW Filter (BW 50/100/150/200/250/300/350/400/450/500Hz)

CW sharp filter characteristic

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

0

10

500450400350300250200150100500

[Hz]

CW Filter (BW 50/100/150/200/250/300/350/400/450/500Hz)

[dB]

CW soft filter characteristics