iiiRadCom ♦ March 2000

Icom IC-756PRO HF & 6m TransceiverIcom IC-756PRO HF& 50MHz TransceiverReviewed by Peter Hart, G3SJX*

SOMEWHAT OVER three years ago,Icom launched the IC-756, a high endHF + 6m base station which I reviewed in

the May 1997 edition of RadCom. Very recentlyIcom have released the IC-756PRO, which fromits number may convey the impression of just anupgraded version. The outward appearance is verysimilar, indeed the same solid multi-partitioneddiecast chassis is used, with the same front and rearpanels. Some of the buttons have been reassignedto provide additional features, but the mouldings areidentical. However, internally the electrical architec-ture is very different, relying heavily on DSP for IFas well as AF processing functions and with anumber of important new features added.

PRINCIPAL FEATURESTHE IC-756PRO is 12V operated, with a peakcurrent drain of up to 23A. It is a table-top sizemeasuring 340 x 111 x 285mm and weighs about9.6kg. USB, LSB, CW, RTTY, AM and FMmodes are selectable, with normal or reversesideband on CW and RTTY. RTTY is FSK withselectable mark frequencies, shifts and polarity,and requires a logic level input on transmit. For usewith AFSK terminal units, AMTOR, PSK31,SSTV etc, voice modes are used. On all the voicemodes, a long push of the mode key selects dataand this mutes the microphone.

The receiver tunes continuously from 30kHz to60MHz, with the transmitter enabled only withinthe exact amateur band allocation (make sure thatyou have the appropriate version for the countryfrom which you wish to operate). Individual but-

tons select the bands, with a triple band-stackingregister employed that is useful if you operatefrequently on three modes, eg SSB, CW and RTTY.One of three last used frequency/mode combina-tions is returned for each press of the band key.

The smooth action tuning knob tunes in 10Hzsteps at 5kHz per knob revolution, with autospeed-up on fast sustained tuning. Faster tuningrates for rapid frequency changes and slower ratesfor precision tuning are also provided. 101 memorychannels are included with the usual facilities, andfrequency and memory channel can be entereddirectly. A memo-pad feature allows frequenciesto be quickly stored and recalled by a simple keypress, on the basis of last-in, first-out. RIT andtransmitter incremental tuning operate over arange of ±9.99kHz and various scan modes areimplemented. Comprehensive switching man-ages access to the VFOs and split frequencyoperation. For repeater use, separate splits areprogrammable on HF and 50MHz and there isalso a CTCSS tone encoder and decoder forrepeater access or tone squelch operation.

The receiver front-end configuration may beoptimised to suit different requirements, with twoselectable preamplifiers, three levels of signal

attenuation and a novel combined RF gain/allmode squelch control. Preamp 1 has a gain of about10dB for use on all frequencies above 1.6MHzand preamp 2 is intended for use on 21MHz andabove with a gain of about 16dB. The CW pitchis adjustable over the range 300-900Hz to suitpersonal preferences. A front panel pushbuttonselects between two antenna sockets and alsoenables external receiver antenna. The last usedantenna / external receive antenna combinationsare stored with the band stores. It can be useful tobe able to select external receive antennas auto-matically for the LF bands for loop antennas or forthose fortunate to have access to Beverages. Anauto ATU is built-in, matching up to 3:1 VSWRand covers all bands including 50MHz. Tuningsettings every 100kHz are stored to enable rapidand accurate reselection.

The transmitter power is variable from about 2-100W on all bands and the analogue meter indi-cates power output, SWR, compression or ALClevel. An IF DSP speech processor is providedwith three bandwidth settings, treble and bassadjust, VOX and a transmit signal monitor. Fulland semi break-in is available on CW, with a frontpanel control for adjustable delay.

The rear panel carries the usual interfaces,common to all Icom radios, for control of linears,ATUs, data terminals, etc. The standard Icom CI-V serial computer interface is provided with data

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TRANSMITTER MEASUREMENTS

CW SSB(PEP) INTERMODULATIONPOWER POWER PRODUCTS

FREQUENCY OUTPUT OUTPUT HARMONICS 3rd order 5th order1.8MHz 106W 106W -64dB -30dB -44dB3.5MHz 108W 108W -64dB -36dB -45dB7MHz 108W 108W -64dB -28dB -42dB10MHz 108W 108W -64dB -32dB -42dB14MHz 108W 108W -64dB -22dB -36dB18MHz 108W 108W -60dB -30dB -40dB21MHz 108W 108W -70dB -35dB -36dB24MHz 108W 107W -70dB -26dB -36dB28MHz 107W 107W -70dB -22dB -38dB50MHz 104W 103W -70dB -20dB -36dB

Carrier suppression: 25dB (see text) Sideband suppression: 70dB @ 1kHzTransmitter AF distortion: <1% Microphone input sensitivity: 3mV for full outputSSB T/R switch speed: mute-TX 12ms, TX-mute 1ms, mute-RX 22ms, RX-mute 1ms

NOTES:All signal input voltages given as PD across antenna terminal.Unless stated otherwise, all measurements made on SSB with receiver preamp 1 switched in.All two-tone transmitter intermodulation products quoted with respect to either originating tone. Top view with covers removed, showing PA, filters

and ATU.

iv RadCom ♦ March 2000

Icom IC-756PRO HF & 6m Transceiver

transfer rates from 300 to 19200baud, which also allowstransceiving with other Icom ra-dios. Two key jacks are fitted, thejack on the front panel for a keyingpaddle to control the internal keyer,and the jack on the rear panel for astraight key or external keyer. Atransverter jack is also providedwhich outputs about -20dBm ontransmit and functions as the re-ceiver input on receive.

DSPTHE MAJOR difference betweenthis radio and the earlier IC-756lies with the DSP circuitry. TheIC-756 used a quadruple conver-sion superhet with the main selec-tivity achieved through crystal/ce-ramic filters at both the 9MHz and455kHz IFs prior to feeding theDSP at 15.6kHz for demodulationand audio processing. The IC-756PRO is a triple conversion superhet with a firstIF of 64.455MHz, a second IF of 455kHz withfairly broad (15kHz) selectivity and a third conver-sion to 36kHz to feed a 32-bit floating point DSPwhich provides all the IF filtering, demodulationand audio processing functions. The channel fil-tering is impressive, providing 41 differentpassband widths on CW and SSB from 50Hz to3600Hz with shape factors superior to any crystal

filters. There are 32 passband widths selectable onRTTY (50-2700Hz) and 3 each on AM (3, 6, 9kHz)and FM (7, 10, 15kHz). Three separate selectablebandwidths are available for each mode by a simplepush of a front panel key, selectable from the availablemenu via the set-up routine.

The DSP also provides the twin passbandtuning function which enables the filter sides to beboth independently moved and narrowed. The

AGC is also implemented by the DSP,with three separate time constantsselectable from the front panel. Thesethree values may be set from a menuof 13 different values (0.1 to 6s) andare set separately for all modes exceptFM. Two separate notch functions areprovided in this transceiver, both us-ing the DSP. A particularly effectivemanually-tuned IF notch with a depthof 70dB is included within the AGCloop, and hence does not result indesensitising with strong carriers. Anauto-notch is implemented at AF andwill automatically attenuate several beatnotes, even if they are moving.

An adjustable DSP noise reduc-tion system is included and there isalso a conventional (not DSP) IFnoise blanker for pulse type interfer-ence such as car ignition. The modu-lated transmit signal is also generatedby the DSP, with adjustable bass andtreble microphone equalisation and

adjustable compressor.

OTHER SPECIAL FEATURESA DUAL-WATCH facility is incorporated via asecond receiver. The two receivers use separatefirst mixers and synthesisers and a common signalpath for all RF, IF and AF circuitry. Hence thesecond receiver will use the same IF bandwidthand mode and must be broadly within the same

Fig 1: Effective selectivity curve on USB.

vRadCom ♦ March 2000

Icom IC-756PRO HF & 6m Transceiver

RECEIVER MEASUREMENTS

SENSITIVITY SSB 10dBs+n:n INPUT FOR S9FREQ PREAMP 1 IN PREAMP OUT PREAMP 1 IN PREAMP OUT136kHz - 2.5µV (-99dBm) - 280µV1.8MHz 0.18µV (-122dBm) 0.45µV (-114dBm) 18µV 56µV3.5MHz 0.18µV (-122dBm) 0.45µV (-114dBm) 18µV 56µV7MHz 0.16µV (-123dBm) 0.35µV (-116dBm) 14µV 45µV10MHz 0.16µV (-123dBm) 0.35µV (-116dBm) 14µV 45µV14MHz 0.16µV (-123dBm) 0.35µV (-116dBm) 13µV 45µV18MHz 0.14µV (-124dBm) 0.35µV (-116dBm) 13µV 45µV21MHz 0.16µV (-123dBm) 0.40µV (-115dBm) 14µV 45µV24MHz 0.16µV (-123dBm) 0.40µV (-115dBm) 16µV 50µV28MHz 0.16µV (-123dBm) 0.45µV (-114dBm) 16µV 56µV50MHz 0.22µV (-120dBm) 0.45µV (-114dBm) 20µV 45µV

AM sensitivity (28MHz): 1µV for 10dBs+n:n at 30% mod depth AGC threshold: 1.3µVFM sensitivity (28MHz): 0.3µV for 12dB SINAD 3kHz pk deviation AGC attack time: see text100dB above AGC threshold for +1dB audio output AGC decay time: see textMax audio before clipping: 1.5W into 8 ohm, 2W at 10% distortionInband intermodulation products: better than -50dB

S-READING INPUT LEVEL SSB FILTER IF BANDWIDTH(7MHz) PREAMP 1 IN PREAMP OUT -6dB -60dBS1 1.6µV 5µV 10kHz 10.9kHz 16.2kHzS3 2.5µV 8µV 6kHz 6.6kHz 10.8kHzS5 4.5µV 14µV 2.4kHz 2520Hz 3560HzS7 8µV 25µV 500Hz 515Hz 1140HzS9 14µV 45µV 250Hz 263Hz 859HzS9+20 63µV 200µV 100Hz 107Hz 216HzS9+40 800µV 2.5mV 50Hz 63Hz 141HzS9+60 6.3mV 20mV

INTERMODULATION (50kHz Tone Spacing) PREAMP 1 IN PREAMP OUT3rd order 2-tone 3rd order 2-tone

Frequency intercept dynamic range intercept dynamic range136kHz - - +16dBm 84dB1.8MHz +3dBm 90dB +13dBm 92dB3.5MHz +3dBm 90dB +13dBm 92dB7MHz +0dBm 89dB +11dBm 92dB14MHz +2dBm 90dB +13dBm 93dB21MHz +0dBm 89dB +13dBm 92dB28MHz -2dBm 88dB +14dBm 92dB50MHz -4dBm 84dB +13dBm 92dB

CLOSE-IN INTERMODULATION ON 7MHz BAND PREAMP 1 IN PREAMP OUT3rd order 2-tone 3rd order 2-tone

Spacing intercept dynamic range intercept dynamic range3kHz -27dBm 71dB -18dBm 72dB5kHz -26dBm 72dB -17dBm 73dB7kHz -26dBm 72dB -16dBm 74dB10kHz -22dBm 74dB -12dBm 76dB15kHz -14dBm 80dB -4dBm 82dB20kHz -8dBm 84dB +3dBm 86dB30kHz -2dBm 88dB +9dBm 90dB40kHz +4dBm 92dB +12dBm 92dB50kHz 0dBm 89dB +11dBm 92dB

RECIPROCALFREQUENCY MIXING FOR BLOCKING BLOCKINGOFFSET 3dB NOISE PREAMP1 IN PREAMP OUT3kHz 83dB -32dBm -22dBm5kHz 86dB -32dBm -22dBm10kHz 92dB -29dBm -19dBm15kHz 96dB -21dBm -10dBm20kHz 98dB -13dBm -3dBm30kHz 101dB -7dBm +2dBm50kHz 106dB -7dBm +2dBm100kHz 111dB -7dBm +2dBm200kHz 115dB -7dBm +2dBm

band, or for the higher frequencies, on adjacentbands. A balance control sets the gain of the twosignal paths via PIN diode attenuators followingthe mixers. A single tuning control is used, assign-able to either receiver and both receive VFOfrequencies are continuously displayed togetherwith the separate selected memories (active or not).A quick dualwatch function equalises the fre-quency of both receivers.

The IC-756PRO includes a built-in RTTY(baudot) demodulator and decoder circuit fordisplaying the received data directly on the displaypanel. This is a new feature, a first in any trans-ceiver. The DSP provides a sharp two-tone dualpeak filter demodulator limited to 2125Hz mark,170Hz shift 45-baud RTTY. A tuning indicator isalso provided on the display panel to assist withtuning RTTY signals.

A built-in full message keyer with some usefulfeatures for contest operation is provided for CW.The keyer operates over a wide range of speeds,with adjustable weighting and a variety of keyingpaddle arrangements. Four memories will eachstore up to 54 characters, with a provision to sendautomatically incrementing serial numbers andauto-repeat after a time delay. The message storesare programmed in text from front panel pushbuttons, not using the keying paddle.

The IC-756PRO also includes a digital voicerecorder with four memory channels on receiveand four on transmit. Each memory store islimited to a maximum message length of 15seconds. This is useful for repeated CQ calls inlong contests and other fixed contest messages.The transmit stores can be tagged with labels upto 20 characters long for easy identification on thedisplay panel. As an optional extra, a voicesynthesiser module can be installed.

MEASUREMENTSMeasurements are given in the table when pow-ered from a 13.8V supply with additional com-ments as follows. Note that I have now includedthe 136kHz band in my standard set of measure-ments.

RECEIVERWith the higher gain preamp in circuit, preamp2, the sensitivity on 21MHz and above wasabout 3dB better than with preamp 1. Note thatno preamp is selectable on 136kHz. The S meterreading is independent of mode. The rejectionof images and all IF frequencies was exception-ally good, in excess of 100dB. The initial AGCattack time was fast, but took around 200ms tosettle the last dB.

The filter bandwidths shown in the table arejust a representative selection from the 40+ filtersavailable. The shape factors and steep skirts arereally excellent, even with bandwidths down to50Hz. This is where DSP scores, as these figuresare unattainable with conventional IF filters. Thereciprocal mixing performance is very good -better than the original IC-756 - and the overalldynamic range figures are reasonable. Even theclose-in dynamic range is quite creditable (see

Fig 1), considering that all the channel selectivityis in the final IF, a potential problem with thisapproach. The in-band intermodulation perform-ance is exceptionally good.

TRANSMITTERThe power output figures in the table weremeasured with the ATU out of circuit. The

ATU introduced an additional loss of about10%. The SSB carrier suppression with nomodulation was greater than 80dB, but de-graded to only 25dB with modulation. The CWrise and fall times were fairly sharp and therewas about 30% character shortening at40WPM on full break-in. There was no char-acter shortening on semi break-in.

vi RadCom ♦ March 2000

Icom IC-756PRO HF & 6m Transceiver

LCD panel showing RTTY decoder display (full screen).

LCD DISPLAY PANELTHE FRONT PANEL display uses a 5-in colourTFT LCD panel. Associated with this are sevenbuttons down the side and five function keysalong the bottom. These buttons and keys areassigned functions according to context. The tophalf of the panel indicates all the frequencies,modes, split and dual-watch status, IF filters etcat all times. A graphical representation of IFbandwidth is also portrayed, responding also toPBT settings. The 24-hour clock time is alsodisplayed and a timer with programmable on/offtimes may also be enabled.

The bottom part of the display has a number offunctions. It can show the spectrum on either sideof the receive frequency, with a selectable span from±12.5 to ±100kHz. The current spectrum trace isshown in green, with a blue background trace forpeak hold. Peak hold can be useful for monitoringtransient openings on 50MHz. The transmit fre-

quency is indicated for split operation or the subreceiver frequency for dual watch. A section of thememory bank can be displayed showing up to 13consecutive locations. Alphanumeric names can beassigned for easy reference and it is a simple matterto scroll through, locate and select channels. Thissection of display also shows the set-up and mes-sage stores for the voice and CW keyers anddecoded RTTY messages.

This radio has a host of user programmablesettings. These are easily accessed and set,without constant reference to the manual, bythe simple presentation in words and the as-signable keys provided on the display. Thissame approach allows the memory keyer,scanning options and other functions to beset up in a very user friendly way.

ON-AIR PERFORMANCETHE FIRST THING you will see when you turnon this radio is the start-up screen saying that theDSP is calibrating. This takes ten seconds. OverallI found the radio an excellent performer, easy touse after initial familiarisation and with all thefeatures that anyone could possibly want and wellimplemented. The filters were really superb. Thenarrower filters offer some interesting possibili-ties for 136kHz DX and narrow data modes suchas PSK31. The performance on 136kHz seemedvery good and although I am currently without myLF antennas, several stations were heard on thisband. The receive performance was very good onall modes, although some strong signal problems LCD panel showing spectrum display.

could be found on 40m with the preamp switchedin circuit. I found the speaker somewhat restrictedin frequency range and lacked bass response, withmuch better quality on headphones. The built-inRTTY decoder performed effectively and thespectrum display was particularly useful for moni-toring 50MHz band activity. The transmit per-formance was good and CW break-in effective.

CONCLUSIONSThe IC-756PRO is an excellent all-round buyfor HF and 50MHz, packed with features anda good electrical performance. The list price is£2099 and, most importantly, there are noexpensive extra filters needed for serious use onCW and RTTY.

ACKNOWLEDGEMENTI would like to thank Waters and Stanton PLC forthe loan of the equipment. ♦