IC-756Pro_review_RC_2000.pdf

iiiRadCom March 2000 Icom IC-756PRO HF & 6m Transceiver Icom IC-756PRO HF & 50MHz Transceiver Reviewed by Peter Hart, G3SJX* S OMEWHAT OVER three years ago, Icom launched the IC-756, a high end HF + 6m base station which I reviewed in the May 1997 edition of RadCom. Very recently Icom have released the IC-756PRO, which from its number may convey the impression of just an upgraded version. The outward appearance is very similar, indeed the same solid multi-partitioned diecast chassis is used, with the same front and rear panels. Some of the buttons have been reassigned to provide additional features, but the mouldings are identical. However, internally the electrical architec- ture is very different, relying heavily on DSP for IF as well as AF processing functions and with a number of important new features added. PRINCIPAL FEATURES THE IC-756PRO is 12V operated, with a peak current drain of up to 23A. It is a table-top size measuring 340 x 111 x 285mm and weighs about 9.6kg. USB, LSB, CW, RTTY, AM and FM modes are selectable, with normal or reverse sideband on CW and RTTY. RTTY is FSK with selectable mark frequencies, shifts and polarity, and requires a logic level input on transmit. For use with AFSK terminal units, AMTOR, PSK31, SSTV etc, voice modes are used. On all the voice modes, a long push of the mode key selects data and this mutes the microphone. The receiver tunes continuously from 30kHz to 60MHz, with the transmitter enabled only within the exact amateur band allocation (make sure that you have the appropriate version for the country from which you wish to operate). Individual but- tons select the bands, with a triple band-stacking register employed that is useful if you operate frequently 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 10Hz steps at 5kHz per knob revolution, with auto speed-up on fast sustained tuning. Faster tuning rates for rapid frequency changes and slower rates for precision tuning are also provided. 101 memory channels are included with the usual facilities, and frequency and memory channel can be entered directly. A memo-pad feature allows frequencies to be quickly stored and recalled by a simple key press, on the basis of last-in, first-out. RIT and transmitter incremental tuning operate over a range of 9.99kHz and various scan modes are implemented. Comprehensive switching man- ages access to the VFOs and split frequency operation. For repeater use, separate splits are programmable on HF and 50MHz and there is also a CTCSS tone encoder and decoder for repeater access or tone squelch operation. The receiver front-end configuration may be optimised to suit different requirements, with two selectable preamplifiers, three levels of signal attenuation and a novel combined RF gain/all mode squelch control. Preamp 1 has a gain of about 10dB for use on all frequencies above 1.6MHz and preamp 2 is intended for use on 21MHz and above with a gain of about 16dB. The CW pitch is adjustable over the range 300-900Hz to suit personal preferences. A front panel pushbutton selects between two antenna sockets and also enables external receiver antenna. The last used antenna / external receive antenna combinations are stored with the band stores. It can be useful to be able to select external receive antennas auto- matically for the LF bands for loop antennas or for those fortunate to have access to Beverages. An auto ATU is built-in, matching up to 3:1 VSWR and covers all bands including 50MHz. Tuning settings every 100kHz are stored to enable rapid and 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 ALC level. An IF DSP speech processor is provided with three bandwidth settings, treble and bass adjust, VOX and a transmit signal monitor. Full and semi break-in is available on CW, with a front panel 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 Equipment Review * The Willows, Paice Lane, Medstead, Alton, hants GU34 9YA. TRANSMITTER MEASUREMENTS CWSSB(PEP) INTERMODULATION POWERPOWER PRODUCTS FREQUENCYOUTPUTOUTPUTHARMONICS3rd order5th order 1.8MHz106W106W-64dB-30dB-44dB 3.5MHz108W108W-64dB-36dB-45dB 7MHz108W108W-64dB-28dB-42dB 10MHz108W108W-64dB-32dB-42dB 14MHz108W108W-64dB-22dB-36dB 18MHz108W108W-60dB-30dB-40dB 21MHz108W108W-70dB-35dB-36dB 24MHz108W107W-70dB-26dB-36dB 28MHz107W107W-70dB-22dB-38dB 50MHz104W103W-70dB-20dB-36dB Carrier suppression: 25dB (see text)Sideband suppression: 70dB 1kHz Transmitter AF distortion: <1%Microphone input sensitivity: 3mV for full output SSB 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. ivRadCom March 2000 Icom IC-756PRO HF & 6m Transceiver transfer rates from 300 to 19200 baud, which also allows transceiving with other Icom ra- dios. Two key jacks are fitted, the jack on the front panel for a keying paddle to control the internal keyer, and the jack on the rear panel for a straight key or external keyer. A transverter jack is also provided which outputs about -20dBm on transmit and functions as the re- ceiver input on receive. DSP THE MAJOR difference between this radio and the earlier IC-756 lies with the DSP circuitry. The IC-756 used a quadruple conver- sion superhet with the main selec- tivity achieved through crystal/ce- ramic filters at both the 9MHz and 455kHz IFs prior to feeding the DSP at 15.6kHz for demodulation and audio processing. The IC- 756PRO is a triple conversion superhet with a first IF of 64.455MHz, a second IF of 455kHz with fairly broad (15kHz) selectivity and a third conver- sion to 36kHz to feed a 32-bit floating point DSP which provides all the IF filtering, demodulation and audio processing functions. The channel fil- tering is impressive, providing 41 different passband widths on CW and SSB from 50Hz to 3600Hz with shape factors superior to any crystal filters. There are 32 passband widths selectable on RTTY (50-2700Hz) and 3 each on AM (3, 6, 9kHz) and FM (7, 10, 15kHz). Three separate selectable bandwidths are available for each mode by a simple push of a front panel key, selectable from the available menu via the set-up routine. The DSP also provides the twin passband tuning function which enables the filter sides to be both independently moved and narrowed. The AGC is also implemented by the DSP, with three separate time constants selectable from the front panel. These three values may be set from a menu of 13 different values (0.1 to 6s) and are set separately for all modes except FM. Two separate notch functions are provided in this transceiver, both us- ing the DSP. A particularly effective manually-tuned IF notch with a depth of 70dB is included within the AGC loop, and hence does not result in desensitising with strong carriers. An auto-notch is implemented at AF and will automatically attenuate several beat notes, even if they are moving. An adjustable DSP noise reduc- tion system is included and there is also a conventional (not DSP) IF noise blanker for pulse type interfer- ence such as car ignition. The modu- lated transmit signal is also generated by the DSP, with adjustable bass and treble microphone equalisation and adjustable compressor. OTHER SPECIAL FEATURES A DUAL-WATCH facility is incorporated via a second receiver. The two receivers use separate first mixers and synthesisers and a common signal path for all RF, IF and AF circuitry. Hence the second receiver will use the same IF bandwidth and 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 S9 FREQPREAMP 1 INPREAMP OUTPREAMP 1 INPREAMP OUT 136kHz -2.5V (-99dBm) - 280V 1.8MHz0.18V (-122dBm)0.45V (-114dBm) 18V 56V 3.5MHz0.18V (-122dBm)0.45V (-114dBm) 18V 56V 7MHz0.16V (-123dBm)0.35V (-116dBm) 14V 45V 10MHz0.16V (-123dBm)0.35V (-116dBm) 14V 45V 14MHz0.16V (-123dBm)0.35V (-116dBm) 13V 45V 18MHz0.14V (-124dBm)0.35V (-116dBm) 13V 45V 21MHz0.16V (-123dBm)0.40V (-115dBm) 14V 45V 24MHz0.16V (-123dBm)0.40V (-115dBm) 16V 50V 28MHz0.16V (-123dBm)0.45V (-114dBm) 16V 56V 50MHz0.22V (-120dBm)0.45V (-114dBm) 20V 45V AM sensitivity (28MHz): 1V for 10dBs+n:n at 30% mod depthAGC threshold: 1.3V FM sensitivity (28MHz): 0.3V for 12dB SINAD 3kHz pk deviationAGC attack time: see text 100dB above AGC threshold for +1dB audio outputAGC decay time: see text Max audio before clipping: 1.5W into 8 ohm, 2W at 10% distortion Inband intermodulation products: better than -50dB S-READING INPUT LEVEL SSBFILTER IF BANDWIDTH (7MHz)PREAMP 1 INPREAMP OUT-6dB-60dB S1 1.6V5V10kHz10.9kHz16.2kHz S3 2.5V8V6kHz6.6kHz10.8kHz S5 4.5V14V2.4kHz2520Hz3560Hz S7 8V25V500Hz515Hz1140Hz S9 14V45V250Hz263Hz859Hz S9+20 63V200V100Hz107Hz216Hz S9+40 800V2.5mV50Hz63Hz141Hz S9+60 6.3mV20mV INTERMODULATION (50kHz Tone Spacing) PREAMP 1 IN PREAMP OUT 3rd order2-tone3rd order2-tone Frequencyinterceptdynamic rangeinterceptdynamic range 136kHz-+16dBm84dB 1.8MHz+3dBm90dB+13dBm92dB 3.5MHz+3dBm90dB+13dBm92dB 7MHz+0dBm89dB+11dBm92dB 14MHz+2dBm90dB+13dBm93dB 21MHz+0dBm89dB+13dBm92dB 28MHz-2dBm88dB+14dBm92dB 50MHz-4dBm84dB+13dBm92dB CLOSE-IN INTERMODULATION ON 7MHz BAND PREAMP 1 IN PREAMP OUT 3rd order2-tone3rd order2-tone Spacinginterceptdynamic rangeinterceptdynamic range 3kHz-27dBm71dB-18dBm72dB 5kHz-26dBm72dB-17dBm73dB 7kHz-26dBm72dB-16dBm74dB 10kHz-22dBm74dB-12dBm76dB 15kHz-14dBm80dB-4dBm82dB 20kHz-8dBm84dB+3dBm86dB 30kHz-2dBm88dB+9dBm90dB 40kHz+4dBm92dB+12dBm92dB 50kHz0dBm89dB+11dBm92dB RECIPROCAL FREQUENCYMIXING FORBLOCKINGBLOCKING OFFSET3dB NOISEPREAMP1 INPREAMP OUT 3kHz83dB-32dBm-22dBm 5kHz86dB-32dBm-22dBm 10kHz92dB-29dBm-19dBm 15kHz96dB-21dBm-10dBm 20kHz98dB-13dBm-3dBm 30kHz101dB-7dBm+2dBm 50kHz106dB-7dBm+2dBm 100kHz111dB-7dBm+2dBm 200kHz115dB-7dBm+2dBm band, or for the higher frequencies, on adjacent bands. A balance control sets the gain of the two signal paths via PIN diode attenuators following the mixers. A single tuning control is used, assign- able to either receiver and both receive VFO frequencies are continuously displayed together with 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 for displaying the received data directly on the display panel. This is a new feature, a first in any trans- ceiver. The DSP provides a sharp two-tone dual peak filter demodulator limited to 2125Hz mark, 170Hz shift 45-baud RTTY. A tuning indicator is also provided on the display panel to assist with tuning RTTY signals. A built-in full message keyer with some useful features for contest operation is provided for CW. The keyer operates over a wide range of speeds, with adjustable weighting and a variety of keying paddle arrangements. Four memories will each store up to 54 characters, with a provision to send automatically incrementing serial numbers and auto-repeat after a time delay. The message stores are programmed in text from front panel push buttons, not using the keying paddle. The IC-756PRO also includes a digital voice recorder with four memory channels on receive and four on transmit. Each memory store is limited to a maximum message length of 15 seconds. This is useful for repeated CQ calls in long contests and other fixed contest messages. The transmit stores can be tagged with labels up to 20 characters long for easy identification on the display panel. As an optional extra, a voice synthesiser module can be installed. MEASUREMENTS Measurements are given in the table when pow- ered from a 13.8V supply with additional com- ments as follows. Note that I have now included the 136kHz band in my standard set of measure- ments. RECEIVER With the higher gain preamp in circuit, preamp 2, the sensitivity on 21MHz and above was about 3dB better than with preamp 1. Note that no preamp is selectable on 136kHz. The S meter reading is independent of mode. The rejection of images and all IF frequencies was exception- ally good, in excess of 100dB. The initial AGC attack time was fast, but took around 200ms to settle the last dB. The filter bandwidths shown in the table are just a representative selection from the 40+ filters available. The shape factors and steep skirts are really excellent, even with bandwidths down to 50Hz. This is where DSP scores, as these figures are unattainable with conventional IF filters. The reciprocal mixing performance is very good - better than the original IC-756 - and the overall dynamic range figures are reasonable. Even the close-in dynamic range is quite creditable (see Fig 1), considering that all the channel selectivity is in the final IF, a potential problem with this approach. The in-band intermodulation perform- ance is exceptionally good. TRANSMITTER The power output figures in the table were measured with the ATU out of circuit. The ATU introduced an additional loss of about 10%. The SSB carrier suppression with no modulation was greater than 80dB, but de- graded to only 25dB with modulation. The CW rise and fall times were fairly sharp and there was about 30% character shortening at 40WPM on full break-in. There was no char- acter shortening on semi break-in. viRadCom March 2000 Icom IC-756PRO HF & 6m Transceiver LCD panel showing RTTY decoder display (full screen). LCD DISPLAY PANEL THE FRONT PANEL display uses a 5-in colour TFT LCD panel. Associated with this are seven buttons down the side and five function keys along the bottom. These buttons and keys are assigned functions according to context. The top half of the panel indicates all the frequencies, modes, split and dual-watch status, IF filters etc at all times. A graphical representation of IF bandwidth is also portrayed, responding also to PBT settings. The 24-hour clock time is also displayed and a timer with programmable on/off times may also be enabled. The bottom part of the display has a number of functions. It can show the spectrum on either side of the receive frequency, with a selectable span from 12.5 to 100kHz. The current spectrum trace is shown in green, with a blue background trace for peak hold. Peak hold can be useful for monitoring transient openings on 50MHz. The