In July 2000, I sold my IC-756, and bought a 756Pro from a local dealer. I am delighted with the Pro. I find the Pro a big improvement over its predecessor, the 756. The Pro receiver seems much quieter than that of the 756 - probably due to a cleaner DDS LO implementation.. The DSP IF filters have much steeper skirts than the analogue crystal filters in the older rig, and are much more effective against adjacent-channel QRM than analogue filters. George, W5YR's IF Filter Page dramatically illustrates this point. Also, read George's "Notes on roofing filters" (below).
The manual pre-AGC IF notch filter (70 dB deep) is dynamite. It makes an S9+20 undesired tone disappear off the S-meter. The DSP-IF filtering, including a tunable notch filter, is all inside the AGC loop (unlike the IC-756). The combination of the DSP-NR and noise blanker renders night-time 40m listening much more pleasant and less fatiguing. I observe significant artifacts under strong-signal conditions only when the noise blanker is enabled. These are clearly due to the NB gating on signal peaks, and are eliminated by switching the NB out.
Overall, the Pro pulls the "weak ones" out of the noise noticeably better than the 756 (or any of its other predecessors in my shack) did. The measured sensitivity on 20m with Pre-amp 1 on, and 500 Hz bandwidth, is 0.1 µV for 10 dB S+N/N (using an HP 8640B generator). I am able to copy easily SSB signals which do not move the S-meter. Those signals would have been barely intelligible on the 756. The ability to tailor the filter passband to the received signal (using the Twin PBT or the filter tables) also provides a superb tool for pulling out the "weak ones". The manual notch is also helpful in improving the SNR of the received signal.
With the Pro, you can optimize the IF bandwidth by tailoring it to the occupied bandwidth of the received signal, thus yielding optimum S/N ratio. Also, the DSP IF filters are inside the AGC loop, so strong signals outside the DSP filter bandwidth will not swamp the receiver. The fact that all the DSP IF filters, including the Manual Notch (but excluding the Auto-Notch) are inside the AGC loop sets the Pro (and the late, lamented Kachina 505) apart from all other amateur HF transceivers on the market.
The vertical sensitivity of the 756Pro Spectrum Scope is significantly higher than that of the 756. A signal of less than 1 uV is visible, whilst the 756 requires at least 20 uV to produce a spike. The only alignment procedure for the IC-756Pro spectrum scope is vertical (amplitude) alignment and calibration. The horizontal (frequency) display is in the digital domain, and thus never goes out of cal. The CAL control on the lower right side of the chassis will center the marker correctly. Incidentally, you can observe the spectral content and occupied bandwidth of your transmitted signal by setting "Scope during Tx = ON" in the "Scope Set" menu.
The transmit speech amplifier of the Pro has a little less gain than that of the 756, requiring a slightly higher MIC GAIN setting when using a Heil microphone with a dynamic insert (HC-4 or HC-5). The new Heil HM-i electret will fully drive the Pro with Mic Gain at around 9 o'clock.
The infinitely-variable DSP IF filters have far better shape factors than classical analogue filters. The tunable IF notch filter is nearly 70 dB deep. And once you have got used to the spectrum scope, you will never wish to be without one again. The AGC voltage is also derived from the DSP.
The IC-756Pro and an Icom amplifier - PW-1, IC-4KL or IC-2KL/AT-500 - make an excellent combination; the amplifier tracks the radio. The Pro can also be interfaced to a Yaesu Quadra. My first impressions of the Quadra are documented here.
The IC-756Pro Monitor is excellent; it consists of a dedicated DAC which decodes the digital Tx IF output of the DSP. At this point, the IF parameters are those of the transmitted signal; the next step in the main signal path is the DAC whose output drives the analogue up-conversion and power-amplification chain.
Using the Monitor and a good pair of headphones, you can set up the Pro for the desired transmit audio quality with very little trouble. Try switching between NAR, MID and WIDE TX occupied bandwidth. View George, W5YR's IC-756Pro Monitor page.
The recommended microphone for the 756Pro is the Heil HM-i. The HM-i is plugged directly into the front-panel [MIC] socket. 756Pro Settings: [MIC GAIN] at 9 o'clock, Treble +5 dB, Bass -2dB, compression OFF and Tx occupied bandwidth = MID (COMP OFF MID). No auxiliary equipment is interposed between the microphone and the radio. If compression is used, set COMP ON MID, and adjust [COMP] for 5 to 10 dB compression, no more. This will avoid overdrive.
In May 2002, I purchased an IC-756Pro II at the Dayton Hamvention. Upon returning home, I installed the Pro II in my station and began evaluating it.
One week later, I can report that the verdict is very favorable. I have been using the Pro II, and have observed quite an improvement in the receiver performance compared to the Pro. So far, I have noticed superior strong-signal handling, DSP IF filtering and DSP noise reduction (NR). The adjustable noise-blanker (NB) threshold is also a big advantage over the fixed NB level in the old Pro.
The improvement in strong-signal handling on the IC-756Pro II is dramatic. My nearest ham neighbor is 1 km down the street from me, and has a 3-element quad at a height of 18m. When he transmits SSB on 20m with approximately 1.2 kW PEP (S9 + 60 dB at my QTH), the peak values of received artifacts are as follows:
| Offset kHz | Strength | Scope |
|---|---|---|
| 10 | S5 | +40 dB |
| 15 | S3 | +20 dB |
| 20 | S2 | +10 dB |
| 35 | S1 | +10 dB |
Front-end settings for this test are Preamp OFF, ATT off, RF Gain 12 o'clock. Spectrum Scope settings are Span 12.5 kHz, ATT off. For offset > 20 kHz, the artifacts are barely audible, and do not degrade the intelligibility of weak SSB signals (S1 ~ S2), despite the increase in scope "grass" level due to this powerful signal.
By contrast, my neighbor's transmissions overloaded the IC-756Pro front end so severely as to render the entire 20m band unusable.
The manual notch is at least as good as that of the Pro. The auto-notch is more effective in suppressing multiple tones, and the received audio seems to me to be totally free of DSP artifacts and "munge". There was occasionally a barely-perceptible trace of such artifacts in the Pro.
Using the Twin PBT with the 250 Hz CW filter selected, one can crank the CW IF filter bandwidth down to 50 Hz (as in the Pro). The CW Pitch control will not put the CW signal out of the IF filter passband, even at 50 Hz bandwidth.
On SSB, the recovered audio sounds a little more "mellow" to my ear with the "SOFT" shape factor selected. On CW, the effect is more subtle. If the signal is in the middle of the filter bandpass, one will probably not notice much change. The "SOFT" CW shape factor has noticeably wider skirts than the "SHARP" setting. (Try this test: Tune in a single-tone signal in "SHARP", with 250 Hz BW. Tune it off (up in frequency) until the signal disappears. Switch to "SOFT". The signal will reappear.)
The CW "SOFT" setting is useful in cases of adjacent-channel interference caused by a nearby signal with severe key-clicks. The key-click sidebands are visible on the spectrum scope, but in some cases the "SOFT" shape factor will eliminate them from the recovered audio.
From my perspective, weak-signal handling appears to be superior to that of the earlier Pro. Very subjectively, I would say that the Pro II receiver is approximately "3 dB quieter" than that of the Pro. This is a function of an even quieter DDS LO, more effective DSP noise-reduction and improved DSP IF filters.
The "grass" (baseline noise) level of the Pro II spectrum scope is lower than that of the Pro. I also find the display somewhat sharper and crisper than on the earlier Pro. In addition, the white meter backlight is a bit more legible.
Matt, KK5DR's Pro II review is very enlightening; I concur with Matt's findings.
The transmitter seems to perform at least as well as the Pro. I have had excellent audio reports with the Heil GoldLine/HC-5.
All in all, I am delighted with the Pro II. I feel that the purchase was well worth the outlay - especially at the Dayton price! As I "put more hours" on the radio, I plan to add to these notes.
I plan to keep the earlier Pro as a secondary HF radio. It has served me flawlessly for 2 years.
In the IC-756Pro, there are 330 ohm resistors in series with the tip & ring leads of the front-panel [PHONES] jack (one resistor in each lead). The corresponding values for the IC-756 are 100 ohms. As a consequence, the headphone level will be approximately 10 dB lower on the Pro than on the 756 (with 8 ohm phones).
You can increase headphone volume by using higher-impedance headphones, by connecting a 2:1 or 4:1 (turns ratio) audio matching transformer between the phone plug and the headphones or (least desirable of all) by disassembling the radio and changing out the resistors (R1 & R2 on the PHONE board).
There appears to be a misprint on pages 12 and 16 of the IC-756Pro user manual. The manual shows the left-most RCA socket on the rear panel (1) as RX ANT, and the socket to its right (2) as XVERTER. On the radio, the reverse is true; (1) is marked X-VERTER, and (2) as RX ANT.
The netting procedure on the IC-756Pro and Pro II is very straightforward. Here are three methods:
1: In BK-IN FULL or SEMI, depress the Morse key, and note the sidetone pitch. Then unkey, and tune the received signal to the same pitch.
2 (Most accurate): Select BK-IN OFF, depress the Morse key, and tune the main VFO knob to zero-beat the sidetone to the received signal. Then select BK-IN FULL or SEMI as desired.
The receive and transmit frequencies will now be exactly equal.
3 (Visual method): In SCOPE/SET, activate "Scope on during Tx". Select SPAN = 12.5 kHz. Tune the received signal so that the peak of the spike is on the scope centre line.
The receive frequency will now be within a few Hz of the transmit frequency.
Note: On the IC-756Pro/Pro II, the transmit CW offset tracks the CW PITCH control. The offset equals the sidetone frequency.
A small group of IC-756Pro owners, including myself, have confirmed that under certain special conditions a low-frequency rumble appears on the external speaker output. This rumble is at a very low level, and is completely masked at normal listening levels.
The apparent cause of this rumble is a low-frequency ripple which the NB superimposes on its own +8V supply rail. This +8V rail also powers low-level audio stages following the DAC. There is a suspicion that the ripple is modulating these stages.
A "fix" for the rumble has just been published. Read "The Icom 756PRO - A Cure for the Rumble", by Tony Brock-Fisher K1KP and Jim Jarvis N2EA. This article appears in "Technical Correspondence", QST, June 2002, page 68.
It is entirely possible that other artifacts observed by some IC-756Pro users are due to inadequate opposite-sideband suppression at the transmitting station. With some operators "tweaking" the carrier set point in the quest for "hi-fi" audio, there's a lot of that going around these days - not to mention degraded carrier suppression. Fortunately for the rest of us on the bands, the IC-756Pro has no carrier set point adjustment, as the DSP performs all modulation tasks. (By the way, the 756Pro also generates mathematically near-perfect AM, FM and CW signals, as well as "textbook" SSB. If you have access to a spectrum analyzer, you can check that out.) Read "Notes on Carrier Set Point and the IC-756Pro" (below).
The DSP modulation process in the IC-756Pro or Pro II generates a mathematically near-perfect double-sideband AM signal. The Spectrum Scope greatly simplifies AM setup.
To set the Pro or Pro II correctly for AM, key the transmitter and set [RF POWER] for 25W unmodulated carrier output. Then whistle into the microphone, and adjust [MIC GAIN] so that the peak sideband amplitude is exactly 6 dB below the carrier amplitude, as displayed on the scope. The transmitter is now correctly set up for AM operation.
The transmitted occupied bandwidth in AM mode is approx. 5.6 kHz. The following receive IF filter bandwidths are available: 9, 6 and 3 kHz. The inner Twin PBT knob functions as an IF Shift control.
It is very refreshing to see the quotation from the ARRL reviews to the effect that at HF, the external (antenna, man-made and sky noise) is many dB higher than the receiver's system (internal) noise.
Recently, I measured the sensitivity of my new IC-756Pro (S/N > 2600) at 28.500 kHz, with 500 Hz BW and Pre-amp 2 engaged, as 0.1 µV for 10 dB S+N/N. The comparable figure for my previous rig, an IC-756, was 0.13 µV. The signal source was a recently calibrated HP 8640B.
An examination of the RF Unit schematic reveals that the Pre-amp 1 and 2 circuits in the two radios are very similar. In both radios, Pre-amp 1 uses a pair of 2SK2171 JFET's in parallel; the two circuits are almost identical, except for RF output transformer type and a few minor differences in component values. Again, in both front-ends, Pre-amp 2 is an NEC uPC1658G MMIC. Judging from differences in connections and external component values, this device is being run in the Pro at a lower Vcc, with lower absolute power output, but slightly higher gain, than for the 756 case. This suggests a lower noise figure and better IP3/IP5 characteristics for the 756Pro case. (Refer to Pages 11-6 and 10-8 of the 756 and Pro service manuals, respectively). All of this suggests that the system noise figure will be comparable for the two transceivers.
According to the ARRL tests, the transmitted composite noise at fo + 2 kHz (fo = 14100 kHz) is -115 dBc for the IC-756, compared to -125 dBc for the 756Pro. This suggests a quieter DDS implementation in the Pro. Assuming a comparable system noise figure for both radios, it should be possible to predict better MDS (and sensitivity) for the Pro. In this light, the 6 dB difference in measured MDS in favor of the 756 is difficult to comprehend, unless the insertion loss of the RF band-pass filters is 6 dB or so higher in the Pro. It is interesting that measurements performed by some other reflector members, and by myself, tend to favor the Pro.
It is well known that the noise figure (NF) of the first stage of a receiver is a close approximation of the system noise figure, assuming substantial gain in the first stage and negligible insertion loss ahead of that stage. So we can say that the pre-amp (actually the RF amplifier - I do wish the amateur HF industry would use that term!) sets the system noise figure. That said, the external noise will always be at least 10 dB above the system noise figure at HF. This can be seen rather graphically on the 756 or Pro, by connecting an antenna and observing the sharp rise in "grass" level on the spectrum scope.
The "bottom line" is that small differences in NF, MDS or 10 dB S+N/N sensitivity amongst HF receivers will, in most cases, be swamped by the external noise - certainly on HF, and often also on 6 meters. We can all take comfort in that as we anxiously hook our new rigs up, first to a signal generator, then to our antenna systems. The sudden roar of noise, and the band of "grass" on the scope, when the antenna is plugged in, should make us feel very good about our receivers!
To protect the keying circuit of the 756Pro, use an auxiliary keying relay, such as the Yaesu FRB-757A, or an small open-frame relay with a 12V coil drawing 100 mA or less. Connect a diode (e.g. 1N4001) across the coil with cathode to +12V. The relay contacts must be rated to carry the keying voltage and current of the amplifier. Here is a good example.
The ALC line should always be connected; when using a solid-state amplifier, ALC is mandatory, as it is the amplifier's first line of defense against abnormal operation. Set the ALC to limit the drive power so as to drive the amplifier to no more than its rated output. This will hold IP3 to an acceptable level - a kindness to one's neighbors on the band. A side benefit is that of keeping the Radio Inspector happy.
I have had my Pro a month now, and am really enjoying it. It does a far better job of pulling out weak SSB signals than the 756 did. I use the MID occupied-bandwidth setting (on the COMP soft-key; hold the key in to change NAR-MID-WIDE) with 2 dB treble boost, and 0 dB bass boost. I am using the Heil Gold-Line mike with the HC-5 and wide-range elements.
You can judge the audio quality via the Monitor function. The 756Pro monitor is excellent; it decodes the digital IF bit-stream generated by the DSP in transmit mode, and down-converts the resulting 36 kHz analogue IF to base-band. The monitor is thus a true representation of the transmitted SSB signal.
There is really no need to use any external audio processing hardware with the Pro. The three occupied-bandwidth selections, together with the boost/cut menu, offer 121 different transmit audio settings.
I sometimes reconfigure the 3 kHz SSB filter to 2 kHz for enhanced adjacent-channel rejection, in cases where the 1.8 kHz filter is a little too narrow. All IF filtering in the Pro is done by the DSP. I run the NR at about 12 o'clock, and use Pre-amp 1 most of the time (Pre-amp 2 for very weak signals). The combination of NR and Noise Blanker is very effective against impulse noise; however, strong signals cause the noise blanker to produce artifacts.
The Manual Notch is about 60 dB deep, and so narrow that it does not degrade the received audio. Manual Notch is also inside the AGC loop (unlike Auto Notch). This allows you to remove a heterodyne without swamping the receiver.
I was aware that there were some differences in BDR, IP3-DR and other "numbers" in favor of the earlier IC-756, but I have a feeling that Icom may have gone through a rev. level change since the release of the early unit which the ARRL Lab tested. My radio is in the S/N 2600 serial number range, vs. 1300 range for the unit tested by the ARRL. In any event, I measured the sensitivity with 500 Hz BW and Pre-amp 2 engaged, as 0.1 µV for 10 dB S+N/N. The comparable figure for my IC-756 was 0.13 µV.
Operationally, by using the Twin PBT or by adjusting the filter bandwidth to match the occupied bandwidth of the signal, I am able to copy easily signals which were unintelligible on the 756. Also, I observe that the vertical sensitivity of the spectrum scope on the Pro is at least 20 dB better than that of the 756. Although 40m is quieter at night out here in the Pacific Northwest than on the East Coast, I have not observed any of the typical overload on that band with the Pro. With the 756, I occasionally heard artifacts from adjacent-channel broadcast stations on 40m.
On the Pro, one can adjust the filter to eliminate "splatter" artifacts. I also observe that the NB-NR combination is more effective on the Pro than on the 756 in eliminating repetitive impulse noise.
To my way of thinking, the architecture of the 756Pro represents a true paradigm shift; the DSP chipset does all filtering, modulation, demodulation, noise reduction, and audio processing. Even the transmit monitor is a digital loop-back from the DSP modulation process, so you are listening to the transmit IF, rather than a sample from the speech amplifier as in the earlier 756. All filtering, including the manual notch (but not the auto-notch) is pre-AGC; this eliminates swamping due to strong signals outside the IF pass-band. Also, there are no filters to buy!
Ah...but there are true IF-DSP's. Both the Icom IC-756Pro and the Kachina down-convert their penultimate IF (455 kHz) to a low final IF (36 kHz in the 756Pro and 40 kHz for the Kachina). The low IF is then fed to an ADC (analogue/digital converter), followed by the DSP chipset. The DSP performs all filtering, signal enhancement and demodulation tasks, including AGC derivation. A DAC (digital/analogue converter) following the DSP then converts the DSP output to AF base-band.
Perhaps a definition is in order: IF-DSP means that the DSP operates at an intermediate frequency (prior to demodulation). The exact frequency involved is not the issue here. AF-DSP means that the DSP operates at audio frequency (post-demodulation). A typical example of this is the UT-106 DSP module for the Icom IC-706 MkII/G. The only reason why IF-DSP has not yet reached the 455 kHz (or higher) IF stage in an amateur HF transceiver is the cost of DSP chipsets at higher speed levels. I am sure that some military HF equipment already has 455 kHz (if not faster) IF-DSP. As with all aspects of the semiconductor industry, it is only a matter of time before the price of such devices comes down to the point where they will show up in our HF rigs.
Another point to be made is that to prevent strong out-of-band signals from swamping a receiver, DSP IF filtering must be within the AGC loop. This is only feasible if the end-to-end group delay across the DSP (ADC input to DAC output) is sufficiently short to prevent instability in the AGC loop. We are already at this point with the Kachina and the 756Pro; DSP-derived AGC will only get better as the chipsets get faster.
In the current, popular competing HF transceiver, the AGC is derived by envelope detection of the analogue IF signal before the ADC, and the DSP is entirely post-AGC. This is rather disappointing in a newly-designed radio.
Blocking dynamic range (also called cross-modulation dynamic range) is defined as follows:
Blocking dynamic range (BDR) is the difference, in dB, between the noise floor and an off-channel signal that causes 1 dB of gain compression in the receiver. It indicates the signal level, above the noise floor, that begins to cause desensitization. BDR is calculated by subtracting the noise floor from the level of undesired signal that produces a 1-dB decrease in a weak desired signal. It is expressed in dB. The greater the dynamic range, (expressed in dB), the better the receiver performance. It is usual for the dynamic range to vary with frequency spacing.
Key Test Conditions: If possible, AGC is normally turned off; the receiver is operated in its linear region. Desired signal set to 10 dB below the 1-dB compression point, or 20 dB above the noise floor in receivers whose AGC cannot be disabled. The receiver bandwidth is set as close as possible to 500 Hz.
To summarize, the blocking (or cross-modulation) dynamic range is the level (relative to the noise floor) of an undesired signal offset 20 (or 50) kHz from the desired signal, at which the desired signal is compressed by 1 dB. Example: The noise floor is -127 dBm (0.1 µV in 50 ohms). Two signal generators (desired & undesired signal) are connected to the receiver input via a combiner. "Desired" is cranked up to the point where 1 dB compression is seen, then backed off 10 dB. "Undesired" is offset 20 kHz, then turned up until 1 dB compression is seen. The "Undesired" level is +10 dBm. Blocking dynamic range is +10 - (-127) = 137 dB.
Now we consider the receiver issues which cause blocking: The first stage in the receiver's RF/IF chain to go into overload is the one responsible for the blocking. The blocking can occur in the RF amplifier (preamp), so the test is conducted with preamp off and on. Now, any stage from the first mixer, through IF amplifiers and downstream mixers, to the demodulator, can go into overload. The overall gain distribution is a significant factor in determining just which stage will hit its non-linear region and overload first.
The IC-765 had particularly good BDR (> 150 dB with preamp off). I believe that this was due to careful balancing of stage gains between first mixer, second mixer and IF chain. The 765 was a conventional analogue radio.
A DSP radio such as the IC-756Pro or Kachina 505 is a totally different animal. The limiting factor for dynamic range is the ADC (analogue/digital converter) which digitizes the IF for delivery to the DSP. The dynamic range of the ADC is determined by the number of bits per sample. One bit (a power of 2) = 2 x amplitude (i.e. 2 x voltage*), or 6 dB. A 16-bit DSP has a theoretical dynamic range of 96 dB; a 24-bit ADC, 144 dB. The 756Pro uses a 24-bit ADC and DAC (digital/analogue converter, and a 32-bit DSP. The absolute maximum amplitude which the ADC can encode corresponds to an encoded binary value of "all ones" , i.e. a hex value of FFFF (16-bit) or FF FF FF (24-bit). Note: Representation of negative values can result in maximum possible positive and negative values, rather than the positive maxima shown here.
In practice, dithering (uncertainty of resolution of Bit 0, the least significant bit) limits the achievable dynamic range to 138 dB - still pretty good. Thus, the receiver designer needs to distribute the gain of the analogue stages preceding the ADC so that no stage overloads before the ADC reaches it own overload point (the upper limit of its resolution). The DSP will then be fully exploited.
The Pro derives its AGC voltage via a secondary DAC, which decodes the processed IF bitstream prior to demodulation (in the DSP). The AGC can do a very good job of protecting the ADC from overload; there are a few software tricks to minimize swamping by strong out-of-band signals. ADC overload is catastrophic, as the DSP now no longer has a useful signal to process.
I am not sure to what extent the conventional method for measuring BDR is valid for a DSP receiver, considering that the "1-dB compression point" may have no meaning at the ADC overload point, when the digital output abruptly ceases to represent the analogue input. Thus, the apparently "poor number" for the Pro in the ARRL test suite may not be a true index of receiver performance. This seems to be borne out by operational history, in which the Pro handles strong out-of-band signals very well - as long as the ADC is not driven to overload. (Note that as the maximum encoded amplitude value that the DSP data bus can present to the DAC is "all ones", the DAC can never be overdriven. However, the analog circuits downstream from the DAC output must have sufficient headroom to accommodate this maximum output amplitude without distortion or clipping.)
Another factor is that the radio tested by the League was an early production unit. Usually, what happens is that the manufacturer submits a first-run unit for test; the test suite often identifies performance issues which are corrected in the next series. This occurred with the earlier IC-756 and several other HF radios of various makes.
The final arbiter of receiver (and transmitter) performance is operational experience. The IC-756Pro has been used very successfully on a number of DXpeditions, and has performed flawlessly.
*The input signal being digitized could be in current rather than voltage form, although this is unlikely.
In July 2000, I was at the West Coast DX Convention in Burnaby, BC, and chatted with a couple of the operators who had participated in the Radio-sport Team Championship in Slovenia earlier this month. They had run two IC-756Pro's at the DXpedition, and reported that the radios operated flawlessly. They even acquitted themselves extremely well on 40m at night, with the European broadcasters almost next door. This sentiment was echoed by one of the Kingman Reef operators, Steve Wright, VE7CT, who gave a presentation on the K5K operation at our local club last February. K5K used a number of 756Pro's, and the operators were very happy with them. They stood up under very arduous operating conditions, in a very hostile RF environment, and no contacts were lost due to unsatisfactory strong-signal handling.
In my station, I have not observed any overload, even during contests or Field Day. During K5K, I saw some nasty 20m signals, 10 to 15 kHz wide, on the spectrum scope of the Pro. I suspected cross-mod in the radio; however, an on-air check with an HP 853A/8558B spectrum analyzer confirmed that the signals were indeed that wide.
DSP is the way of the future. It offers a cost-effective way to design, implement and optimize radio designs which are as close as possible to the limit of what can be achieved theoretically. Those who continue to disparage DSP radio architecture and digital filters will eventually go the way of the proponents of the horse-'n'-buggy, the manual telephone exchange, the vacuum tube, the vinyl LP, etc. A radio such as the IC-756Pro is the way of the future; the concept can only improve as 32-bit ADC/DAC chips, and faster DSP's, become available at prices we hams can afford. A 32-bit ADC has a theoretical dynamic range of 192 dB, and will be limited by the noise floor of the receiver front end.
When you are "shopping" for a high-end HF transceiver, I would recommend that you consider the Pro. In comparing the Pro to its popular competitors, consider this; in the Pro, the DSP does everything - IF filtering, IF notch (> 60 dB deep!), all modulation & demodulation, noise reduction, transmit audio equalization and AGC. The IF-processing part of the DSP (filtering and notch) is inside the AGC loop. By contrast, the competing product is just another analogue radio with limited DSP tacked onto the back end. The DSP is wholly outside the AGC loop, so the DSP filters cannot remove undesired signals before they develop AGC voltage and swamp the receiver.
And then there is the spectrum scope - difficult to do without once you have got used to it. The visual display of signals in the portion of the band that one is tuned to, or working, is a very powerful operating tool.
The inherent 6-meter coverage of the Pro is an added bonus. The receiver noise figure is excellent, and no $1000 transverters are required!
In the choice between an IC-756Pro and an IC-756, my recommendation is to get a 756Pro. Failing that, the 756 is a very acceptable alternative (or "backup" rig).
The two most significant deficiencies in the 756 which I had prior to the Pro were the lack of an IF notch filter inside the AGC loop, and the relatively poor vertical sensitivity of the scope. I had fitted the FL-222/223 narrow SSB filter combination; these filters greatly improved adjacent-channel selectivity, and tightened up the Twin PBT. The narrow filter pair is essential if you operate mostly SSB; the filters will cost $250 to $300 (new), on top of your $1200~1300 radio. Use of the Twin PBT to narrow the IF bandwidth comes with a price; the overall shape factor is degraded. Cascaded narrow filters avoid this. To ensure correct Twin PBT operation, the optional filters should have comparable bandwidths and shape factors. View the bandpass curves of popular Icom filters.
By the time you accumulate filters for SSB and digital modes, you will be close to the price of a 756Pro. And then there is the hassle of opening up the 756 to swap out filters according to changes in operating practice (e.g. mostly-SSB vs. mostly-digital).
Q. Is there any way to adjust the carrier point oscillator in the 756Pro?
A. The IC-756Pro has no carrier insertion oscillator, and therefore no set point adjustment. The DSP performs all modulation and demodulation tasks. In SSB mode, the DSP models an idealized phasing-type exciter and demodulator, for transmitting and receiving respectively. The DSP algorithms are hard-coded in firmware, and are not user-modifiable.
In SSB transmit, the frequency relationship between the virtual carrier and the generated LSB or USB spectrum is fixed. Three occupied-bandwidth settings (NAR 2.2 kHz, MID 2.4 kHz, WIDE 2.8 kHz) are available. George, W5YR's occupied bandwidth measurements confirm these values.
The WIDE transmit AF response at the front-panel MIC socket is approximately 150 Hz ~ 2.8 kHz @ -6 dB. It is possible to "stretch" the low-end response down to approx. 90 Hz @ -6 dB by applying the transmit audio at the Pin 4 of rear-panel ACC1 socket, with WIDE selected.
In receive, the IF filter bandwidth is infinitely variable, in the range 50 Hz ~ 3.6 kHz. In addition, the lower and upper flanks of the IF bandpass can be shifted by up to +/- 600 Hz relative to virtual carrier via the Twin PBT controls.
The IC-756Pro and Pro II, and the IC-746Pro (IC-7400) are a "wholly different animal" compared to conventional radios. The DSP chipset is actually a fully-functional fixed-frequency transceiver operating at 36 kHz. RF/IF amplifiers, up/down converters and DDS frequency generators interface this transceiver to the operating frequency domain.
Measurements performed by George, W5YR confirm that for the WIDE occupied-bandwidth setting, the low-end audio response is lower at ACC1 Pin 4 (90 Hz at -6 dB) than at the MIC input (150 Hz at -6 dB). These measurements also show that for the MID and NAR settings, the difference in low-end response between ACC1 Pin 4 and the MIC input is negligible. The explanation for this is as follows:
From a look at the transmit speech-amplifier circuit (IC451 on the MAIN UNIT board), there is an RC feedback network in the first stage (AMP). This network is a high-pass filter (HPF) whose -6 dB cutoff frequency is approx. 150 Hz. The HPF accounts for the steeper low-frequency roll-off at the MIC input compared to the ACC1 Pin 4 input (significant only in the WIDE setting). The second stage (VCA, voltage-controlled amplifier) has a flat response. (Refer to IC-756Pro service manual, Page 10-4.)
The design intent in this circuit is to cut off low-frequency components of the microphone output (below 150 ~ 200 Hz). These components contribute virtually nothing to articulation at the receiver, but rob transmitter power that can be more usefully employed to transmit the midband and upper frequency sub-bands which determine the articulation index at the receiver. Note that if bass-boost is applied to the microphone audio fed to the front-panel MIC socket, IC451 will severely attenuate all components below 150 ~ 200 Hz.
The AMOD (ACC1 Pin 4) input bypasses IC451 entirely. The outputs of IC451 and the AMOD buffer are summed, and fed to the ADC via the DTAF line. The design intent here is probably to pass AFSK signals injected via AMOD without distorting the frequency transitions, which have a low-frequency component. (Note: The transmit audio signal path does not feed a "balanced modulator". There is no such "animal" in the IC-756Pro or Pro II.)
Icom's engineers probably felt that this analogue solution would be more cost-effective and less risky (in terms of potential bugs) than separate, selectable DSP modulation algorithms for voice and data transmission.
Now, in the DSP, the low and high cutoff frequencies of the PSN modulation algorithm are hard-coded in the DSP firmware. This applies for all three occupied-bandwidth settings: WIDE, MID and NAR. As the audio signal path from ACC1 Pin 4 to the ADC is flat, its frequency response is determined solely by the DSP.
We have already seen from the above discussion that the audio signal path from the MIC socket to the speech amplifier (IC451) is 6 dB down at 150 Hz. In the WIDE setting, the DSP "modulator" is 6 dB down at 90 Hz.
Thus, it is possible to extend low-frequency transmitted audio response by selecting the WIDE occupied-bandwidth setting, and injecting microphone audio at line level via ACC1 Pin 4. The above caveat concerning articulation still holds true.
(Contributed by George T. Baker, W5YR. Refer also to George's Notes on Filtering & DSP.)
At the first IF of 64.455 MHz, there is a crystal 15 KHz roofing filter at the input of the IF amplifier. At the second IF of 455 KHz, a 15KHz ceramic filter follows the second mixer. this filter feeds the Noise Blanker Gate which in turn feeds a second 15 KHz ceramic filter preceding the third mixer to the final (DSP) IF of 36 KHz.
Admittedly, these are not “narrow filters” by classical analog-radio standards, but they are adequately narrow to ultimately prevent overload of the DSP A/D converter(s) and that is all that is required. In a linear system, the ultimate band-limiting can take place at any point provided that system operation is linear up to that point. Proper distribution of stage gains and appropriate AGC loop operation virtually ensure linearity in the PRO up to the DSP stage.
It is also of dubious merit to ascribe performance ills to a lack of "classical" narrow IF filters - i.e. typical 2.5 - 3.0 KHz IF crystal filters - in the PRO.
While it is “conventional wisdom” that such narrow filters are all to the good, despite their phase delay, uneven amplitude response and other negative effects, recall that we once had radios built to the “conventional wisdom” that two r-f amplifier stages are better than one, since “sensitivity” was the principal measure of a good receiver.
Furthermore, the properties of conventional crystal and mechanical filters have now met with such intense competition from the DSP approach that one can readily project that in a few years there will be little or no need or requirement for crystal filters since their performance will be - and already is - inferior to the DSP equivalent. Have another look at the IF Filter Page and compare. Here is a test which you can run on your PRO or PRO II to put the DSP IF filters through their paces.
An examination of the IC-756Pro block diagram reveals that the PRO has a 15 kHz crystal roofing filter at the first IF - immediately after the 1st-mixer output combiner.
The scope system in the PRO is fed from the combiner output (before the roofing filter) at an IF of 64.455 KHz via separate IF stages to a mixer, which with a 77.8 MHz LO, down-converts to 13.345 MHz for the scope circuitry. There are two 13.35 MHz ceramic bandpass filters in cascade in the mixer output which feeds another mixer with a 12.89 MHz +/- 100 KHz LO to produce an output which via a 455 kHz ceramic bandpass filter drives the scope IF system. Note that all of the above circuitry for the scope operation is completely independent of the circuitry following the first mixer which drives the actual receiver IF and subsequent circuitry, including the DSP. No compromises in receiver IF design are required to support scope operation.
Times change, and so does the architecture and design of our radios. Conventional radios such as the earlier Kenwoods are vastly different in many respects from the PROs, the Kachina 505DSP, and even the Ten-Tec Pegasus and Jupiter. I don’t think that it is any accident that most of the really high-line commercial and military radios (e.g. Rohde & Schwarz, Rockwell-Collins, Harris etc.) have used extensive digital filtering and signal processing for the past several years.
The Kachina 505 uses 16-bit ADC/DAC, 24-bit processing and a DSP IF of 40
KHz. It has analog AGC with selectable time constants, etc. plus a Digital AGC
which works in tandem with the analog AGC. The Ten-Tec product line uses 16-bit
ADC/DAC. The Icom IC-756Pro, Pro II and IC746 Pro all use 24-bit ADC/DAC and a
32-bit DSP processor.
1. "HF Radio Systems
& Circuits", Sabin & Schoenike, editors. Noble,
1998.
This textbook was written by members of the Engineering
Staff, Collins Divisions, Rockwell Corporation. Chapter 8 is an exhaustive
treatment of DSP radio design concepts.
2. "The ARRL Handbook for the Radio Amateur", 2001 Edition, Chapter 18, Digital Signal Processing.
3. "The Scientist's and Engineer's Guide to Digital Signal Processing", by Steven W. Smith, Ph.D.
4. "A High-Performance Digital Transceiver Design, Part 1", by James Scarlett KD7O, QEX, July/August 2002.
Copyright © 2000 A. Farson VA7OJ/AB4OJ. All rights reserved.
