An examination of the IC-756Pro or IC-756Pro II 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/PRO II is fed from the combiner output (before the 64.455 MHz roofing filter) at an IF of 64.455 KHz via a PIN diode attenuator and 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 (CFJ455K8) 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. (Note: The CFJ455K8 filter defines the scope's resolution bandwidth as 1 kHz at -6 dB.)
As the IF pickoff point for the scope is also before the AGC gain-control point, neither the RF Gain control setting nor AGC action affects the vertical sensitivity of the scope.
With Preamp 2 enabled, 0 dB attenuation (RF and scope) and autotuner disengaged , the Pro II spectrum scope will display a visible spike with 0.12 μV (-125 dBm) at the antenna socket. (This was measured with a Cushman CE-31A service monitor in CW mode, calibrated against a Rohde & Schwarz URV4 RF power meter.) The radio was also in CW mode, with the 500 Hz filter selected.
Thus, it can be seen:
that there are three roofing filters in the main receiver channel and three more in the scope channel.
that none of the main receiver's roofing filters are involved in the scope operation, since the scope operates as a completely independent receiver.
that the presence of the scope has no negative effect whatsoever on the design and architecture of the “main” channel of the receiver.
Q: Can I configure the Spectrum Scope to display the spectrum of my transmitted signal?
Please refer to page 3-9 of the IC-756Pro II Service Manual. Whilst transmitting, a portion of the 64.455 MHz 1st IF signal from the transmit IF amplifier output is passed through the PIN attenuator to the scope IF amplifier chain. Assuming that the stages following the transmit IF amplifier (3rd transmit mixer, RF power-amplifier chain) are driven within their linear operating region, the transmit spectral display will be an acceptably close replica of the actual transmitted signal.
Using the "Scope during Tx" display, I am able to estimate the -6 dB occupied bandwidth of my transmitted signal quite accurately.
Q. Is it desirable to "reinitialize" the NR periodically by toggling it on and off, or by momentarily pulsing the PTT line?
A. This is largely unneeded since the NR logic is only doing its job. NR in the Pro2 is a time-varying adaptive digital filtering operation which alters its response according to the statistics of the noise it encounters within the IF passband. The statistics can and do vary with time and the noise reduction algorithms react accordingly.
When first activated, NR appears to consider everything "noise" and thus lowers gain across the entire IF passband considerably giving the perception of a very quiet receiver . This can be interpreted as effective NR operation, but that is probably a premature conclusion...
As time goes on, however, the NR logic has more statistical history to work with and begins to alter its response accordingly. The actual noise within the passband that meets the rejection criteria of the NR logic continues to be suppressed while other frequency components are restored approximately to their original level.
One result is that the perceived "noise" (audio) level increases and one could conclude that the NR is no longer functioning. This may or may not be the case, depending upon the statistical nature of the noise.
I find it difficult to accept the premise that Icom failed to execute the NR function properly in the Pro2 and earlier models. If one examines the IF/audio spectral response of the Pro2 with the NR operating, the change in noise response of the passband can be seen to vary with time.
Initially the whole passband is diminished in level and then usually (per the type of noise I have locally) the low and high ends of the spectrum quickly "grow" in level while the midrange tends to diminish. The increased signal levels at the extremes of the passband appear to account for the apparent increase in audio level, which some are interpreting as a loss of effectiveness of the NR function.
As to "defeating" this effect with periodic PTT operation, one could probably simulate the same outcome by periodically decreasing the AF gain and then slowly bringing it back up. The PTT operating restores the NR logic to its original "starting" point so it begins anew "thinking" that most of what is in the passband is noise and lowering the overall response level. Continued PTT operation merely restarts the process and keeps the NR logic from reaching and maintaining its steady state response condition.
I apologize if any of this sounds negative or even argumentative - that is not my intent - but I think that efforts to circumvent the programmed operation of the Pro2 NR logic, while subjectively appealing, actually accomplish little toward improving the noise reduction capabilities of the receiver.
An interesting exercise would be to bury various signal waveforms in "noise" and to determine the S/N ratios for each as a function of time with NR activated. The final arbiter is whether NR improves S/N to make a weak signal more readable, not whether the receiver sounds "quieter" with NR activated. (IF I can ever find time, I would like to conduct such a test.)
A final note: the NR function is most effective with the wider IF bandwidths. As bandwidth is reduced, the noise statistics are changed and the NR has less to work with. At very small bandwidths, NR is essentially ineffective since even pure noise appears to be a randomly modulated sine wave whose frequency is related to the bandwidth. Using NR with a 100 Hz filter for CW produces little useful effect, for example.
Notes on internal autotuners, July 2004:
Q: I only use resonant antennas. Should I enable my internal autotuner?
A: The purpose of the autotuner is to keep the PA happy by ensuring that it is always presented with a 50Ω resistive load for optimum power transfer and best linearity. (See Reference 17).
It goes without saying that the low-pass filters must also be correctly terminated in 50Ω resistive to fulfil the above requirement.
Even a resonant antenna will present precisely 50 + j0Ω to the transmitter at only one frequency. The mission of the autotuner is to ensure that the transmitter sees 50 + j0Ω over a reasonable VSWR excursion, i.e. a reasonable frequency band. Another advantage of enabling the autotuner is that it remains in the RF signal path on receive, thus providing additional preselection.
Q: But if I activate the tuner to match a slightly elevated VSWR, say around 1.5:1, won't the tuner's insertion loss exceed any mismatch loss in my feedline?
A: This will be true for some cases. However, it should be noted that the reflectometer at the PA LPF output (which reports forward and reflected power to the control processor) will start folding back the RF drive at VSWR > 1.5:1. This is yet one more reason for ensuring that the PA is terminated in a 50 + j0Ω load.
Note on "RF" vs. "IF" DSP (March 2005):
An interesting comparison of two approaches to DSP has emerged from a recent discussion with Bob, W4ATM. It is tempting to think of an RF-level DSP design such as the RF Space SDR-14 as the be-all and end-all. However, its 14-bit ADC which is fed directly with the entire MF/HF band 0.5 ~ 33 MHz, and samples at 66.7 MHz, has an ENOB (effective number of bits) of approx. 12.3. Each bit (power of 2) equates to 6 dB of dynamic range; thus, 12.3 X 6 = 74 dB dynamic range (from MDS to ADC "all 1's" point). The only analogue RF circuitry at the ADC input is a wideband variable-gain preamp followed by an LPF. (Read comments by Pieter, N4IP).
A digital down-converter (DDC) following the ADC processes the digital ADC output into I/Q format, and translates the effective bandwidth down to 150 kHz (typical) within the 33 MHz band. The resulting DDC processing gain* increases the overall dynamic range by 10 log10 (33/0.15) = 23 dB. The resulting dynamic range is now (74 + 23) = 97 dB.
*Processing gain is not "gain" in the sense of amplification; it is a reduction of noise, and thus an increase in S/N ratio, due to the bandwidth reduction.
The overall dynamic range can be improved further by decreasing the bandwidth of the DDC output band segment. The limiting case is determined by the mathematical precision of the post-processor. The I/Q outputs of the DDC drive a PC sound-card via a USB 1.1 interface. The PC performs all demodulation tasks.
By contrast, the lower-cost 24-bit, 36kHz-sampling ADC used in the Icom Pro series has an ENOB of at least 20 (120 dB dynamic range). This is close to the effective blocking dynamic range of the RF/IF chain ahead of the ADC. Thus, Icom has done a trade-off; the engineers in Osaka realised that they would be better off at this stage of the game by not allowing a dynamic-range bottleneck at the ADC, and by designing the analogue RF/IF chain for a dynamic range as close as possible to that attainable in the ADC.
Fast ADC and DDC IC's are becoming more cost-effective, as are post-processors with high mathematical precision. These trends may allow RF-DSP receiver designs to eclipse the current analogue front end/IF-DSP topology in a few years. However, there is still the concern that the composite power of a number of strong signals in the sampling bandwidth of a wide-band ADC will drive the converter beyond its "all 1's" point. Until high-resolution ADC's with high sampling rates become cost-effective, the hybrid topology (incorporating a roofing filter to reduce the statistical likelihood of ADC overload by strong undesired signals) will remain in favour.
Icom is using ADC's and DAC's designed for use in wireless phones and DVD/CD players. The ADC used in the IC-756Pro II and IC-756Pro III has 120 dB dynamic range, compared to 112 dB for the chip used in the IC-746Pro.
Comment by George, W5YR: Adam, I agree with your later note concerning the merit of the Icom approach vs. the SDR14 re sampling rates, bit levels, etc. It is still hard to beat Armstrong's original concept of taking everything down to a common fixed frequency for gain and selectivity.
The four evolutionary phases of the "DSP" HF amateur transceiver:
The audio DSP add-on, such as the Ten-Tec Omni V and VI series. These radios incorporated an OEM audio DSP board manufactured for Ten-Tec by JPS. Other examples are the IC-706 Mk II, IC-703 and R-75 receiver, with optional DSP module.
The limited post-AGC final-IF DSP implementation. Examples in historical order: IC-775DSP, Yaesu FT-1000MP, IC-756, Yaesu FT-1000MP MkV. In these radios, the DSP typically performs modulation, demodulation, NR, some audio filtering, TX EQ and auto-notch. The DSP, consisting of an ADC, DSP processor and DAC, is in the final IF but post-AGC. In the IC-775,and the Yaesu radios, an analogue back end is fitted, and touted as an "alternative" to the DSP. (Good marketing propaganda; the fact is that leaving the analogue circuitry in was cheaper than designing it out!) The final IF is in the range 10 ~ 15 kHz.
The "true IF-DSP" radio, in which DSP now does all IF filtering (including a tuneable notch), AGC derivation and TX compression, as well as the functions mentioned in (2). Here we have the Kenwood TS-870, the Kachina 505, the IC-756Pro, the Ten-Tec RX-340, Pegasus and Jupiter, the IC-756Pro II and the IC-7400 (IC-746Pro). The IF filtering functions are now within the AGC loop. Also, due to advances in chip speed, the final IF is now typically 36 or 40 kHz (the earliest implementation, the TS-870, used 11 kHz).
The "RF-DSP" radio, in which the DSP is clocked at a frequency above the highest RF operating frequency, and processes the entire HF band. This approach is at the leading edge; it is dependent on the speed and price/performance ratio of fast ADC, DSP and DAC chips. Refer to Note on RF vs. IF DSP above.
The Kachina 505 uses 16-bit ADC/DAC, 24-bit processing and a DSP IF of 40 KHz. It has analogue AGC with selectable time constants, etc. plus a Digital AGC which works in tandem with the analogue 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.
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 above article is now also available in Russian.
Read my IC-756Pro III User Review.
"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, of which this excerpt gives an example.
"The ARRL Handbook for the Radio Amateur", 2001 Edition, Chapter 18, Digital Signal Processing.
"The Scientist's and Engineer's Guide to Digital Signal Processing", by Steven W. Smith, Ph.D.
"A High-Performance Digital Transceiver Design, Part 1", by James Scarlett KD7O, QEX, July/August 2002.