[Icom] re pro output power]

[Bill Tarkington]

George, thanks for a most exhaustive examination of the issue.  I find your
results to be revealing but most satisfactory as well.  I operate my Pro
using the SM 8, and until now often wondered if the SM 20 would give me
anything significant that the SM 8 does not.  Your test seems to answer that
question as well as the one I posed to you. I like the SM 8 because using
the two cords, I can also operate my IC 275H sitting right next to the Pro.
I want to take the time here to thank not only you, but so many others like
Adam Farson, and others, that I have found to be quite reliable sources of
information.  All of you exhibit a high level of trustworthy expertise, that
I can only wish that I had both the smarts and equipment to emulate.  As a
group, you make it highly worthwhile to belong to this list. So, thanks
again so much for your individual and collective contributions. Bill/KC5ICR.
----- Original Message -----
From: "George, W5YR" <w5yr@att.net>
To: <icom@mailman.qth.net>
Sent: Tuesday, March 12, 2002 3:33 PM
Subject: e: [Icom] re pro output power]


| Bill, your note prompted me to revisit the SSB performance of my PRO using
| various mics and adjustments. The results confirm that indeed the SM-8 can
| provide full output from the PRO, and I would expect the same of the
SM-20,
| although I did not have one to test.
|
| First though, we must be careful to note that there are at least four ways
| to measure the SSB power output of the PRO or any other transmitter:
|
| 1. oscilloscope trace of the output r-f envelope
|
| 2. transmitter panel meter (and with the PRO, the digital meters as well)
|
| 3. an external average-power meter
|
| 4. an external peak-power meter
|
| Of these, only the first is capable of responding accurately and
completely
| to the question "How much SSB output power can the transmitter generate?"
| followed closely by an equally important proviso " and maintain a clean
| output waveshape with no clipping or flat-topping."
|
| The remaining methods each have inherent errors associated with
calibration
| accuracy, meter dynamics such as response time and damping factor, and the
| like. Thus, we must be careful in interpreting the readings of any of the
| metered approaches in comparison with the scope approach.
|
| For these tests, I adjusted the PRO to produce 100 watts RTTY
| single-frequency output as measured on the MFJ 989C wattmeter. This meter
| tracks the output power of the Kachina 505 DSP at any reading from 10
watts
| to 100 watts to within the width of the indicator needle. The 505 output
| power is controlled digitally to within 0.1 db. Hence, I am confident in
| relying upon the 989C wattmeter for these largely comparative readings of
| both average and peak envelope power.
|
| The scope gain was set to provide a display of the unmodulated r-f output
| envelope that was 4 cm peak to peak. Under these conditions, any SSB
| modulation waveform that created peaks reaching the 4 cm limits was by
| definition 100% modulating the transmitter, i.e., producing 100 watts peak
| envelope power. By the same token, any signal reaching those limits and
| evidencing clipping distortion disclosed a condition of overdrive
| distortion in the transmitter.
|
| This all too frequent situation with SSB transmitters makes the meters
read
| higher but unfortunately the added power readings are largely the result
of
| distortion products. So, my boundary condition for "100% modulation" was
to
| observe that the voice input waveform caused peaks that reached the 4 cm
| boundaries and yet remained clean and unclipped at those levels.
|
| The Transmit EQ for the PRO was set at 0 dB for the low end and +5 dB for
| the high end. Four microphones were used: the SM-8; the HM-36 hand mic
| provided with the PRO; the Heil HM-i; and the Heil GM-5 Goldline. The
first
| three were plugged directly into the front panel mic jack and the internal
| mic amplifier circuitry was used. The Goldline was operated through a
| Behringer MX602A Mixer/EQ which was set for flat EQ; only the output level
| was varied to control the input signal level to the ACC1 connector on the
| rear panel of the PRO.
|
| The audio signal used for these tests was yours truly speaking the
| time-honored and consistent word "FIIIIVVVVVEEEE" as a test waveform. This
| word has the unique property of providing salient peaks which are easily
| observed and allow for ready detection of clipping distortion. All audio
| was generated at about the same voice level with the mics about 3-4 inches
| from my lips.
|
| The followng table compares the results of operation with these four
| different mics:
|
|  MIC MIC GAIN  PEP (Scope) PEP (PRO Meter*)  PEP (MFJ Meter) AVG (MFJ
| Meter)
| -----    --------  -----------  ---------------  ---------------
| ---------------
| HM-36  11 o'clock  100 watts        75 watts       50-60 watts     30-35
| watts
|
| SM-8     9    "      "    "           "   "           60     "      30-40
| "
|
| HM-i     3    "      "    "           "   "            "     "      20-30
| "
|
| GM-5     N/A         "    "           "   "          50-60   "      25-30
| "
|
| -----------
| * In every instance, the PRO digital meter and the analog panel meter
| rapidly rose to a reading of about 3/4 full scale or 75 watts with each
| input word. The analog meter, in particular appears to be very lightly
| damped compared with the MFJ meter. Because of the rapidity of the PRO
| meter movements both up and back, their maximum reading could only be
| estimated, as inertia and damping prevented a 100% indication.
|
| These results are remarkably consistent, considering the inevitable
| variability of the speech sounds used for excitation. In particular,
| comparing the scope waveforms with the meter readings, one can conclude
| that
|
| 1. the PRO is capable of being driven to 100 watts PEP clean SSB output
| from voice input with any of the four mics that were tested using
| reasonable mic gain settings and speech levels.
|
| 2. when driven to 100 watts PEP output, the PRO output signal remains
clean
| and unclipped; for all tests, the digital ALC reading was driven to the
| maximum and the lack of peak clipping is a fair indication of the
| smoothness and effectiveness of the ALC action.
|
| 3. analog meters fail to read the full 100% PEP with normal speech input
| since not all speech segments contain peaks that consistently reach 100%
| PEP, and few analog meters are so lightly damped that they can follow
| speech elements closely enough to give accurate readings. Professional
"VU"
| meters are the exception, but the PRO analog meter is surprisingly agile
in
| this respect.
|
| 4. due to the peak to average power ratio of some 10-14 db in the typical
| unprocessed male voice, it is not surprising to find that SSB analog power
| meter readings can be quite low with normal speech compared to actual PEP
| output. It is not uncommon for a typical average-power wattmeter to read
in
| the vicinity of only 10-15 watts when a 100-watt transmitter is being
| driven to its full PEP output.
|
| After doing these tests, I have concluded that Julius Jones' remark about
| the PRO being "set up for only 60 watts SSB output" was probably based
upon
| observations of a peak-reading analog wattmeter. The PRO *is* fully
capable
| of being driven to 100 watts PEP output power with any conventional mic,
it
| would appear, but one must bear in mind that analog metered output
| indications may be substantially lower than the 100 watts actually being
| developed on modulation waveform peaks.
|
| One final point: although the PRO is designed and rated for 100 watts PEP
| output within acceptable distortion specifications, any effort to extend
| the power output - such as by changing internal pot settings, etc. - to
| gain a few extra watts will (a) cause the output device temperatures to
| rise rapidly and (b) the distortion products to rise even more rapidly.
| This effect is clearly pronounced when operating PSK31 where a 40-50 watt
| PEP output is about the limit if IMD readings of -30 db or better are
| desired.
|
| I hope that this more detailed quantitative information will make up for
| any confusion my earlier posting may have caused.
|
| 73/72/oo, George W5YR - the Yellow Rose of Texas
| Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
| Amateur Radio W5YR, in the 56th year and it just keeps getting better!
| QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
| Icom IC-756PRO #02121  Kachina 505 DSP  #91900556  Icom IC-765 #02437
|
| All outgoing email virus-checked by Norton Anti-Virus 2002
|
| Bill Tarkington wrote:
| >
| > George, realize your comments are relative to the factory hand mike's
| > insufficiency.  For those of us who are using the front panel mike
| > connector, can we safely assume the SM20 and/or SM8 desk mikes will
drive
| > the transmitter to full output (assumed to be 100 watts)?
|
|
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