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Author Topic: Lowering output power from SB200  (Read 14488 times)
WB2WIK
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Posts: 21837




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« Reply #75 on: March 15, 2009, 07:50:19 AM »

I have to ask: What owner's manual do you have that says to reduce output power on SSB, you should turn down the mike gain?

None of my manuals say that, and none of the manuals I could find for my older gear (even stuff I no longer have) say that either.

If you can be specific, I'll look it up.

Thanks,

Steve WB2WIK/6
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WB2WIK
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Posts: 21837




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« Reply #76 on: March 15, 2009, 08:09:57 AM »

Anything less than 1% distortion for voice communications is so low it cannot be detected and certainly cannot be "heard" by listening to it.

I designed RF signal generators for a living in the past.  They typically had <0.05% total THD when amplitude modulated with 100% modulation, and they ALL employed full-time ALC.  All lab quality signal generators employ full-time ALC, that's how they achieve "output leveling," so when you change frequencies you don't have to re-set the output level.  Our generators achieved level stability of better than 0.25 dB from 130 kHz to 1.2 GHz.

Here's how we did it: We created a signal in a variety of ways (often, a tunable microwave oscillator/VCO mixed against a variety of fixed-frequency bandswitched signals derived via synthesis from a standard TCXO, followed by switchable bandpass filters); that signal would be amplified to a high level like 300mVrms or so, followed by a PIN diode modulator/variable attenuator, followed by a high gain amplifier to create an even higher level signal like 3-10Vrms.  Following the wideband output amplifier would be a 50 Ohm microwave series resistor, then the selectable output attenuator and output port.

The ALC loop was formed by a (biased) linear detector at the output of the final PA, a diff amp, and ALC amp controlling bias to the PIN diode attenuator between the driver and PA.  The PIN diode attenuator was also the AM modulator when the generator needed to be amplitude modulated.  

The challenge was making all this work over several decades of RF, especially since PINs can fall apart below 2 MHz or so: These were very specially selected PINs with a huge lifetime (made by our sister division HPA in those days), and were effective and linear down to a couple hundred kHz.

The ALC loop had to continuously control the attenuation of the multi-stage PIN modulator to keep RF output level constant as frequency was changed, and the input level signals from the first mixed filter band and driver stage varied all over the place with frequency: 10-20 dB changes in drive were very common, but the output level signal had to remain constant within 0.25 dB by specification, and in reality usually leveled to within 0.1 dB.

A low distortion audio signal from a Wien bridge oscillator was applied to the PIN modulator for AM service, and could modulate the generator to 100.0% modulation, with <0.5% THD.  All this, in an active ALC leveled system.  (THD measured using HP distortion analyzers, which are really just very sharp tunable filters that compare the amplitude at the desired frequency to the aggregated signal level of all frequencies outside of the desired frequency, even those within 100 Hz.)

Amazing.  There was no way to ever turn off the ALC, it had to be there, as it was part of the generator and the AM modulator was in series with the ALC loop.

The generator produced lower distortion than the best ham gear run at "below ALC" levels.  Considerably lower.  But it always used ALC.

Lab quality generators are still built this way.  There aren't any that don't use ALC continuously, and the good ones can be 100% modulated down to 30 Hz and up to 20 kHz with far less than 1% distortion all the while actively controlling the output signal amplitude.

WB2WIK/6

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K7GRR
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Posts: 50




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« Reply #77 on: March 15, 2009, 09:52:26 AM »

::
I have to ask: What owner's manual do you have that says to reduce output power on SSB, you should turn down the mike gain?

None of my manuals say that, and none of the manuals I could find for my older gear (even stuff I no longer have) say that either.

If you can be specific, I'll look it up

>> FT-757.  On page 8, control number 9, it says that to reduce power in SSB mode, use the mike gain.  In other modes, use the Drive control, which is disabled in SSB.

Yaesu no longer has the manual on line, but I found it at http://www.scribd.com/doc/3292400/Yaesu-FT757GX-Manual

I'm not saying that is a "good" method, but it is what the manual says...

-T
K7GRR
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WB2WIK
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Posts: 21837




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« Reply #78 on: March 16, 2009, 08:49:23 AM »

K7GRR: Thanks for that!  I've never actually seen any manual recommend this method, and it's really not a good idea.  Of course they say, "with the processor turned off" as a qualifier.

There's a good reason for speech processors and there's a good reason turning down the gain won't work when you're using one.

Most HF military SSB gear has no provision for turning the speech processor off!  There's a good reason for that, too.

;-)

WB2WIK/6
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KE7VOB
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Posts: 22




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« Reply #79 on: March 16, 2009, 09:32:41 AM »

Timothy,

I do still follow this thread. I do see now that the manual does say to reduce mic gain. However, had I never asked the original question I wouldn't know why it isn't a good idea. I learned a lot from the various replies to this post.

Why would I hesitate to ask another question when I have one?

My ( new to me ) FT-990 is on the UPS truck and the doorbell should be ringing anytime. Hopefully it will arrive in good condition. I am keeping the FT-757 and will not be reducing mic gain to reduce power.
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WB2WIK
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Posts: 21837




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« Reply #80 on: March 16, 2009, 11:06:13 AM »

KE7VOB,

Good choice, and you're going to like that FT-990 (assuming it's in good shape!).  I had one back when it was new and it was an excellent rig in most respects...and it does have an output power level control that works on all modes!  ;-)
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W9PMZ
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Posts: 661


WWW

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« Reply #81 on: March 16, 2009, 02:11:05 PM »

WIK - who did you design sources for?

73,

Carl - W9PMZ
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WB2WIK
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Posts: 21837




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« Reply #82 on: March 16, 2009, 02:54:03 PM »

Carl, for both Hewlett Packard Co. and Boonton Electronics Corp.

73,

Steve WB2WIK/6
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KE7VOB
Member

Posts: 22




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« Reply #83 on: March 16, 2009, 05:33:59 PM »

Off topic but I received the radio in excellent condition. Wow what an improvement.
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WB2WIK
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Posts: 21837




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« Reply #84 on: March 17, 2009, 08:31:23 AM »

VOB, congratulations!  That was (and still is) an excellent rig (FT-990).

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W9PMZ
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« Reply #85 on: March 17, 2009, 11:19:35 AM »

I hope you weren't one of the design guys with the ALC fiasco on the HP ESG.

Your discription earlier about one of your projects kind of sounds like the ESG scheme.

Now, I like HP stuff, I like their new MXA, especially the ability to measure CDMA signals in different integration bandwidths at the same time.  I like their new E5071C ENA, it has 10dB or so more dynamic range that the 8753 series, make for easier tuning of amplifiers with large amounts of gain.

But the ESG is a dog when it comes to ALC Leveling and some other quirks.  It was so bad that we now have taken Agilent out of the picture in favor of the SMATE.  I do like Agilent's replacement, the MXG, but I did not have a say on choosing this equipment.

The darn things keep failing at our manufacturing partner in Thailand.  It came down to HP having to keep seed stock in Thailand for our use as the ESGs failed with their ALC going unleveled.

Their attenuator scheme on the ESG wasn't that great as well, jumping in some regions took out some of my amps during tuning.

And finally, the darn thing compresses badly much above -15dBm with modulation.  This causes an issue with the calibration of our radio equipment.  With the instrument compressing absolute calibration was impossible.  We resorted to installing a Mini Circuts ZHL42 to keep the ESG out of compression.

73,

Carl - W9PMZ
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N3JBH
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Posts: 2358




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« Reply #86 on: March 18, 2009, 05:49:13 AM »

Can it make 10 pages?
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WB2WIK
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Posts: 21837




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« Reply #87 on: March 18, 2009, 08:34:54 AM »

Carl,

My involvement at HP goes way back before the problems you describe, and way back before "Agilent" existed.

It was HP back then, and the project was the 8640A/B signal generator.  Its ALC is excellent, and its functionality is much more similar to an amateur radio transmitter in that it is AM or CW (or FM, of course), with very limited pulse modulation capabilities (only low prr, narrow bandwidth).  Very similar to a ham transmitter and nothing like the modern stuff.

73

Steve WB2WIK/6
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W9PMZ
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Posts: 661


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« Reply #88 on: March 18, 2009, 08:46:02 AM »

Was just curious.

I hate the ESG box as it has caused so many problems.

Even my rep calls it a dog.

73,

Carl - W9PMZ
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NB3O
Member

Posts: 53




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« Reply #89 on: March 18, 2009, 12:07:29 PM »

"Can it make 10 pages? "
What's even better, is I've been trying to drop the subscription since page 5, but it ain't working.
I guess it's a handy way to determine whether or not my mail server is functioning properly.
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