[ARC5] Lopsided modulation
AKLDGUY .
neilb0627 at gmail.com
Sat Feb 24 19:09:12 EST 2018
You may be onto something with the B+ kicking downward on modulation. I
notice that the dynamotor voltage does kick down significantly. This drop
may be causing the plate voltage to bottom out, or even go negative!, on
modulation peaks.
The MD7 modulator schematic doesn't show significant capacitance decoupling
the bottom of the mod tranny feed to the 1625 final stage - about 1.2 uF
IIRC, and I followed that when building my own modulator. I suspect it may
not be sufficient decoupling.
But even if the final plate voltage is bottoming out or going negative,
where is the literature that says this causes lopsided modulation?
Neil ZL1ANM
On 25/02/2018 12:27 PM, "Richard Knoppow" <1oldlens1 at ix.netcom.com> wrote:
> If you can tune in the oscillator on a receiver see if you hear any
> modulation when you modulate the transmitter. A variation in AM sidebands
> comes from phase modulation so I am not sure you would see it on a
> waterfall display. I know this is covered in some older textbooks but I
> don't have a ready citation. My idea is that the modulation is causing a
> change in the B+ voltage which is, in turn, modulating the oscillator.
> However, there are other causes such as some effects of tuned circuits.
> I think its unlikely an antenna can cause the problem unless its a VLF
> antenna with extremely high Q. Even when AT&T used single sideband
> transmission to England in the late 1920s and early 1930s a filter was used
> to suppress the unwanted sidband. Fairly complete descriptions of this
> system exist in the Bell System Technical Journal, which is available free
> on the web.
>
> There is a little on incidental FM in the handbook at:
> http://www.darc.de/fileadmin/_migrated/content_uploads/Guide
> _toSpectrum-and_SignalAnalysis__engl..pdf
>
> Also see
> http://www2.ee.ufpe.br/codec/HP_AM_FM.pdf
>
> Also
>
> https://books.google.com/books?id=BgLyCAAAQBAJ&pg=PA177&lpg=
> PA177&dq=incidental+fm&source=bl&ots=U5pLyPPC9v&sig=
> p7yTUPEGxoCi2pRp3s_FOCi-oyY&hl=en&sa=X&ved=0ahUKEwiUrf7h2L
> _ZAhUr0YMKHbB8DN44ChDoAQgzMAI#v=onepage&q=incidental%20fm&f=false
>
> Lots of other stuff via Google.
>
>
>
> On 2/24/2018 2:07 PM, AKLDGUY . wrote:
>
>> FM does not seem to be present in either his signal or mine. The carriers
>> are straight lines on the waterfall display. Actually, there is one
>> operator on the net whose carrier is a fuzzy line when he speaks, but his
>> sidebands are symmetrical. I'm stumped.
>>
>> Neil ZL1ANM
>>
>> On 25/02/2018 10:30 AM, "Richard Knoppow" <1oldlens1 at ix.netcom.com
>> <mailto:1oldlens1 at ix.netcom.com>> wrote:
>>
>> That affects the upward vs downward modulation but not
>> the strength of the sidebands. A frequent cause of
>> asymetrical sidebands is incidental FM. If the oscillator is
>> being affected by the modulation it can be the cause.
>> The scope display will not show assymetry of the upper
>> vs lower sidebands although it will certainly show assymetry
>> of upward vs downward modulation.
>>
>> On 2/24/2018 1:18 PM, Jim Wiley wrote:
>>
>> One reason could be that your voice in itself is
>> non-symmetrical. If you can do so easily, try reversing
>> the "polarity" if the modulation transformer, and see if
>> that helps. If it does, your can leave it set the way
>> that works best for you. Monitoring the modulation
>> envelope with an oscilloscope is always the best way.
>>
>> - Jim, KL7CC
>>
>>
>>
>> On 2/24/2018 11:38 AM, AKLDGUY . wrote:
>>
>> A friend and I are running Command transmitters on
>> the local 75 meter AM net, with full plate and screen
>> modulation. We monitor our transmissions on the local
>> SDR receiver site which has a waterfall display. In
>> both cases, the lower sideband shows as considerably
>> stronger than the upper, and this has us perplexed.
>> Our stations are the only ones affected.
>>
>> I have never seen anything in the literature to
>> explain this. Downward modulation is a problem with
>> AM, but articles do not indicate whether lopsided
>> modulation results.
>>
>> My transmitter is the AN/ARC-5, absolutely unmodified
>> and running 40W input due to dynamotor constraint.
>> His is the SCR-274 modified for plate & screen
>> modulation and running twice that.
>>
>> Anyone suggest a reason for the unequal sidebands?
>>
>> Neil ZL1ANM
>>
>>
>>
>> -- Richard Knoppow
>> 1oldlens1 at ix.netcom.com <mailto:1oldlens1 at ix.netcom.com>
>> WB6KBL
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> --
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
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