[Boatanchors] Using a SG-230 tuner on AM
Gary Schafer
garyschafer at comcast.net
Mon Aug 10 14:36:45 EDT 2009
A spectrum analyzer in the peak hold mode will not read PEP. It will only
show the maximum levels of each individual frequency average power contained
in them. With a steady tone modulating an AM transmitter there are still 3
individual frequencies involved. But come to think of it, if you use a wider
bandwidth and the correct sweep speed it may show the sum of all and add
them. If it did show the PEP there would be a 6 db increase with modulation
over carrier power level.
A field strength meter is normally going to read average power unless you
have one with the time constant and detector circuit that is capable of PEP
with an AM signal. Otherwise you will see a 50% increase in power with
modulation verses no modulation.
The easiest way to see PEP is with an ordinary scope coupled to the antenna
line. With carrier only set the scale to read say 2 centimeters. With 100%
modulation you will see the voltage double to 4 centimeters. 2x the voltage
across a fixed resistance is 4 times the power. But this power is only there
on modulation envelope peaks.
A lot of AM transmitters are not capable of full PEP. The peaks flatten out
due to poor power supplies, final tubes that are soft or are not capable of
the peak power in the first place. If you don't see a doubling of the RF
voltage on a scope then the transmitter isn't reaching the full PEP.
A 100 watt input transmitter requires a modulation power of 50 watts AVERAGE
power. Some would call this RMS power but RMS power doesn't exist. That's
another thread!
So 100 watts are contained in the carrier and 25 watts in each sideband.
Ignoring the transmitter efficiency for the moment.
50 watts average power of audio across say 5000 ohms plate load is around
500 volts RMS. That is a peak audio voltage of (500 x 1.414) 707 volts.
So let's say that our transmitter's final plate voltage is also 707 volts
DC.
The definition of 100% positive plate modulation is when the peak modulation
voltage equals the amplifier plate voltage in the positive direction. When
the peak modulation voltage in the negative direction (-707 volts) is the
same level as the DC plate voltage the resultant plate voltage is zero which
is 100% negative modulation.
The modulation voltage adds or subtracts from the applied DC plate voltage
on the finals. So on positive audio peaks the plate voltage doubles and on
negative audio peaks the plate voltage swings to zero.
Now if the final plate voltage doubles, as it does with 100% modulation, we
can calculate the power at that instant the audio voltage is causing the
voltage to double. E squared divided by R = power. Our peak plate voltage
supplied by the DC plate voltage plus the peak audio voltage is 707 + 707 or
1414 volts. 1414 squared = 1999396 divide by 5000 ohms = 399.88 watts Peak
envelope power.
Remember the definition of peak envelope power: the average power at the
crest (peak) of the modulation envelope over at least one RF cycle. The
above has satisfied that.
There will be many RF cycles contained at the peak of any audio peak as the
RF is at a much higher rate than the audio rate.
When finding PEP you can not just add average powers together. You must
first find the voltage of each, add those voltages together and then
calculate the power.
Note that all of the above is considering INPUT power to the amplifier. The
total output power would of course be a percentage depending on transmitter
efficiency.
73
Gary K4FMX
> -----Original Message-----
> From: Carl [mailto:km1h at jeremy.mv.com]
> Sent: Monday, August 10, 2009 11:09 AM
> To: Gary Schafer; 'D C *Mac* Macdonald'; rbethman at comcast.net;
> boatanchors at mailman.qth.net
> Subject: Re: [Boatanchors] Using a SG-230 tuner on AM
>
> OK, I'll have to set up a different test.
>
> However a SA set to peak hold with a steady tone should be able to read
> true
> PEP....correct? I only see about 2X with a 1000 Hz sine wave.
>
> So if I set up a calibrated FS meter I should then see a 6dB increase when
> 100% modulated with a steady tone
>
> What I dont understand and hasnt been answered is the modulator power. Is
> a
> modulator output measured in peak, RMS or average power? IOW, a modulator
> rated at 100W is doing what? Since a 100W modulator xfmr is self limiting
> (core saturation) with 100W tube capability so it cant provide more than
> rated power. OTOH my PP 304TL's using a 5KW BCB TX xfmr knows no limits
> (-;
>
> I remain confused!
> Carl
> KM1H
>
> ----- Original Message -----
> From: "Gary Schafer" <garyschafer at comcast.net>
> To: "'D C *Mac* Macdonald'" <k2gkk at hotmail.com>; "'KM1H Carl Huether'"
> <km1h at jeremy.mv.com>; <rbethman at comcast.net>;
> <boatanchors at mailman.qth.net>
> Sent: Sunday, August 09, 2009 11:44 PM
> Subject: RE: [Boatanchors] Using a SG-230 tuner on AM
>
>
> > Actually the doubled current (and voltage) comes from the modulator
> during
> > modulation. If the modulator has its own high voltage supply all the
> extra
> > power comes from it. If the modulator shares the same supply then of
> > course
> > it all comes from the same supply.
> >
> > Spectrum analyzers are really power meters. They measure the voltage
> > across
> > 50 ohms and are calibrated in power levels.
> >
> > Peak envelope power is not visible on a spectrum analyzer because all
> the
> > power is not on the same frequency. A spectrum analyzer displays the
> power
> > levels in each frequency component.
> >
> > An average reading wattmeter will read the composite of the carrier and
> > side
> > bands together. A 100% modulated transmitter will show 1.5 times the
> > average
> > carrier power when modulated by a steady tone.
> >
> > When using a peak reading wattmeter on SSB it will read the true peak
> > envelope power of the transmitter due to the time constant of the
> metering
> > circuit.
> >
> > A PEP wattmeter such as the Bird will read ok on SSB but will not give a
> > true PEP reading with an AM signal. The reason being that the carrier
> > causes
> > a bias on the detector so that it can not detect the full peak voltages
> > present.
> >
> > Some of the digital wattmeters are able to give true PEP readings on AM
> > signals. The other way to read true PEP is with a scope.
> >
> > 73
> > Gary K4FMX
> >
> >
> >> -----Original Message-----
> >> From: boatanchors-bounces at mailman.qth.net [mailto:boatanchors-
> >> bounces at mailman.qth.net] On Behalf Of D C *Mac* Macdonald
> >> Sent: Sunday, August 09, 2009 9:56 PM
> >> To: KM1H Carl Huether; rbethman at comcast.net;
> boatanchors at mailman.qth.net
> >> Subject: Re: [Boatanchors] Using a SG-230 tuner on AM
> >>
> >>
> >> The doubled current IS supplied by the power supply,
> >> but NOT just the transformer but the filter capacitors
> >> on a VERY short term basis AND the "spikes" supplied
> >> by the modulator and ITS power supply.
> >>
> >> 73 - Mac, K2GKK/5
> >> (Since 30 Nov 53)
> >> OKlahoma City, OK
> >>
> >>
> >> p.s. Spectrum analyzers with which I'm familiar
> >> shows ONLY voltage level. I believe that an
> >> average-reading Wattmeter would show 1.5 times
> >> the carrier reading with 100% modulation by a
> >> single tone. It HAS been a while since I have
> >> made that test on a spectrum analyzer, but I
> >> service FAA's VHF and UHF transmitters which
> >> are set for 10W carrier power. Set up for 90%
> >> modulation with 1 kHz tone, the Marconi (and other)
> >> service monitors show approximately 14W. They
> >> will show 15W at 100% modulation.
> >>
> >> p.p.s. I am not familiar with using a peak-reading
> >> wattmeter, depending on the decay time-delay of the
> >> particular wattmeter, I can see where you would never
> >> see the actual peak ENVELOPE power.
> >>
> >>
> >>
> >> ----------------------------------------
> >> > From: km1h at jeremy.mv.com
> >> > To: k2gkk at hotmail.com; rbethman at comcast.net;
> >> > boatanchors at mailman.qth.net
> >> > Subject: Re: [Boatanchors] Using a SG-230 tuner on AM
> >> > Date: Sun, 9 Aug 2009 19:36:30 -0400
> >> >
> >> > Where is this doubled current coming from? There is only one voltage
> >> source
> >> > feeding the final and that certainly isnt pulling 2X key down.
> >> >
> >> > I dont see 4X power on a peak reading wattmeter or spectrum analyzer.
> >> >
> >> > Carl
> >> > KM1H
> >> >
> >> > ----- Original Message -----
> >> > From: "D C *Mac* Macdonald"
> >> > To: ;
> >> > Sent: Sunday, August 09, 2009 4:15 PM
> >> > Subject: Re: [Boatanchors] Using a SG-230 tuner on AM
> >> >
> >> >
> >> >>
> >> >> I'm certainly not looking to start a "firefight."
> >> >>
> >> >> HOWEVER, if you will display an unmodulated "carrier" on an
> >> >> o'scope, then modulate it 100%, the VOLTAGE when amplified
> >> >> will double. Since ohms law states that if you double the
> >> >> voltage you will also double the current.
> >> >>
> >> >> Since P = I x E, 2I x 2E will give you 4P.
> >> >>
> >> >> It's basic Electronics 101.
> >> >>
> >> >>
> >> >> 73 - Mac, K2GKK/5
> >> >> Oklahoma City, OK
> >> >>
> >> >>
> >> >>
> >> >> ----------------------------------------
> >> >>> Date: Sun, 9 Aug 2009 16:11:02 -0400
> >> >>> From: rbethman at comcast.net
> >> >>> To: boatanchors at mailman.qth.net
> >> >>> Subject: Re: [Boatanchors] Using a SG-230 tuner on AM
> >> >>>
> >> >>> This always starts "some" sort of a firefight.
> >> >>>
> >> >>> I can argue either side. The basic reasoning gets down to WHAT
> >> >>> measurement INSTRUMENTS are being used.
> >> >>>
> >> >>> The current 1500W limit is to be measured in PEP. Darn few folks do
> >> >>> so. They are still using equipment that doesn't get the job done
> >> >>> correctly.
> >> >>>
> >> >>> The only thing I could remotely suggest is to TRY the tuner on a
> >> single
> >> >>> antenna with a radio of the desired output. Brief duration as I
> know
> >> >>> Carl WOULD do.
> >> >>>
> >> >>> Then go from there.
> >> >>>
> >> >>> I'm going to leave the firefight. Been in to many real ones.
> >> >>>
> >> >>> Bob - N0DGN
> >> >>>
> >> >>> D C *Mac* Macdonald wrote:
> >> >>>> At 100% modulation peak, the instantaneous voltage
> >> >>>> AND current are both double the static values.
> >> >>>>
> >> >>>> V^2 times I^2 gives you four times the static power.
> >> >>>>
> >> >>>> Current FCC rules allow for 1.5 kW output power.
> >> >>>> That is why the old kilowatt rigs such as Collins
> >> >>>> KW-1 and Johnson Desk Kilowatt can now only be run
> >> >>>> at 375 Watts carrier power.
> >> >>>>
> >> >>>> 73 - Mac, K2GKK/5
> >> >>>> Oklahoma City, OK
> >> >>>>
> >> >>>
> >> >>> ______________________________________________________________
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> >> >>>
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