A "conventional combiner" is more often than not a "zero-degree combining
network".
For a through discussion of power dividers and combiners take a look at:
http://www.minicircuits.com/appnote/an10006.pdf, and also pp 498 - 510
in "Single Sideband Systems & Circuits" 2nd edition.
It is the designer's choice if the combining system should be zero-degree or
180-degree,
but as it simplifies construction and testing somewhat the zero-degree
approach appears to be usually taken.
A special case is when you use 90-degree hybrids to absorb load reflections, but
they seem to be not so very common for multi-octave combiner networks, as those
encountered in HF
amplifiers.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "Will Matney" <craxd@engineer.com>
To: <amps@contesting.com>
Sent: Tuesday, July 26, 2005 12:23 PM
Subject: Re: [Amps] Amp question
> Karl,
>
> Please see below;
>
> >
> > The reason why this works is that the output combining network in
> > most solid-state PA's are
> > "zero-degree hybrids", which means that they expect the voltages
> > (or currents) driving them to be
> > in-phase to provide a combined output. Output transformers in p-p
> > stages can on the other hand
> > be classified as "180-degree hybrids", which require
> > opposite-phase driving voltages to operate properly.
>
>
> By looking over the schematic of this amp, everything looks the same as one
> for 180 degrees phase difference except it being minus a splitter in the
> front end. To me if it were to be zero degrees you wouldn't need a splitter
> or combiner, just merely paralell the two modules the same as paralelling
> tubes to add up the power output. They have two conventional RF output and
> input transformers per module. These work in push-pull to provide a linear
> signal being it's class AB1. Then each output from the RF transformer goes to
> either end of the combiner transformer just like all the others being 180
> degrees out of phase. However, on the schematic, they dont show any phasing
> dots on the combiner. Still though, I would think it would still have to be
> out of phase if it's wound the same as a conventional combiner. The only way
> it could be different is the two coils be in phase instead out of phase? It
> has to be one of the two by the schematic they show. I still just can't see
> how it
's
> doing it by having them both on at the same time and not buck each other at
> the end. I'll have to do some research on this as I've not seen that
> arrangement in the Motorola app notes, nor in any book I have on the subject.
>
> >
> > As the phase shifts in the two amplifier halves can be assumed to be equal,
> > it remains to drive the halves with in-phase voltages.
> >
> > The RC-network on the input side just acts as a resistive power
> > dividing network with its outputs in-phase,
> > and as the amplifier power gains decrease with frequency, capactive
> > shunt elements
> > can compensate for this decrease, as the insertion loss of the
> > network also will decrease with frequency.
>
>
> Agreed.
>
> >
> > Finally, the reason for using 200 ohms for the balancing resistor
> > is simply that the designer has chosen a 100 ohm
> > input design impedance for the combining network, which requires
> > the amplifier outputs to be 100 instead of 50 ohms.
> > This is "perfectly legal" in an environment where you are at
> > liberty to choose the design impedances freely, and do not have to
> > take long runs of interconnecting RF cables into account. But the
> > stray and winding capacitances present can be more difficult to
> > handle in a system with a higher design impedance.
>
>
> Exactly. I figured it was because of it going into the low pass filters the
> way they had it designed, and allowing them to finish stepping it down to 50
> ohms. But, like you said, using 100 ohms increases the windings on every
> transformer by X2.
>
> >
> > In professional circles this is not very common, as most
> > solid-state professional PA's are built from modules
> > having a design impedance of 50 ohms.
>
> Agreed. That just didn't make a lot of sense to me.
>
> Best,
>
> Will
>
> >
> >
> > 73/
> >
> > Karl-Arne
> > SM0AOM
> >
> > ----- Original Message -----
> > From: "Will Matney" <craxd@engineer.com>
> > To: <amps@contesting.com>
> > Sent: Tuesday, July 26, 2005 6:57 AM
> > Subject: [Amps] Amp question
> >
> >
> > > All,
> > >
> > > Ok, most know how splitter/combiner transformers work in solid
> > > state amps. Here's the question, how can an amp work using two
> > > sets of push-pull amps, having a conventional combiner, but not
> > > have a splitter in the front end? What this amp has is a divider
> > > network made up of a RC network using several resistors in
> > > paralell with a capacitor on either side of the RF input. In
> > > other words, when the RF comes into this RC divider, it applies
> > > RF equally to both sets of push-pull amps at one time. There is
> > > no splitter transformer there. How can this work by having both
> > > push-pull sets on where they should be 180 degrees out of phase,
> > > and one being off while the other is on? To use a combiner
> > > transformer, the RF from one set makes 1/2 wave and so from the
> > > other set. The combiner then adds these two halves together
> > > making a full sine wave. So, how can this work having RF going to
> > > both sets at once instead of using a splitter which would put
> > > them 180 Deg out of phase? This absolutely has me stumped. By
> > > the way, it's for an Ameritron ALS-500M.
> > >
> > > One last question, most combiners/splitters use a 100 ohm
> > > resistor in paralell with it's inputs for a 50 ohm load. How can
> > > this work if the resistance is 200 ohms? That would be for a 100
> > > ohm load wouldn't it?
> > >
> > > Best,
> > >
> > > Will
> > >
> > > -- ___________________________________________________________
> > > Sign-up for Ads Free at Mail.com
> > > http://promo.mail.com/adsfreejump.htm
> > >
>
>
>
>
> --
> ___________________________________________________________
> Sign-up for Ads Free at Mail.com
> http://promo.mail.com/adsfreejump.htm
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
> --
> No virus found in this incoming message.
> Checked by AVG Anti-Virus.
> Version: 7.0.338 / Virus Database: 267.9.5/58 - Release Date: 2005-07-25
>
--
No virus found in this outgoing message.
Checked by AVG Anti-Virus.
Version: 7.0.338 / Virus Database: 267.9.5/58 - Release Date: 2005-07-25
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|