Nice article Jeff! I got through part of it but it takes some digesting!
I think that the kind of IM products that we are most concerned about in our
amps is based on 2xF1 +- 1xF2 for 3rd order IM rather than the harmonics of
the fundamental. This assumes two signals being amplified by the amplifier
as in a two tone test. Of course things become a little more complicated as
the number of tones increase.
Of course we are also concerned with the higher orders of IM also such as
5th, 7th etc. as they all can mix with one another once the amp is operating
in the non-linear mode caused by any order of IM products.
I don't think that the phase of the output of the amp has much to do with IM
with narrow band signals such as we use for SSB. When the bandwidth is much
wider such as that used by cellular transmitter amplifiers maintaining a
near constant phase match over the bandwidth seems to be more important.
Our output matching network is not providing an impedance match to our
amplifiers as the actual device (tube or transistor) output impedance is
typically much lower than what our network is set for to obtain maximum
output. We really do not provide an impedance match we just provide an
optimum load for the tube to work into.
I would agree with Jeff that voltage swing compression has a sharper knee
than current limiting. When lightly loaded an amp can easily reach the peak
voltage swing and go abruptly into compression.
When the amp is loaded heavier than the maximum voltage swing it will not go
into sharp compression. The only thing that will cause non-linear operation
then will be emission capability of the tube and or getting outside of the
linear plate curve range of the tube. This happens more gradually than
voltage limitation does with an under loaded amp.
The reason to over couple the output slightly is to compensate for any
increased drive or increased voltage when in actual operation verses tune up
conditions as these tend to increase slightly with SSB operation compared to
a steady carrier.
If you do not increase the loading past the point of maximum output then
turn down the drive so that the peaks never hit the maximum output that the
amp is tuned for. It will accomplish the same thing of not pushing the
amplifier into compression.
I would always re-peak the plate tuning cap as the last thing after
increasing the loading control. That is the only way to know for sure that
you have set the loading control properly because when you change one side
of the matching network it also affects the other side.(it is no longer in
resonance). The heavier loaded an amp is the less effect the plate tuning
will have (it will be much broader). Depending on the Q of the network it
may be broad enough not to make a noticeable difference or it could make a
substantial difference. Either way the proper way is to re-adjust the plate
tune last.
As described above, over coupling an amplifier to keep it from going into
compression is no different than reducing the drive to keep it from going
into compression once tuned at a particular power level.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of jeff millar
> Sent: Sunday, December 25, 2011 1:32 PM
> To: Amps
> Subject: [Amps] Fwd: Re: Fwd: Linear Amplifier Tuning---PROPERLY!
>
>
> Here's more about IMD than you may want to know...
>
> I used to work with very low IMD amplifiers for the cellular industry
> and spent a lot oftime studying IMD sources and means of control.
> I learned the most about the causes of IMD from a PhD thesis, and
> later book, by Joel Vuolevi...
>
> http://herkules.oulu.fi/isbn9514265149/
>
> Most literature describes Intermodulation distortion as coming from
> harmonics of the fundamental mixing with the main signal.
> 3rd order IMD means
>
> 2 x fundamental +/- 1 x fundamental. (2+1 = 3rd order)
>
> But, the total contribution to the IMD also comes from 3 F +/- 2F,
> 4F +/- 3F, etc up to the 9th harmonic and beyond. All those products
> stack up on the same frequency at the output.
>
> Each of these products has a phase and the resulting IMD depends on
> how all the phases line up. This can make the IMD level very sensitive
> to the test signal, power level, temperature, bias, etc. It also can
> make the high and low side IMD have different levels.
>
> The FCC recognizes this effect and mandates that RF transmitter tests
> uses a random modulation to measure IMD, so the testers don't tweak the
> test to find a sweet spot in IMD performance where some of the
> components
> cancel out.
>
> The level of harmonics and their phases in the output matching
> network affects the total resulting IMD. But it's nearly impossible to
> design for or control all those effects. Every matching and bias
> network
> presents a different impedance at all those harmonics.
>
> Most UHF power transistors designed for very low IMD (-50 dB) have an
> output match specification that is _NOT_ the same at the optimum power
> transfer match.
>
> So, the optimum IMD setting of the tune capacitor is _probably_ not
> peak power output but there's not enough information available to set
> it...and it would drift around anyway.
>
> The biggest contributor to IMD is the amplifier going into compression
> on peaks. This produces a nonlinear input vs output response and those
> non-linearities produce the harmonic mixing effects that create IMD.
>
> The shape of the compression curve affects the level of IMD. A curve
> with a sharp break has more IMD than a more gradual curve. As a guess,
> a more heavily load amplifier will have a softer compression curve and
> produce less IMD when driven into compression.
>
> A heavily loaded amplifier limits on current and a lightly loaded amp
> limits on voltage swing. Intuitively, voltage clipping seems "sharper"
> than current limiting.
>
> The cellular industry implements predistortion of the input signal
> to compensate for amplifier compression. The amplifier compression
> curve bends down, so the predistortion circuit bends the the gain
> curve up to compensate.
>
> The future of amplifiers for _should_ include an IMD monitoring
> circuit and predistortion. This would produce significantly more power
> with a smaller and more efficient amplifier.
>
> IMD levels should be set to about -40 to -50 dB. This would have to
> apply to both transceivers and amplifiers, because much of the IMD these
> days comes from the crummy amplifiers in the transceiver.
>
> Crowded bands would seem completely different if everybody's IMD
> dropped 10-20 dB.
>
> jeff, wa1hco
> -------- Original Message --------
>
>
> ________________________________________
> From: amps-bounces@contesting.com [amps-bounces@contesting.com] On
> Behalf Of TexasRF@aol.com [TexasRF@aol.com]
> Sent: Wednesday, December 21, 2011 8:12 PM
> To: deswynar@xplornet.ca; donroden@hiwaay.net
> Cc: amps@contesting.com
> Subject: Re: [Amps] Fwd: Linear Amplifier Tuning---PROPERLY!
>
> Hi All, it makes no sense to not peak the tune C as the last step in a
> matching procedure.
>
> The tuning network is fully adjustable, allowing a match from your
> nominal
> 50 ohm load to what ever plate load impedance you want to use. If the
> tube
> needs 2000 ohms for best efficiency and power output, it is a simple
> matter
> to provide that. But, the transformed impedance is resistive only when
> the
> complete network is resonant. The only way that can happen is when the
> plate tune C is adjusted last, for maximum power transfer. The complete
> network includes all sources of reactance, including tube output C and
> stray C as
> well as the effect of the rf choke and tank inductor and tune C.
>
> When you tweak the tune C for maximum output, you are making all of the
> reactance contributors parallel resonant, leaving only a resistive load
> for
> the tube.
>
> Sure the plate load impedance changes when you tweak the tune C, that is
> what is supposed to happen. Over coupling slightly is just another way
> to say
> that the plate load impedance is reduced slightly. If you don't end up
> with the desired over coupled value, then the plate load C is not set
> to the
> required capacity and needs further adjustment.
>
> Operating a tube into a reactive load just doesn't make any sense in my
> mind. It may not be reactive enough to hurt anything but how can it help
> in
> any way?
>
> Standing by for other view points and arguments.
>
> 73,
> Gerald K5GW
>
>
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|