Not being an RF PA expert I¡¯d be interested to know why the centre tap of the QMX RF PA output balun (T501) is not decoupled.
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I accept that the RF PA output is symmetrical, and thus ¡®balanced¡¯ in the sense that the positive and negative half-cycles are nominally the same. And I accept that such balance ought to give even harmonic suppression. But it doesn¡¯t automatically follow from any such ¡®balance¡¯ that the instantaneous currents in each half will sum to a constant DC value. That would only be true if the rising current edge in one half of the PA were matched (in both shape and time) by the falling current edge in the other. And that is not the same thing. (If that¡¯s not clear think how the total instantaneous current would look if each half of the PA turned on for just a quarter of the RF cycle instead of half of it).
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In practice it seems unlikely that the rising and falling current edges will be well matched. So it is unlikely that the current in each half will sum to a genuine DC. There are likely to be brief ¡®spikes¡¯ in the total PA current draw, coinciding with the falling and rising current edges. The total PA current has to pass through L502. That will present a significant impedance to the current spikes, and thus result in a ¡®spiky¡¯ voltage pattern at the centre tap.
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Will those voltage ¡®spikes¡¯ at the centre tap add to the extremes of voltage that the PA devices have to handle? And if so ought the T501 centre tap to have been decoupled?
