Even if the filters are perfect, the passband is not perfectly flat. If we were using Butterworth low pass filters it would be, but most of our ham designs are Chebyshev or Cauer filters that have some passband ripple. With ideal components the ripple would be only a fraction of a dB, but it could be larger if the component values are not perfect. Optimizing the filters could be?moving the ripples around so that the desired output frequency is at one of the peaks.
Hi Wes
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Syd, adding turns to a inductor used in a LPF will NEVER result in increased output. Take a look at the response curve for a LPF, there is a knee in the curve, below which there is full output, and above which the output reduces. All that happens when you add turns is the knee frequency decreases, when you remove turns the frequency increases.?
Respectfully I disagree... What?really happens in practice is that quite often you find that increasing the inductance does increase the output power. I see this all the time. *IF* an LPF is perfectly designed and perfectly?constructed with perfect components, and with perfect 50-ohm source and load, then yes the frequency response will be a flat line until there's a knee and at that point there will be an increasing attenuation with frequency; the 3dB point is called the cut-off frequency.?
If the inductances are anything off perfect, then you can easily end up with a response which has a peak in it, and a few dB insertion loss at lower frequencies. If that peak of the response is not aligned with the operating frequency then you see low power. If the inductances are too low for the design, then increasing them to the design value can definitely increase the output power.?
What happens in QCX is even more complex because there is normally not a perfect impedance match between the PA and the LPF input. When you tweak inductance values the output power can be increased if the impedance match is improved.?
Practically speaking, when tuning up QCX LPFs I have often found that increasing the inductance of L2 improves everything. I know it is slightly counter-intuitive but this is the reality.?
Generally I think the correct approach is to see what happens to the output power when you squeeze the turns and when you spread the turns. If the power increases as you squeeze the turns, this means that increasing inductance is increasing the output power. If the maximum output power occurs when you bunched together the turns as much as you could, then the tuning range available from turns squeezing is not enough; so in that case try adding on another turn or two, then try again. Conversely if the maximum power occurs when all the turns are spread out evenly around 100% of the toroid core, then this means that the adjustment range is not enough in the other direction - and in that case try removing a turn or two, then try again.?
73 Hans G0UPL
Shirley Dulcey KE1L