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On Tue, 2 Nov 2004 11:58:17 -0600, "Steve Nosko"
wrote: Why both? It is the total series cap which is of concern. One cap would simply be half the value of each of the two. 2 x 5pf = 1 x 2.5pf, no? Yes, it is doable with one fixed capacitor, however, the voltages across the stray capacitances would be different and their effect would be harder to predict. Of course, the series resistor will reduce the tuning range. I don't see this. It would be just like the one shown. 100k or 1M. It is for DC and is large enough to be neglegible at RF. (strays acknowledged) I meant to say series _capacitance_ (not resistance), sorry for the confusion. THough I don't think it is necessary, if you require symmetry. Another option: +----+ Control voltage | | R R | | +----||-+-||-+--||---+ | 2.5pf | | | | | | | | | | | +--LLLLLLLL+LLLLLLLLL--+ This would be an elegant solution. Another option: +-----------+ Control voltage | | R R | | +-||-+-||-+-||-+--||-+ | 5pf | 5pf | | | | | gnd | | | | | | | +--LLLLLLLL+LLLLLLLLL--+ As well as this. To reduce the Z in a resonator, you will have to reduce both the inductive Xl and capacitive Xc reactance by reducing the inductance and increasing the capacitance (e.g. by multiple varactors). So now the varactor must be _more_ of the overall capacitance to get the desired tuning range, no. If you want minimal side effects, then the varactors need to be just barely inthe circuit, so to speak, which means that they will have small voltages. For a given total capacitance, putting more varactors in parallel means that the capacitance each varactor must produce is reduced, thus the tuning voltage must be increased, which is a good thing, since this improves the ratio between the tuning voltage and RF voltage. Dropping the impedance levels and hence increasing the capacitance required also means that even more varactors can be connected in parallel. However, I think that the OP should also study of making a shortened 1/4 (stripline or microstrip) resonator, with very wide resonators clip Something you should not reject is a "ranged" system. Tune a smaller range with the varactors and switch in/out other caps for larger shifts. The OP clearly had something similar in mind, since he originally asked for a 2:1 tuning range, but now he is asking for the 30-500 MHz range. I do not think that such huge range can be handled by just switching base capacitors. With such large frequency range, it would make more sense, to build at least three completely independent filters, say 30-90 MHz, 90-270 MHz and 270-500 MHz, while each filter could switch in base capacitances. The two lower filters could be lumped LC filters, while the last range could be limited to less than one octave, so that strip line filters could be used. Paul OH3LWR |
"Paul Keinanen" wrote in message
The OP clearly had something similar in mind Yes, that was my plan. since he originally asked for a 2:1 tuning range, but now he is asking for the 30-500 MHz range. I do not think that such huge range can be handled by just switching base capacitors. With such large frequency range, it would make more sense, to build at least three completely independent filters, say 30-90 MHz, 90-270 MHz and 270-500 MHz, while each filter could switch in base capacitances. I was thinking of two... 30-125MHz and 125-500MHz. I.e., each one tunes a range of about 4:1. Something like 3-5 base capacitors would provide the broad tuning, with varactors doing the fine tuning. The two lower filters could be lumped LC filters, while the last range could be limited to less than one octave, so that strip line filters could be used. The problem is that even though the 125-500MHz filter only needs to create a bandstop region somewhere in the 125-500MHz range, it must otherwise still pass the complete 30-500MHz range. As far as I can tell, this still eliminates all distributed-style filters from consideration. ---Joel |
"Paul Keinanen" wrote in message ... On Tue, 2 Nov 2004 11:58:17 -0600, "Steve Nosko" wrote: Why both? It is the total series cap which is of concern. ... Yes, it is doable with one fixed capacitor, however, the voltages across the stray capacitances would be different and their effect would be harder to predict. Yes, at the higher freqs, strays become significant, as I remarked. .... For a given total capacitance, putting more varactors in parallel means that the capacitance each varactor must produce is reduced, thus the tuning voltage must be increased, which is a good thing, since this improves the ratio between the tuning voltage and RF voltage. Dropping the impedance levels and hence increasing the capacitance required also means that even more varactors can be connected in parallel. .... Let me ponder this... By golly, running several parallel varactors at fifty volts does sound good, doesn't it. You are away from the high non-linear region near zero. Never had that option, I guess. -- Steve N, K,9;d, c. i My email has no u's. |
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