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On Mon, 1 Nov 2004 17:24:10 -0600, "Steve Nosko"
wrote: The "back to back" configuration I am familiar with is like this: + Control voltage | R | +----||-----+---||---+ | | | | +--LLLLLLLL+LLLLLLLLL--+ There could also be a series cap in this configuration--except a resistor or choke would be required on one of the varactors. (no different than the one to the control line) You would need fixed capacitors on _both_ anodes and also resistors to ground from both anodes to get the DC bias. The small series capacitors are essential, since they take the most part of the RF voltage. Of course, the series resistor will reduce the tuning range. You could get away with the series capacitor and put multiple (maybe 10) varactors in series. Thus, the RF voltage across each varactor would be low. You will need a high tuning voltage, perhaps 100 V. The worst problem is how to get the DC voltage distributed over the varactors. The simple solution would be to put resistors across each varactor to form a voltage divider. Apart from possible thermal noise problems (if weak signals are also involved), the nasty thing is the resistors will have a parasitic capacitance across the ends of the resistor. This capacitance is in parallel with the varactor, forming a significant base capacitance. Also the losses (and hence Q) of these parallel parasitic capacitances may degrade the total Q of the resonant circuit. Putting multiple varactors in series also increase the total inductance, which would not be so nice in this case, since the inductance levels are already low. However, if the parasitic capacitance/diode is much less than the varactor minimum capacitance, quite large tuning ranges could be obtained. I also don't get this talk about the filter Z. Since you'd need to Z match in/out of the filter, it seems to me the varactor voltages will be the same for any Zin/out since this will be determined by how "tightly" they are coupled into the resonant circuit and not the Zin/Zout, no? The Z match will just change the Vin/out. Think about two resonant circuits coupled by a small capacitance at the top. Connect the signal from the input line to the first resonator using inductive coupling (transformer with untuned primary). By selecting the number of turns on the primary, you can get any impedance transformation ratio, so you can match the 50 ohm line to any low impedance resonator. On the output side on the other resonator, you can do the opposite with the other transformer and restore the impedance to 50 ohms for the output line. 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). Rhode wrote an article in QST a few years ago about running the HF varactor tuning front end at a lower impedance level to avoid the high RF voltages on the varactors. In VHF/UHF reducing the inductance to enable larger capacitances and thus lower impedance and RF voltages is problematic, since the inductance is already extremely small. In principle it should be possible to connect several "coils" in parallel (actual wire loops across the capacitor) and this is how many text books explain how the cavity resonators are formed by adding further and further wire loops surrounding the capacitor. However, I have never seen parallel "coils" in any practical circuit, apparently there are some parasitic capacitance problems. However, I think that the OP should also study of making a shortened 1/4 (stripline or microstrip) resonator, with very wide resonators (and thus low impedance levels) and do the impedance transformation at the input and output coupling. If multiple stage filtering is needed, look for interdigital filters and again design for low resonator impedances to reduce the RF voltage across the tuning capacitors. These might be more practical for the intended frequencies than ordinary LC filters. If the tuning speed is not very large, look for some mechanical tuning at the end of the stripline resonator, such as moving the grounding electrode closer to the resonator hot end by a piezoelectric crystal etc., thus increasing the capacitance. Paul OH3LWR |
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