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On Sat, 30 Oct 2004 09:59:15 -0700, "Joel Kolstad"
wrote: I'm not quite following this (I don't know what a 'varactor varacitor is'?). Something like: signal -- | | -- || -- gnd Where | | is the 5pF capacitor and || is the varactor with anode facing 'left'? I'm not following how the 'back to back' configuration works. Something like this (fixed font): Vt +-+ | | | R | | +----||------+---||---+ | | | | R | | | | +----||---+------||---+ | | | | +--LLLLLLLL+LLLLLLLLL--+ | --- Gnd The cent er of the inductance L is grounded to get ground reference for the tuning voltage Vt. With a small series capacitance and a large varactor capacitance, most of the RF voltage is over the fixed capacitor. The anti-parallel structure should reduce the distortion. You need to bring the tuning voltages Vt to the varactors through separate resistors R (or through inductor resistor combinations). Paul OH3LWR |
#2
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Thanks Paul, much appreciated!
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#3
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"Paul Keinanen" wrote in message ... On Sat, 30 Oct 2004 09:59:15 -0700, "Joel Kolstad" wrote: I'm not quite following this (I don't know what a 'varactor varacitor is'?). Something like: signal -- | | -- || -- gnd Where | | is the 5pF capacitor and || is the varactor with anode facing 'left'? I'm not following how the 'back to back' configuration works. Something like this (fixed font): Vt +-+ | | | R | | +----||------+---||---+ | | | | R | | | | +----||---+------||---+ | | | | +--LLLLLLLL+LLLLLLLLL--+ | --- Gnd The cent er of the inductance L is grounded to get ground reference for the tuning voltage Vt. With a small series capacitance and a large varactor capacitance, most of the RF voltage is over the fixed capacitor. The anti-parallel structure should reduce the distortion. You need to bring the tuning voltages Vt to the varactors through separate resistors R (or through inductor resistor combinations). Paul OH3LWR Paul, 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) I don't know the advantages of either config. as the RF cycle effects each diode the same way "on opposite half cycles" in either. 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. Steve K9DCI |
#4
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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 |
#5
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Hi Paul,
Thanks again for the suggestions; I need to do a few simulations to see how viable some of the approaches are. 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. The problem with this approach is that I've yet to see a stripline/microstrip resonator design that -- by design -- doesn't have re-entrant modes well before covering a 16.7:1 range (30-500MHz, in my case). Otherwise I'd be all for it! If the tuning speed is not very large It's not, 'some low number of seconds' to re-tune is fine. I don't suppose anyone makes motorized piston trimmer caps? Sure would be nice... ---Joel |
#6
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"Paul Keinanen" wrote in message ... 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--... 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. Hi Paul, 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? 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 think there are many two diode configurations. One option: + Control voltage | R 2.5pf | +-||-+-||---+---||---+ | | | | R | | | | | gnd | | | | | +--LLLLLLLL+LLLLLLLLL--+ DC gnd down here THough I don't think it is necessary, if you require symmetry. Another option: +----+ Control voltage | | R R | | +----||-+-||-+--||---+ | 2.5pf | | | | | | | | | | | +--LLLLLLLL+LLLLLLLLL--+ Another option: +-----------+ Control voltage | | R R | | +-||-+-||-+-||-+--||-+ | 5pf | 5pf | | | | | gnd | | | | | | | +--LLLLLLLL+LLLLLLLLL--+ [[I am obviously ignoring parasitics of the diode or cap body to ground. More "stuff" of any kind in tthe in the circuit = more paracitic capacitance to fight. That is a mechanical RF layout issue and, of course, not to be ignored.]] 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. I agree, Clearly a complex arrangement. However, is there really an advantage to having low voltage if you now have so many contributors to the problems... ... 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. [describes z matching in/out of a filter...] That's my point. Once you select an inductor type, you have fixed a number of future decisions. The transformation can get you to wherever you need to be *IN* the resonator....see next 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. 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. Probably necessary reading, but I guess that I am trying to point out that this is one of those "no free lunch" situations. That a wide tuning range AND good IM, temp performance are each pulling the design in opposite directions? I mean: Wide tuning - means - varactors need to be a larger fraction of the total capacitance... But this means that the non linearity and temp drift of the diode has more effect. If you put series caps to reduce voltage, although you are reducing the effect of cap non linearity and temp drift because the diode is now "decoupled" from the tuned circuit, you are reducing the potential tuning range. I have no significant comments on most of the rest...except... ...However, I have never seen parallel "coils" in any practical circuit, apparently there are some parasitic capacitance problems. Side bar: My first "short wave" receiver did this. Take one 50's tube AM radio. Parallel both the RF and LO coils with outboard coils to get 75 Meter phone band (it was mostly AM then). I think it was in Popular Electronics or some such mag. However, I think that the OP should also study of making a shortened 1/4 (stripline or microstrip) resonator, with very wide resonators Been there, done that (not varactor tuned, though). The needed direction. (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 Sure, Interdigital or combline... 220, 221. Whatever it takes... If the tuning speed is not very large, look for some mechanical tuning at the end of the stripline resonator, 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. 73, -- Steve N, K,9;d, c. i My email has no u's. |
#7
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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 |
#8
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"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 |
#9
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"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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