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Varactor tuning
Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. p. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Paul,
The amount of tuning range is a function of the ratio of Cmax/Cmin. If you parallel varactors, Cmax will double, but so will Cmin. The ratio hasn't changed. If you're not already using a "hyper-abrupt" type of varactor, you should look into one. They offer a wider capacitance range. What type of varactor are you using, and what's the frequency of the resonator? What's the application...linear frequency modulation like FM or data keying like FSK??? Joe W3JDR "Paul Burridge" wrote in message ... Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. p. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Paul,
The amount of tuning range is a function of the ratio of Cmax/Cmin. If you parallel varactors, Cmax will double, but so will Cmin. The ratio hasn't changed. If you're not already using a "hyper-abrupt" type of varactor, you should look into one. They offer a wider capacitance range. What type of varactor are you using, and what's the frequency of the resonator? What's the application...linear frequency modulation like FM or data keying like FSK??? Joe W3JDR "Paul Burridge" wrote in message ... Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. p. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Wed, 10 Dec 2003 00:07:59 +0000, Paul Burridge
wrote: Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. p. Paul, That will work, but will double *both* min and max capacitance. But I'm puzzled: "I can't get enough shift with the available bias voltage" implies you're on the *low* end of capacitance (highest voltage). ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food. |
On Wed, 10 Dec 2003 00:07:59 +0000, Paul Burridge
wrote: Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. p. Paul, That will work, but will double *both* min and max capacitance. But I'm puzzled: "I can't get enough shift with the available bias voltage" implies you're on the *low* end of capacitance (highest voltage). ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food. |
On Wed, 10 Dec 2003 00:07:59 +0000, Paul Burridge
wrote: Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. p. Sure. Is this a coaxial ceramic resonator, or one of the low-freq piezo things? What's the frequency, Kenneth? [1] John [1] old Dan Rather joke, sorry. |
On Wed, 10 Dec 2003 00:07:59 +0000, Paul Burridge
wrote: Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. p. Sure. Is this a coaxial ceramic resonator, or one of the low-freq piezo things? What's the frequency, Kenneth? [1] John [1] old Dan Rather joke, sorry. |
that would the more total capacitance but not any larger dC/dV.....
you need even better tricks :-) Marco "Paul Burridge" wrote in message ... Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. p. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
that would the more total capacitance but not any larger dC/dV.....
you need even better tricks :-) Marco "Paul Burridge" wrote in message ... Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. p. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Tom Bruhns wrote:
So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Cheers, Phil Hobbs |
Tom Bruhns wrote:
So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Cheers, Phil Hobbs |
Paul Burridge wrote:
An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Thanks, is this the kind of thing you mean? +-------+ | | | | | | C| | L1 C| | C| | | | | | V | D1 - | | C| Applied DC control voltage | C| L2 Line --------------------+ C| | | | | D2 - | ^ | | | | | | | +-------+ View in FP font. created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de Right. I might make the series inductors symmetrical, if I needed the tap point to be near signal ground (e.g. with a centertapped coil or a differential pair driving it). Last time I used this trick was in a 160-MHz phase shifter. The two inductors will generally be about the same size for best results with a hyperabrupt varactor--5 minutes with a math program will give you the right values. Generally you need to keep the reactance capacitive if you're resonating a crystal against this combination--there are multiple operating frequencies otherwise, since the resonator will look capacitive almost everywhere. Cheers, Phil Hobbs |
Paul Burridge wrote:
An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Thanks, is this the kind of thing you mean? +-------+ | | | | | | C| | L1 C| | C| | | | | | V | D1 - | | C| Applied DC control voltage | C| L2 Line --------------------+ C| | | | | D2 - | ^ | | | | | | | +-------+ View in FP font. created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de Right. I might make the series inductors symmetrical, if I needed the tap point to be near signal ground (e.g. with a centertapped coil or a differential pair driving it). Last time I used this trick was in a 160-MHz phase shifter. The two inductors will generally be about the same size for best results with a hyperabrupt varactor--5 minutes with a math program will give you the right values. Generally you need to keep the reactance capacitive if you're resonating a crystal against this combination--there are multiple operating frequencies otherwise, since the resonator will look capacitive almost everywhere. Cheers, Phil Hobbs |
Paul Burridge wrote in message . ..
Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. Cheers, Tom |
Paul Burridge wrote in message . ..
Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. Cheers, Tom |
On Wed, 10 Dec 2003 00:26:34 GMT, "W3JDR" wrote:
Paul, The amount of tuning range is a function of the ratio of Cmax/Cmin. If you parallel varactors, Cmax will double, but so will Cmin. The ratio hasn't changed. Oh bugger. Well how about using varactors with a higher C/V ratio? If you're not already using a "hyper-abrupt" type of varactor, you should look into one. They offer a wider capacitance range. Not sure what you mean by that term, but imagine it amounts to simply a type with a higher capacitive reaction to applied voltage - as I mentioned above. What type of varactor are you using, and what's the frequency of the resonator? What's the application...linear frequency modulation like FM or data keying like FSK??? I'm currently using BB149A diodes, but I've got some BBY40s as well, which might offer more shift per volt; I haven't checked the spec yet. The fundamental frequency is 8.00Mhz and I need to pull it by +/-32khz for tuning rather than modulating purposes. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Wed, 10 Dec 2003 00:26:34 GMT, "W3JDR" wrote:
Paul, The amount of tuning range is a function of the ratio of Cmax/Cmin. If you parallel varactors, Cmax will double, but so will Cmin. The ratio hasn't changed. Oh bugger. Well how about using varactors with a higher C/V ratio? If you're not already using a "hyper-abrupt" type of varactor, you should look into one. They offer a wider capacitance range. Not sure what you mean by that term, but imagine it amounts to simply a type with a higher capacitive reaction to applied voltage - as I mentioned above. What type of varactor are you using, and what's the frequency of the resonator? What's the application...linear frequency modulation like FM or data keying like FSK??? I'm currently using BB149A diodes, but I've got some BBY40s as well, which might offer more shift per volt; I haven't checked the spec yet. The fundamental frequency is 8.00Mhz and I need to pull it by +/-32khz for tuning rather than modulating purposes. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Tue, 09 Dec 2003 17:28:22 -0700, Jim Thompson
wrote: Paul, That will work, but will double *both* min and max capacitance. But I'm puzzled: "I can't get enough shift with the available bias voltage" implies you're on the *low* end of capacitance (highest voltage). I'm using a 555 timer to generate a sawtooth waveform to feed the diodes, so I get a constant frequency sweep at the vcxo's output. Main problem is the limited voltage output range; starts above zero volts and peaks well before supply rail. So not much of a ramp; just around 4 or 5 volts, I guess. I could try changing the diodes for more responsive ones but they're SMDs and I really hate messin' with 'em. :-( -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Tue, 09 Dec 2003 17:28:22 -0700, Jim Thompson
wrote: Paul, That will work, but will double *both* min and max capacitance. But I'm puzzled: "I can't get enough shift with the available bias voltage" implies you're on the *low* end of capacitance (highest voltage). I'm using a 555 timer to generate a sawtooth waveform to feed the diodes, so I get a constant frequency sweep at the vcxo's output. Main problem is the limited voltage output range; starts above zero volts and peaks well before supply rail. So not much of a ramp; just around 4 or 5 volts, I guess. I could try changing the diodes for more responsive ones but they're SMDs and I really hate messin' with 'em. :-( -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Phil Hobbs wrote...
Tom Bruhns wrote: So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Sounds good. How about a specific example? Thanks, - Win whill_at_picovolt-dot-com |
Phil Hobbs wrote...
Tom Bruhns wrote: So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Sounds good. How about a specific example? Thanks, - Win whill_at_picovolt-dot-com |
On Wed, 10 Dec 2003 14:39:22 +0000, Phil Hobbs
wrote: Tom Bruhns wrote: So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Thanks, is this the kind of thing you mean? +-------+ | | | | | | C| | L1 C| | C| | | | | | V | D1 - | | C| Applied DC control voltage | C| L2 Line --------------------+ C| | | | | D2 - | ^ | | | | | | | +-------+ View in FP font. created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Wed, 10 Dec 2003 14:39:22 +0000, Phil Hobbs
wrote: Tom Bruhns wrote: So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Thanks, is this the kind of thing you mean? +-------+ | | | | | | C| | L1 C| | C| | | | | | V | D1 - | | C| Applied DC control voltage | C| L2 Line --------------------+ C| | | | | D2 - | ^ | | | | | | | +-------+ View in FP font. created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Wed, 10 Dec 2003 19:53:30 +0000, Paul Burridge
wrote: On Tue, 09 Dec 2003 17:28:22 -0700, Jim Thompson wrote: I'm using a 555 timer to generate a sawtooth waveform to feed the diodes, so I get a constant frequency sweep at the vcxo's output. Main problem is the limited voltage output range; starts above zero volts and peaks well before supply rail. So not much of a ramp; just around 4 or 5 volts, I guess. I could try changing the diodes for more The 555 thresholds are set to charge and discharge the timing cap at 1/3 and 2/3rds the supply voltage. If you using that directly the easiest way to get more "swing" is : Use an opamp to translate the voltage lower and add some gain (use at least 12v on the opamp). That can get you a Tuning voltage that is near 0 to near 12V (that should help). OR use a higher Vcc on the 555, say 12v. That will get your total swing to about 4V and the low will be 4v and the peak will be 8v. There are tricks that can be used to "offset" that 1/3 and 2/3 point but the total swing is usually the same. That can help as operating the Varicap closer to 0V will allow you to use more of it's capacitance range though it's usualy less linear at the bottom. Myself I'd use an opamp to create a saw generator and then I can control the swings. Allison |
On Wed, 10 Dec 2003 19:53:30 +0000, Paul Burridge
wrote: On Tue, 09 Dec 2003 17:28:22 -0700, Jim Thompson wrote: I'm using a 555 timer to generate a sawtooth waveform to feed the diodes, so I get a constant frequency sweep at the vcxo's output. Main problem is the limited voltage output range; starts above zero volts and peaks well before supply rail. So not much of a ramp; just around 4 or 5 volts, I guess. I could try changing the diodes for more The 555 thresholds are set to charge and discharge the timing cap at 1/3 and 2/3rds the supply voltage. If you using that directly the easiest way to get more "swing" is : Use an opamp to translate the voltage lower and add some gain (use at least 12v on the opamp). That can get you a Tuning voltage that is near 0 to near 12V (that should help). OR use a higher Vcc on the 555, say 12v. That will get your total swing to about 4V and the low will be 4v and the peak will be 8v. There are tricks that can be used to "offset" that 1/3 and 2/3 point but the total swing is usually the same. That can help as operating the Varicap closer to 0V will allow you to use more of it's capacitance range though it's usualy less linear at the bottom. Myself I'd use an opamp to create a saw generator and then I can control the swings. Allison |
Winfield Hill wrote:
Phil Hobbs wrote... Tom Bruhns wrote: So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Sounds good. How about a specific example? Thanks, - Win whill_at_picovolt-dot-com Last time I used this was with an MV104 common-cathode dual hyperabrupt, to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature hybrid in the usual way, coming in the 0 degree port, coming out the 180 degree port, and hanging matched reactances on the 90 degree ports. Each section had its own inductors, and the cathodes were bypassed heavily (1000 pF) to ground so that the two sides didn't interact too much. The component values were 45 nH in series and 43 nH in parallel. It was linear to within +-4 degrees, and the one section gave phase shifts from 12 to 164 degrees, both dramatically better than I could get with a bare varactor. The idea is to have the varactor resonate with the series inductor just off the low-voltage end of the range, and have the series combination resonate with the parallel L just off the high-voltage end of the range. Since the series-resonance doesn't even notice the parallel L, the design equations decouple nicely, too. You adjust the placement of the resonances to get the range and linearity desired. Cheers, Phil Hobbs |
Winfield Hill wrote:
Phil Hobbs wrote... Tom Bruhns wrote: So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Sounds good. How about a specific example? Thanks, - Win whill_at_picovolt-dot-com Last time I used this was with an MV104 common-cathode dual hyperabrupt, to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature hybrid in the usual way, coming in the 0 degree port, coming out the 180 degree port, and hanging matched reactances on the 90 degree ports. Each section had its own inductors, and the cathodes were bypassed heavily (1000 pF) to ground so that the two sides didn't interact too much. The component values were 45 nH in series and 43 nH in parallel. It was linear to within +-4 degrees, and the one section gave phase shifts from 12 to 164 degrees, both dramatically better than I could get with a bare varactor. The idea is to have the varactor resonate with the series inductor just off the low-voltage end of the range, and have the series combination resonate with the parallel L just off the high-voltage end of the range. Since the series-resonance doesn't even notice the parallel L, the design equations decouple nicely, too. You adjust the placement of the resonances to get the range and linearity desired. Cheers, Phil Hobbs |
Winfield Hill wrote:
Phil Hobbs wrote... Tom Bruhns wrote: So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Sounds good. How about a specific example? Thanks, - Win whill_at_picovolt-dot-com Last time I used this was with an MV104 common-cathode dual hyperabrupt, to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature hybrid in the usual way, coming in the 0 degree port, coming out the 180 degree port, and hanging matched reactances on the 90 degree ports. Each section had its own inductors, and the cathodes were bypassed heavily (1000 pF) to ground so that the two sides didn't interact too much. The component values were 45 nH in series and 43 nH in parallel. It was linear to within +-4 degrees, and the one section gave phase shifts from 12 to 164 degrees, both dramatically better than I could get with a bare varactor. The idea is to have the varactor resonate with the series inductor just off the low-voltage end of the range, and have the series combination resonate with the parallel L just off the high-voltage end of the range. Since the series-resonance doesn't even notice the parallel L, the design equations decouple nicely, too. You adjust the placement of the resonances to get the range and linearity desired. Cheers, Phil Hobbs |
Winfield Hill wrote:
Phil Hobbs wrote... Tom Bruhns wrote: So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Sounds good. How about a specific example? Thanks, - Win whill_at_picovolt-dot-com Last time I used this was with an MV104 common-cathode dual hyperabrupt, to make a 110-MHz phase shifter. It used a Mini-Circuits quadrature hybrid in the usual way, coming in the 0 degree port, coming out the 180 degree port, and hanging matched reactances on the 90 degree ports. Each section had its own inductors, and the cathodes were bypassed heavily (1000 pF) to ground so that the two sides didn't interact too much. The component values were 45 nH in series and 43 nH in parallel. It was linear to within +-4 degrees, and the one section gave phase shifts from 12 to 164 degrees, both dramatically better than I could get with a bare varactor. The idea is to have the varactor resonate with the series inductor just off the low-voltage end of the range, and have the series combination resonate with the parallel L just off the high-voltage end of the range. Since the series-resonance doesn't even notice the parallel L, the design equations decouple nicely, too. You adjust the placement of the resonances to get the range and linearity desired. Cheers, Phil Hobbs |
On Wed, 10 Dec 2003 23:28:06 GMT, wrote:
Myself I'd use an opamp to create a saw generator and then I can control the swings. Thanks, nospam. I can't imagine why I didn't think of this before. Got just the circuit lying about for a 741 sawtooth too! -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
In article , Winfield Hill
writes Phil Hobbs wrote... Tom Bruhns wrote: So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Sounds good. How about a specific example? Thanks, - Win whill_at_picovolt-dot-com Thats the way its done to pull crystals a long way. The thought/real experiment to assist is to assume that the acoustic resonator is resistive (zero phase) at series resonance. The maintaining circuit can then be replaced by an equivalent resistor. The circuit with resistor should oscillate at approx the resonator frequency.The inductor across the varicap is selected to almost tune out/parallel resonate with the varicap. The series inductor is phase retard to ensure the maintaining circuit tis zero phase . Adjustment of the varicap then moves the circuit above and below the series resonance of the resonator. -- ddwyer |
In article , Winfield Hill
writes Phil Hobbs wrote... Tom Bruhns wrote: So the right way to do this is to lower the _effective_ minimum capacitance. You can do that by adding an inductor, to cancel out capacitance. You can end up making the tuning range as wide as you want, but at the expense of the crystal (ceramic resonator in your case) being less of the overall frequency determination. In other words, there comes a point where you'd be as well off to just do an LC oscillator. But to double, say, the range, it's a good way to go. I guess I re-discovered what was already well known, but a few years ago I designed such a VCXO, and was amazed how linear the freq-vs-controlvoltage curve was (a good thing for use in a PLL). Don't know what range you're trying to achieve, but I had no trouble getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a crystal. An inductor in series with the varactors, then another one in parallel with the series combo can get you a very wide range of impedance from a decent varactor. Sounds good. How about a specific example? Thanks, - Win whill_at_picovolt-dot-com Thats the way its done to pull crystals a long way. The thought/real experiment to assist is to assume that the acoustic resonator is resistive (zero phase) at series resonance. The maintaining circuit can then be replaced by an equivalent resistor. The circuit with resistor should oscillate at approx the resonator frequency.The inductor across the varicap is selected to almost tune out/parallel resonate with the varicap. The series inductor is phase retard to ensure the maintaining circuit tis zero phase . Adjustment of the varicap then moves the circuit above and below the series resonance of the resonator. -- ddwyer |
W3JDR wrote:
Paul, The amount of tuning range is a function of the ratio of Cmax/Cmin. If you parallel varactors, Cmax will double, but so will Cmin. The ratio hasn't changed. If you're not already using a "hyper-abrupt" type of varactor, you should look into one. They offer a wider capacitance range. What type of varactor are you using, and what's the frequency of the resonator? What's the application...linear frequency modulation like FM or data keying like FSK??? Joe W3JDR "Paul Burridge" wrote in message ... Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. p. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill If he puts two diodes in parallel he will double the capacitance and will have to reduce the amount of inductance to have the same min. frequency. Im not sure that the high end won't be greater than before, even though the capacitance ratio is the same, since the fixed inductance is lower. Another idea would be to put the two varicaps in parallel, but switch one of them out as you approach the upper frequency. |
W3JDR wrote:
Paul, The amount of tuning range is a function of the ratio of Cmax/Cmin. If you parallel varactors, Cmax will double, but so will Cmin. The ratio hasn't changed. If you're not already using a "hyper-abrupt" type of varactor, you should look into one. They offer a wider capacitance range. What type of varactor are you using, and what's the frequency of the resonator? What's the application...linear frequency modulation like FM or data keying like FSK??? Joe W3JDR "Paul Burridge" wrote in message ... Hi, I'm currently working on this VCXO that achieves frequency shift by applying DC bias to two varactor diodes connected cathode to cathode (bias applied to the junction between them). If I can't get enough shift with the available bias voltage, is there any problem with just putting another pair of the same diodes in parallel with the existing ones? This is a ceramic resonator oscillator, BTW, so will stand a lot more 'pulling' than a xtal would, so don't worry about that aspect of it. p. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill If he puts two diodes in parallel he will double the capacitance and will have to reduce the amount of inductance to have the same min. frequency. Im not sure that the high end won't be greater than before, even though the capacitance ratio is the same, since the fixed inductance is lower. Another idea would be to put the two varicaps in parallel, but switch one of them out as you approach the upper frequency. |
If he puts two diodes in parallel he will double the capacitance and
will have to reduce the amount of inductance to have the same min. frequency. Im not sure that the high end won't be greater than before, even though the capacitance ratio is the same, since the fixed inductance is lower. Another idea would be to put the two varicaps in parallel, but switch one of them out as you approach the upper frequency. Kenneth, F=1/((2*PI)*SQR(L*C)) If you double C, you have to halve L to maintain the same frequency. If you do this, you only changed the LC ratio, not the delta tuning range. The only way to get more delta F is to get more delta C. If you use the switching technique, you'll have a discontinuous tuning curve (Vtune vs Freq) which makes it hard to implement a closed loop tuning system. It can be done, but the control loop gets complicated. A previous poster suggested what I'd called a "synthetic reactance", which is a series-parallel LC combination. This technique can produce very large effective-capacitance changes with a modest varactor range, however it also comes with the susceptibility of mode-jumping in the output frequency. Joe W3JDR |
If he puts two diodes in parallel he will double the capacitance and
will have to reduce the amount of inductance to have the same min. frequency. Im not sure that the high end won't be greater than before, even though the capacitance ratio is the same, since the fixed inductance is lower. Another idea would be to put the two varicaps in parallel, but switch one of them out as you approach the upper frequency. Kenneth, F=1/((2*PI)*SQR(L*C)) If you double C, you have to halve L to maintain the same frequency. If you do this, you only changed the LC ratio, not the delta tuning range. The only way to get more delta F is to get more delta C. If you use the switching technique, you'll have a discontinuous tuning curve (Vtune vs Freq) which makes it hard to implement a closed loop tuning system. It can be done, but the control loop gets complicated. A previous poster suggested what I'd called a "synthetic reactance", which is a series-parallel LC combination. This technique can produce very large effective-capacitance changes with a modest varactor range, however it also comes with the susceptibility of mode-jumping in the output frequency. Joe W3JDR |
On Sat, 13 Dec 2003 23:27:24 GMT, "W3JDR" wrote:
Another idea would be to put the two varicaps in parallel, but switch one of them out as you approach the upper frequency. Kenneth, F=1/((2*PI)*SQR(L*C)) If you double C, you have to halve L to maintain the same frequency. If you do this, you only changed the LC ratio, not the delta tuning range. The only way to get more delta F is to get more delta C. Suppose you mean greater Cmax/Cmin, larger delta C was already achieved above, but as you say "it doesn't work" The tuning range can be calculated as follows: (Cmax/Cmin)^2 = Fmax/Fmin If you use the switching technique, you'll have a discontinuous tuning curve (Vtune vs Freq) which makes it hard to implement a closed loop tuning system. It can be done, but the control loop gets complicated. Joe W3JDR You could always divide the tuning into two ranges and it shouldn't be too difficult to adjust the trimmer capacitors 73 LA8AK -- remove ,xnd to reply (Spam precaution!) |
On Sat, 13 Dec 2003 23:27:24 GMT, "W3JDR" wrote:
Another idea would be to put the two varicaps in parallel, but switch one of them out as you approach the upper frequency. Kenneth, F=1/((2*PI)*SQR(L*C)) If you double C, you have to halve L to maintain the same frequency. If you do this, you only changed the LC ratio, not the delta tuning range. The only way to get more delta F is to get more delta C. Suppose you mean greater Cmax/Cmin, larger delta C was already achieved above, but as you say "it doesn't work" The tuning range can be calculated as follows: (Cmax/Cmin)^2 = Fmax/Fmin If you use the switching technique, you'll have a discontinuous tuning curve (Vtune vs Freq) which makes it hard to implement a closed loop tuning system. It can be done, but the control loop gets complicated. Joe W3JDR You could always divide the tuning into two ranges and it shouldn't be too difficult to adjust the trimmer capacitors 73 LA8AK -- remove ,xnd to reply (Spam precaution!) |
A previous poster suggested what I'd called a "synthetic reactance", which
is a series-parallel LC combination. This technique can produce very large effective-capacitance changes with a modest varactor range, however it also comes with the susceptibility of mode-jumping in the output frequency. Joe W3JDR =================================== But perhaps the most important effect of this method of creating a multi-band or wideband tuned circuit is big deterioration in effective operating Q. Once upon a time it was a popular PA tuned-tank, minimum-dip, arrangement. At the higher frequencies the coils got hot. Too high a circulating current in the tank. Poor efficiency. They didn't catch on! Varactor diodes used in receiver and local oscillator circuits have a relatively poor Q to begin with. Perhaps lower than coil Q. Series tuned circuits in parallel with shunt tuned circuits only magnify adverse varactor effects. ---- Reg, G4FGQ |
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