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Varactor tuning diodes.
Hello,
I have a general question regarding tuning diodes: Does the oscillation voltage alter their capacitance, or is capacitance a slow changing value - like resistance and PIN diodes? The reason I ask is that I am spicing (simulation) a VCO I wish to build (wide range 140-240MHz) and get an oscillation voltage of 20 volts peak (maybe the Q is too high). This high oscillation voltage would easily swamp the tuning voltage, 3-15 volts, and forward bias the diode itself. If anyone wishs to reply directly they must remove the "xxx" from the return address or write to: james dot fenech at nec dot com dot au Thanks, James. |
The varactor capacitance change is instantaneous -- think of a capacitor
who's plate spacing is dependent on it's voltage. This is why varactors are used for frequency multiplication -- that capacitance change "squeezes" the pulse to generate lots of harmonics. So your oscillation voltage will certainly affect the tuning, and the forward bias effects will be most unfortunate. I have modelled varactor diodes by dinking with the area parameter of the SPICE diode model, but I'm not a pro in that regard and don't know the "right" way to do it. SPICE is also not the tool for determining the behavior of nonlinear high-Q circuits (I believe the best tool still involves FR-4 and lots of cussing). "James Fenech" wrote in message ... Hello, I have a general question regarding tuning diodes: Does the oscillation voltage alter their capacitance, or is capacitance a slow changing value - like resistance and PIN diodes? The reason I ask is that I am spicing (simulation) a VCO I wish to build (wide range 140-240MHz) and get an oscillation voltage of 20 volts peak (maybe the Q is too high). This high oscillation voltage would easily swamp the tuning voltage, 3-15 volts, and forward bias the diode itself. If anyone wishs to reply directly they must remove the "xxx" from the return address or write to: james dot fenech at nec dot com dot au Thanks, James. |
The varactor capacitance change is instantaneous -- think of a capacitor
who's plate spacing is dependent on it's voltage. This is why varactors are used for frequency multiplication -- that capacitance change "squeezes" the pulse to generate lots of harmonics. So your oscillation voltage will certainly affect the tuning, and the forward bias effects will be most unfortunate. I have modelled varactor diodes by dinking with the area parameter of the SPICE diode model, but I'm not a pro in that regard and don't know the "right" way to do it. SPICE is also not the tool for determining the behavior of nonlinear high-Q circuits (I believe the best tool still involves FR-4 and lots of cussing). "James Fenech" wrote in message ... Hello, I have a general question regarding tuning diodes: Does the oscillation voltage alter their capacitance, or is capacitance a slow changing value - like resistance and PIN diodes? The reason I ask is that I am spicing (simulation) a VCO I wish to build (wide range 140-240MHz) and get an oscillation voltage of 20 volts peak (maybe the Q is too high). This high oscillation voltage would easily swamp the tuning voltage, 3-15 volts, and forward bias the diode itself. If anyone wishs to reply directly they must remove the "xxx" from the return address or write to: james dot fenech at nec dot com dot au Thanks, James. |
On Sun, 4 Jan 2004 16:22:32 -0800, "Tim Wescott"
wrote: The varactor capacitance change is instantaneous -- think of a capacitor who's plate spacing is dependent on it's voltage. This is why varactors are used for frequency multiplication -- that capacitance change "squeezes" the pulse to generate lots of harmonics. Rubbish. Varactors are not used for freqency mulitplication. They are freequenty used in VXCOs for modulating (or otherwise varying) the oscillator output frequency by means of applying a DC bias voltage which alters the capacitance of the PN junction. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Sun, 4 Jan 2004 16:22:32 -0800, "Tim Wescott"
wrote: The varactor capacitance change is instantaneous -- think of a capacitor who's plate spacing is dependent on it's voltage. This is why varactors are used for frequency multiplication -- that capacitance change "squeezes" the pulse to generate lots of harmonics. Rubbish. Varactors are not used for freqency mulitplication. They are freequenty used in VXCOs for modulating (or otherwise varying) the oscillator output frequency by means of applying a DC bias voltage which alters the capacitance of the PN junction. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Paul Burridge wrote...
Tim Wescott wrote: The varactor capacitance change is instantaneous -- think of a capacitor who's plate spacing is dependent on it's voltage. This is why varactors are used for frequency multiplication -- that capacitance change "squeezes" the pulse to generate lots of harmonics. Rubbish. Varactors are not used for freqency mulitplication. They are freequenty used in VXCOs for modulating (or otherwise varying) the oscillator output frequency by means of applying a DC bias voltage which alters the capacitance of the PN junction. Your own rubbish, Paul. In truth of fact, varactors are nicely used for frequency multiplication, just as Tim stated. They also work well for VCO FM modulation, but users must keep in mind their instantaneous capacitance-change property and be careful to bias them in appropriate higher-voltage bias regions, or risk unintended distortion. When used as multipliers, low-voltage bias with large signals enhances the effect. Thanks, - Win whill_at_picovolt-dot-com |
Paul Burridge wrote...
Tim Wescott wrote: The varactor capacitance change is instantaneous -- think of a capacitor who's plate spacing is dependent on it's voltage. This is why varactors are used for frequency multiplication -- that capacitance change "squeezes" the pulse to generate lots of harmonics. Rubbish. Varactors are not used for freqency mulitplication. They are freequenty used in VXCOs for modulating (or otherwise varying) the oscillator output frequency by means of applying a DC bias voltage which alters the capacitance of the PN junction. Your own rubbish, Paul. In truth of fact, varactors are nicely used for frequency multiplication, just as Tim stated. They also work well for VCO FM modulation, but users must keep in mind their instantaneous capacitance-change property and be careful to bias them in appropriate higher-voltage bias regions, or risk unintended distortion. When used as multipliers, low-voltage bias with large signals enhances the effect. Thanks, - Win whill_at_picovolt-dot-com |
On Mon, 05 Jan 2004 02:40:07 +0000, Paul Burridge
wrote: Rubbish. Varactors are not used for freqency mulitplication. If you are determined to be arrogant, it helps to occasionally be right. Varactors are still used for frequency multiplication. They were also commonly used as parametric amplifiers and mixers before other semiconductors got fast enough to work at microwave frequencies. Varactors were even used as paramps in the front ends of opamps. Google "varactor frequency multiplier" and "parametric amplifier" and maybe learn something. John |
On Mon, 05 Jan 2004 02:40:07 +0000, Paul Burridge
wrote: Rubbish. Varactors are not used for freqency mulitplication. If you are determined to be arrogant, it helps to occasionally be right. Varactors are still used for frequency multiplication. They were also commonly used as parametric amplifiers and mixers before other semiconductors got fast enough to work at microwave frequencies. Varactors were even used as paramps in the front ends of opamps. Google "varactor frequency multiplier" and "parametric amplifier" and maybe learn something. John |
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Rubbish. Varactors are not used for freqency mulitplication. Google"varactor multipliers", still used. Popular technique in the 60's for getting 70 cm output from 2m TX. As osc control keep RF across diode low, high levels produce harmonics and reduce tuning range. Most tuning varieties have max V = 30. At low freqs power diodes work ok and have high V rev. |
Rubbish. Varactors are not used for freqency mulitplication. Google"varactor multipliers", still used. Popular technique in the 60's for getting 70 cm output from 2m TX. As osc control keep RF across diode low, high levels produce harmonics and reduce tuning range. Most tuning varieties have max V = 30. At low freqs power diodes work ok and have high V rev. |
On 4 Jan 2004 19:27:25 -0800, Winfield Hill
wrote: Your own rubbish, Paul. In truth of fact, varactors are nicely used for frequency multiplication, just as Tim stated. You're right. I can't believe I posted that load of nonsense. Maybe it's a case of "late onset mad cow disease." :-| -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On 4 Jan 2004 19:27:25 -0800, Winfield Hill
wrote: Your own rubbish, Paul. In truth of fact, varactors are nicely used for frequency multiplication, just as Tim stated. You're right. I can't believe I posted that load of nonsense. Maybe it's a case of "late onset mad cow disease." :-| -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Paul Burridge wrote...
You're right. I can't believe I posted that load of nonsense. Maybe it's a case of "late onset mad cow disease." :-| OK, you are forgiven, my son. Go and sin no more. Thanks, - Win whill_at_picovolt-dot-com |
Paul Burridge wrote...
You're right. I can't believe I posted that load of nonsense. Maybe it's a case of "late onset mad cow disease." :-| OK, you are forgiven, my son. Go and sin no more. Thanks, - Win whill_at_picovolt-dot-com |
If you aren't using the turnip truck you just fell from, could I borrow it for a
while? {;-) Jim Paul Burridge shared these priceless pearls of wisdom: - -Rubbish. Varactors are not used for freqency mulitplication. Jim Weir, VP Eng. RST Eng. WX6RST A&P, CFI, and other good alphabet soup |
If you aren't using the turnip truck you just fell from, could I borrow it for a
while? {;-) Jim Paul Burridge shared these priceless pearls of wisdom: - -Rubbish. Varactors are not used for freqency mulitplication. Jim Weir, VP Eng. RST Eng. WX6RST A&P, CFI, and other good alphabet soup |
On 5 Jan 2004 08:23:16 -0800, Winfield Hill
wrote: Paul Burridge wrote... You're right. I can't believe I posted that load of nonsense. Maybe it's a case of "late onset mad cow disease." :-| OK, you are forgiven, my son. Go and sin no more. Yes, m'lud. In mitigation I'd like to plead Jack Daniels. :-) -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On 5 Jan 2004 08:23:16 -0800, Winfield Hill
wrote: Paul Burridge wrote... You're right. I can't believe I posted that load of nonsense. Maybe it's a case of "late onset mad cow disease." :-| OK, you are forgiven, my son. Go and sin no more. Yes, m'lud. In mitigation I'd like to plead Jack Daniels. :-) -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Mon, 05 Jan 2004 09:14:28 -0800, Jim Weir wrote:
If you aren't using the turnip truck you just fell from, could I borrow it for a while? {;-) Hehe! Fair comment. Thank God I don't have any reputation to lose on this group. :-) -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Mon, 05 Jan 2004 09:14:28 -0800, Jim Weir wrote:
If you aren't using the turnip truck you just fell from, could I borrow it for a while? {;-) Hehe! Fair comment. Thank God I don't have any reputation to lose on this group. :-) -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
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Motorola's first solid state 450 car phone, the MK, used a 50 W 150 MHz PA
followed by a varactor tripler for 10W at 450. Varactor triplers are tricky however. Steve K;9;d/c/i "Paul Burridge" wrote in message ... On Sun, 4 Jan 2004 16:22:32 -0800, "Tim Wescott" wrote: The varactor capacitance change is instantaneous -- think of a capacitor who's plate spacing is dependent on it's voltage. This is why varactors are used for frequency multiplication -- that capacitance change "squeezes" the pulse to generate lots of harmonics. Rubbish. Varactors are not used for freqency mulitplication. They are freequenty used in VXCOs for modulating (or otherwise varying) the oscillator output frequency by means of applying a DC bias voltage which alters the capacitance of the PN junction. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Motorola's first solid state 450 car phone, the MK, used a 50 W 150 MHz PA
followed by a varactor tripler for 10W at 450. Varactor triplers are tricky however. Steve K;9;d/c/i "Paul Burridge" wrote in message ... On Sun, 4 Jan 2004 16:22:32 -0800, "Tim Wescott" wrote: The varactor capacitance change is instantaneous -- think of a capacitor who's plate spacing is dependent on it's voltage. This is why varactors are used for frequency multiplication -- that capacitance change "squeezes" the pulse to generate lots of harmonics. Rubbish. Varactors are not used for freqency mulitplication. They are freequenty used in VXCOs for modulating (or otherwise varying) the oscillator output frequency by means of applying a DC bias voltage which alters the capacitance of the PN junction. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Tue, 6 Jan 2004 17:26:59 -0600, "Steve Nosko"
wrote: Motorola's first solid state 450 car phone, the MK, used a 50 W 150 MHz PA followed by a varactor tripler for 10W at 450. Varactor triplers are tricky however. Steve K;9;d/c/i The worst is suppression of 150MHz, it is almost impossible to suppress it. Bought some UK commercial integrated modules for 432MHz which was fed from 144MHz 1W, had a 20W amplifier within a very well screened and silvered box, but radiation on 144MHz were almost impossible to suppress, the only way out was to place it deep at the lowest point of the basement with ferrite chokes on the cables LA8AK -- Amount of SPAM is so large that MailWasher must delete 99% of the incoming mails Cannot check every email manually. Please use intelligent title for email. Mails without titles or using just "hi" is deleted |
On Tue, 6 Jan 2004 17:26:59 -0600, "Steve Nosko"
wrote: Motorola's first solid state 450 car phone, the MK, used a 50 W 150 MHz PA followed by a varactor tripler for 10W at 450. Varactor triplers are tricky however. Steve K;9;d/c/i The worst is suppression of 150MHz, it is almost impossible to suppress it. Bought some UK commercial integrated modules for 432MHz which was fed from 144MHz 1W, had a 20W amplifier within a very well screened and silvered box, but radiation on 144MHz were almost impossible to suppress, the only way out was to place it deep at the lowest point of the basement with ferrite chokes on the cables LA8AK -- Amount of SPAM is so large that MailWasher must delete 99% of the incoming mails Cannot check every email manually. Please use intelligent title for email. Mails without titles or using just "hi" is deleted |
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