On 7/27/2015 2:17 PM, John Larkin wrote:> On Sun, 26 Jul 2015 14:20:19 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 7/26/2015 2:15 PM, bitrex wrote: >>> On 7/26/2015 8:02 AM, Phil Hobbs wrote: >>>> On 7/26/2015 12:17 AM, John Larkin wrote: >>>>> On Sat, 25 Jul 2015 23:53:34 -0400, bitrex >>>>> <bitrex@de.lete.earthlink.net> wrote: >>>>> >>>>>> On 7/25/2015 11:07 PM, John Larkin wrote: >>>>>>> On Sat, 25 Jul 2015 21:17:32 -0400, bitrex >>>>>>> <bitrex@de.lete.earthlink.net> wrote: >>>>>>> >>>>>>>> On 7/25/2015 7:46 PM, John Larkin wrote: >>>>>>>>> On Sat, 25 Jul 2015 17:14:35 -0400, bitrex >>>>>>>>> <bitrex@de.lete.earthlink.net> wrote: >>>>>>>>> >>>>>>>>>> On 7/25/2015 12:12 PM, John Larkin wrote: >>>>>>>>>>> On Sat, 25 Jul 2015 10:56:05 -0400, bitrex >>>>>>>>>>> <bitrex@de.lete.earthlink.net> wrote: >>>>>>>>>>> >>>>>>>>>>>> I'm trying to simulate a low frequency Colpitts oscillator in >>>>>>>>>>>> LTSpice >>>>>>>>>>>> using an NPN darlington and a crystal, and I'm having a lot of >>>>>>>>>>>> trouble >>>>>>>>>>>> getting it to start up. Does anyone have a working one of >>>>>>>>>>>> these? I'm >>>>>>>>>>>> just using a capacitor to simulate the crystal, the parameters >>>>>>>>>>>> I am >>>>>>>>>>>> working with are something like: >>>>>>>>>>>> >>>>>>>>>>>> motional capacitance: 5.3ff >>>>>>>>>>>> ESR = 11k >>>>>>>>>>>> ESI ~ 4500 henries >>>>>>>>>>>> EPR = 100 meg >>>>>>>>>>>> EPC = 1.5pf >>>>>>>>>>>> >>>>>>>>>>>> Does this seem plausible? >>>>>>>>>>> >>>>>>>>>>> Select "skip initial operating point" in the transient >>>>>>>>>>> analysis, or it >>>>>>>>>>> will never start. Or goose it as Phil suggests. >>>>>>>>>>> >>>>>>>>>>> XOs are terrible to sim in time domain. The sims take forever and >>>>>>>>>>> there's no reasonable way to measure the frequency to PPM >>>>>>>>>>> resolution. >>>>>>>>>>> Once I get one running, I cut over to AC loop analysis to >>>>>>>>>>> fine-tune >>>>>>>>>>> things. >>>>>>>>>>> >>>>>>>>>>> Post your netlist so people can play with it. >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>> >>>>>>>>>> Here's a Dropbox link to the files: >>>>>>>>>> >>>>>>>>>> https://www.dropbox.com/sh/txmcmhn6p7fhuh2/AACKLnAh-41bI2jO7UT4wjCza?dl=0 >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> I'm actually attempting to use the Darlington buffer inside a >>>>>>>>>> LM13700 >>>>>>>>>> section to build a Colpitts crystal oscillator, which will then >>>>>>>>>> be 2 >>>>>>>>>> quadrant multiplied by an external signal within the >>>>>>>>>> transconductance >>>>>>>>>> amp itself... >>>>>>>>> >>>>>>>>> I'd guess that C2 and C3 are way too big. >>>>>>>>> >>>>>>>>> >>>>>>>> >>>>>>>> Lowering C2 and C3 to 470p, making the emitter resistor 4.7k, keeping >>>>>>>> the feedback cap the same and setting the crystal resonant >>>>>>>> frequency to >>>>>>>> 500kHz makes it start up. >>>>>>> >>>>>>> Don't quit now; keep going. >>>>>>> >>>>>>> And maybe make the upper one a bit smaller than the lower one. But I >>>>>>> never understood Colpitts oscillators. >>>>>>> >>>>>>> I did recently design one using a coaxial ceramic resonator, at 600 >>>>>>> MHz. It works fine even if I don't understand it. >>>>>> >>>>>> Wes Hayward's book "Radio Frequency Design" has a pretty good >>>>>> section on >>>>>> analysis of the various LC oscillator topologies on pages 265-290, and >>>>>> how to plot the gain and phase angle as a function of emitter current. >>>>>> >>>>>> The book is very math-heavy in general, it's like the anti-AoE. >>>>> >>>>> Spice has made all sorts of math unnecessary. Math tends to sputter >>>>> out when things get nonlinear anyhow. >>>>> >>>>> >>>> >>>> Oscillator and high frequency amplifier design are special cases, where >>>> grinding out the math is important. Even in a classical Colpitts, the >>>> choice of collector current, tank impedance, the ratio of the tank caps, >>>> and the size of the emitter and base coupling caps have a lot of >>>> influence on frequency stability and phase noise. >>>> >>>> That's a 5-D search space, so getting it right can take awhile >>>> numerically, and you can never really be sure you're not on a local >>>> optimum. >>>> >>>> You only have to do it once for each topology, then you just use Mathcad >>>> or Mathematica or something to apply it to the case at hand. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>>> >>> >>> I'm pretty skilled with Numpy and SciPy. Do you have an interesting set >>> of equations I can set it to work grinding on? >> >> Not right handy--I haven't done a crystal oscillator in years. >> > > It's really easier to buy oscillators. The phase noise and suchlike > specs are right there already. A pretty good OCXO can be cheaper than > buying just an SC-cut crystal. > >Yup. I mostly build oscillators in protos these days, or if I need to do something fancy, such as tuning it faster than the RC network on the varactor wants to let me, or getting more than an octave of tuning. (Both from recent examples--LC not crystal, obviously.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Colpitts crystal oscillator in LTSPICE
Started by ●July 25, 2015
Reply by ●July 27, 20152015-07-27
Reply by ●July 27, 20152015-07-27
On Mon, 27 Jul 2015 17:30:35 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>>> >> >> It's really easier to buy oscillators. The phase noise and suchlike >> specs are right there already. A pretty good OCXO can be cheaper than >> buying just an SC-cut crystal. >> >> >Yup. I mostly build oscillators in protos these days, or if I need to >do something fancy, such as tuning it faster than the RC network on the >varactor wants to let me, or getting more than an octave of tuning. >(Both from recent examples--LC not crystal, obviously.) > >Cheers > >Phil HobbsVCOs tend to have miserable modulation bandwidths, if specified at all. I'd like to make or buy a VCO that runs at 1.5 or 2 GHz maybe, with 500 MHz mod bandwidth. Looks difficult. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●July 27, 20152015-07-27
On 7/27/2015 6:59 PM, John Larkin wrote:> On Mon, 27 Jul 2015 17:30:35 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >>>> >>> >>> It's really easier to buy oscillators. The phase noise and suchlike >>> specs are right there already. A pretty good OCXO can be cheaper than >>> buying just an SC-cut crystal. >>> >>> >> Yup. I mostly build oscillators in protos these days, or if I need to >> do something fancy, such as tuning it faster than the RC network on the >> varactor wants to let me, or getting more than an octave of tuning. >> (Both from recent examples--LC not crystal, obviously.) >> >> Cheers >> >> Phil Hobbs > > VCOs tend to have miserable modulation bandwidths, if specified at > all.A lot of them have time constants in the 10-100 microsecond range, even for a UHF VCO. Pathetic.> > I'd like to make or buy a VCO that runs at 1.5 or 2 GHz maybe, with > 500 MHz mod bandwidth. Looks difficult.There are a bunch of parametric effects that you have to worry about in a case like that. What do you want it for? Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●July 28, 20152015-07-28
On Mon, 27 Jul 2015 21:22:40 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 7/27/2015 6:59 PM, John Larkin wrote: >> On Mon, 27 Jul 2015 17:30:35 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>>>> >>>> >>>> It's really easier to buy oscillators. The phase noise and suchlike >>>> specs are right there already. A pretty good OCXO can be cheaper than >>>> buying just an SC-cut crystal. >>>> >>>> >>> Yup. I mostly build oscillators in protos these days, or if I need to >>> do something fancy, such as tuning it faster than the RC network on the >>> varactor wants to let me, or getting more than an octave of tuning. >>> (Both from recent examples--LC not crystal, obviously.) >>> >>> Cheers >>> >>> Phil Hobbs >> >> VCOs tend to have miserable modulation bandwidths, if specified at >> all. > >A lot of them have time constants in the 10-100 microsecond range, even >for a UHF VCO. Pathetic. > >> >> I'd like to make or buy a VCO that runs at 1.5 or 2 GHz maybe, with >> 500 MHz mod bandwidth. Looks difficult. > >There are a bunch of parametric effects that you have to worry about in >a case like that. What do you want it for? > >Cheers > >Phil HobbsI want to transport some analog signals out of a high EMI environment, over fiber. The customer wants at least 500 MHz bandwidth. Think fiber-optic scope probe. I got some fiber-coupled LEDs, which will modulate (baseband, pure analog) nicely without mode jumps or interference wobbles, but they are too slow. I was thinking PWM or FM with a laser, but the FM thing will be difficult, as you note. Blindingly fast PWM is a possibility. The budget doesn't include lithium niobate modulators. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●July 28, 20152015-07-28
On 07/28/2015 01:43 PM, John Larkin wrote:> On Mon, 27 Jul 2015 21:22:40 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 7/27/2015 6:59 PM, John Larkin wrote: >>> On Mon, 27 Jul 2015 17:30:35 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>>>> >>>>> >>>>> It's really easier to buy oscillators. The phase noise and suchlike >>>>> specs are right there already. A pretty good OCXO can be cheaper than >>>>> buying just an SC-cut crystal. >>>>> >>>>> >>>> Yup. I mostly build oscillators in protos these days, or if I need to >>>> do something fancy, such as tuning it faster than the RC network on the >>>> varactor wants to let me, or getting more than an octave of tuning. >>>> (Both from recent examples--LC not crystal, obviously.) >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> VCOs tend to have miserable modulation bandwidths, if specified at >>> all. >> >> A lot of them have time constants in the 10-100 microsecond range, even >> for a UHF VCO. Pathetic. >> >>> >>> I'd like to make or buy a VCO that runs at 1.5 or 2 GHz maybe, with >>> 500 MHz mod bandwidth. Looks difficult. >> >> There are a bunch of parametric effects that you have to worry about in >> a case like that. What do you want it for? >> >> Cheers >> >> Phil Hobbs > > I want to transport some analog signals out of a high EMI environment, > over fiber. The customer wants at least 500 MHz bandwidth. Think > fiber-optic scope probe. > > I got some fiber-coupled LEDs, which will modulate (baseband, pure > analog) nicely without mode jumps or interference wobbles, but they > are too slow. I was thinking PWM or FM with a laser, but the FM thing > will be difficult, as you note. Blindingly fast PWM is a possibility. > The budget doesn't include lithium niobate modulators. > >Near a CO2 laser? One approach is to current-tune a laser, and demodulate it using a delay discriminator. You can use the delay discrim for AFC as well, just to maintain an operating point. The AFC can be really slow, so you can filter the daylights out of it. The delay discriminator will get you a full range signal for a small tuning input, but of course it'll be sinusoidal, so it'll need calibrating. That can all be done easily with connectorized fibre parts--you just need two 50:50 couplers with a few centimetres of path difference. Another approach would be to put AM on a subcarrier, and modulate the diode with that. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●July 28, 20152015-07-28
On Tue, 28 Jul 2015 13:53:56 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 07/28/2015 01:43 PM, John Larkin wrote: >> On Mon, 27 Jul 2015 21:22:40 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 7/27/2015 6:59 PM, John Larkin wrote: >>>> On Mon, 27 Jul 2015 17:30:35 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>>>>> >>>>>> >>>>>> It's really easier to buy oscillators. The phase noise and suchlike >>>>>> specs are right there already. A pretty good OCXO can be cheaper than >>>>>> buying just an SC-cut crystal. >>>>>> >>>>>> >>>>> Yup. I mostly build oscillators in protos these days, or if I need to >>>>> do something fancy, such as tuning it faster than the RC network on the >>>>> varactor wants to let me, or getting more than an octave of tuning. >>>>> (Both from recent examples--LC not crystal, obviously.) >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>> >>>> VCOs tend to have miserable modulation bandwidths, if specified at >>>> all. >>> >>> A lot of them have time constants in the 10-100 microsecond range, even >>> for a UHF VCO. Pathetic. >>> >>>> >>>> I'd like to make or buy a VCO that runs at 1.5 or 2 GHz maybe, with >>>> 500 MHz mod bandwidth. Looks difficult. >>> >>> There are a bunch of parametric effects that you have to worry about in >>> a case like that. What do you want it for? >>> >>> Cheers >>> >>> Phil Hobbs >> >> I want to transport some analog signals out of a high EMI environment, >> over fiber. The customer wants at least 500 MHz bandwidth. Think >> fiber-optic scope probe. >> >> I got some fiber-coupled LEDs, which will modulate (baseband, pure >> analog) nicely without mode jumps or interference wobbles, but they >> are too slow. I was thinking PWM or FM with a laser, but the FM thing >> will be difficult, as you note. Blindingly fast PWM is a possibility. >> The budget doesn't include lithium niobate modulators. >> >> >Near a CO2 laser?No. I don't know the actual application, other than the EMI environment is extreme. I assume that battery power should be an option. But we get occasional requests for this sort of thing, so I'm playing with it. Slower stuff we could just digitize.> >One approach is to current-tune a laser, and demodulate it using a delay >discriminator. You can use the delay discrim for AFC as well, just to >maintain an operating point. The AFC can be really slow, so you can >filter the daylights out of it. The delay discriminator will get you a >full range signal for a small tuning input, but of course it'll be >sinusoidal, so it'll need calibrating. > >That can all be done easily with connectorized fibre parts--you just >need two 50:50 couplers with a few centimetres of path difference. > >Another approach would be to put AM on a subcarrier, and modulate the >diode with that.We might AM modulate a laser directly, if we can avoid mode jumps and interferance effects. FM or PWM would make gain independent of fiber and connector losses, which would be nice. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●July 28, 20152015-07-28
On 07/28/2015 02:05 PM, John Larkin wrote:> On Tue, 28 Jul 2015 13:53:56 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 07/28/2015 01:43 PM, John Larkin wrote: >>> On Mon, 27 Jul 2015 21:22:40 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 7/27/2015 6:59 PM, John Larkin wrote: >>>>> On Mon, 27 Jul 2015 17:30:35 -0400, Phil Hobbs >>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>> >>>>>>>> >>>>>>> >>>>>>> It's really easier to buy oscillators. The phase noise and suchlike >>>>>>> specs are right there already. A pretty good OCXO can be cheaper than >>>>>>> buying just an SC-cut crystal. >>>>>>> >>>>>>> >>>>>> Yup. I mostly build oscillators in protos these days, or if I need to >>>>>> do something fancy, such as tuning it faster than the RC network on the >>>>>> varactor wants to let me, or getting more than an octave of tuning. >>>>>> (Both from recent examples--LC not crystal, obviously.) >>>>>> >>>>>> Cheers >>>>>> >>>>>> Phil Hobbs >>>>> >>>>> VCOs tend to have miserable modulation bandwidths, if specified at >>>>> all. >>>> >>>> A lot of them have time constants in the 10-100 microsecond range, even >>>> for a UHF VCO. Pathetic. >>>> >>>>> >>>>> I'd like to make or buy a VCO that runs at 1.5 or 2 GHz maybe, with >>>>> 500 MHz mod bandwidth. Looks difficult. >>>> >>>> There are a bunch of parametric effects that you have to worry about in >>>> a case like that. What do you want it for? >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> I want to transport some analog signals out of a high EMI environment, >>> over fiber. The customer wants at least 500 MHz bandwidth. Think >>> fiber-optic scope probe. >>> >>> I got some fiber-coupled LEDs, which will modulate (baseband, pure >>> analog) nicely without mode jumps or interference wobbles, but they >>> are too slow. I was thinking PWM or FM with a laser, but the FM thing >>> will be difficult, as you note. Blindingly fast PWM is a possibility. >>> The budget doesn't include lithium niobate modulators. >>> >>> >> Near a CO2 laser? > > No. I don't know the actual application, other than the EMI > environment is extreme. I assume that battery power should be an > option. But we get occasional requests for this sort of thing, so I'm > playing with it. Slower stuff we could just digitize. > >> >> One approach is to current-tune a laser, and demodulate it using a delay >> discriminator. You can use the delay discrim for AFC as well, just to >> maintain an operating point. The AFC can be really slow, so you can >> filter the daylights out of it. The delay discriminator will get you a >> full range signal for a small tuning input, but of course it'll be >> sinusoidal, so it'll need calibrating. >> >> That can all be done easily with connectorized fibre parts--you just >> need two 50:50 couplers with a few centimetres of path difference. >> >> Another approach would be to put AM on a subcarrier, and modulate the >> diode with that. > > We might AM modulate a laser directly, if we can avoid mode jumps and > interferance effects. FM or PWM would make gain independent of fiber > and connector losses, which would be nice.You can't do that over a very wide amplitude range, though, whereas with FM you get a zero-centred output basically for free, and the tuning range is narrow enough (some fraction of a wave number, maybe 10 GHz p-p) that you don't have to worry about mode hops much. Connectorized fibre couplers are pretty cheap if you get them from China, and they'll make you the unbalanced Mach-Zehnder as a single small assembly, complete with two photodiodes, for a reasonable price in quantities of > 10 or so. (Check out Photop, who are the ones I've used. Great outfit.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●July 28, 20152015-07-28
On Mon, 27 Jul 2015 15:59:25 -0700, John Larkin <jlarkin@highlandtechnology.com> wrote:> >I'd like to make or buy a VCO that runs at 1.5 or 2 GHz maybe, with >500 MHz mod bandwidth. Looks difficult.Run the VCO at say 4.5-5 GHz, mix it down with a fixed 3 GHz oscillator to get 1.5-2 GHz. It should be fairly simple to filter out the sum (7.5-8 GHz). The 3 GHz is outside both the desired frequency range as well as the actual VCO range.
Reply by ●July 28, 20152015-07-28
On Tue, 28 Jul 2015 10:43:59 -0700, John Larkin <jlarkin@highlandtechnology.com> wrote:>On Mon, 27 Jul 2015 21:22:40 -0400, Phil Hobbs ><pcdhSpamMeSenseless@electrooptical.net> wrote: > >>On 7/27/2015 6:59 PM, John Larkin wrote: >>> On Mon, 27 Jul 2015 17:30:35 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>>>> >>>>> >>>>> It's really easier to buy oscillators. The phase noise and suchlike >>>>> specs are right there already. A pretty good OCXO can be cheaper than >>>>> buying just an SC-cut crystal. >>>>> >>>>> >>>> Yup. I mostly build oscillators in protos these days, or if I need to >>>> do something fancy, such as tuning it faster than the RC network on the >>>> varactor wants to let me, or getting more than an octave of tuning. >>>> (Both from recent examples--LC not crystal, obviously.) >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> VCOs tend to have miserable modulation bandwidths, if specified at >>> all. >> >>A lot of them have time constants in the 10-100 microsecond range, even >>for a UHF VCO. Pathetic. >> >>> >>> I'd like to make or buy a VCO that runs at 1.5 or 2 GHz maybe, with >>> 500 MHz mod bandwidth. Looks difficult. >> >>There are a bunch of parametric effects that you have to worry about in >>a case like that. What do you want it for? >> >>Cheers >> >>Phil Hobbs > >I want to transport some analog signals out of a high EMI environment, >over fiber. The customer wants at least 500 MHz bandwidth. Think >fiber-optic scope probe. > >I got some fiber-coupled LEDs, which will modulate (baseband, pure >analog) nicely without mode jumps or interference wobbles, but they >are too slow. I was thinking PWM or FM with a laser, but the FM thing >will be difficult, as you note. Blindingly fast PWM is a possibility. >The budget doesn't include lithium niobate modulators.What did they used to send analog CATV signals over long distances using fibre ? Apparently they just amplitude modulated the light using the full CATV downlink frequency range.
Reply by ●July 28, 20152015-07-28
On Wed, 29 Jul 2015 00:29:39 +0300, upsidedown@downunder.com wrote:>On Tue, 28 Jul 2015 10:43:59 -0700, John Larkin ><jlarkin@highlandtechnology.com> wrote: > >>On Mon, 27 Jul 2015 21:22:40 -0400, Phil Hobbs >><pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>>On 7/27/2015 6:59 PM, John Larkin wrote: >>>> On Mon, 27 Jul 2015 17:30:35 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>>>>> >>>>>> >>>>>> It's really easier to buy oscillators. The phase noise and suchlike >>>>>> specs are right there already. A pretty good OCXO can be cheaper than >>>>>> buying just an SC-cut crystal. >>>>>> >>>>>> >>>>> Yup. I mostly build oscillators in protos these days, or if I need to >>>>> do something fancy, such as tuning it faster than the RC network on the >>>>> varactor wants to let me, or getting more than an octave of tuning. >>>>> (Both from recent examples--LC not crystal, obviously.) >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>> >>>> VCOs tend to have miserable modulation bandwidths, if specified at >>>> all. >>> >>>A lot of them have time constants in the 10-100 microsecond range, even >>>for a UHF VCO. Pathetic. >>> >>>> >>>> I'd like to make or buy a VCO that runs at 1.5 or 2 GHz maybe, with >>>> 500 MHz mod bandwidth. Looks difficult. >>> >>>There are a bunch of parametric effects that you have to worry about in >>>a case like that. What do you want it for? >>> >>>Cheers >>> >>>Phil Hobbs >> >>I want to transport some analog signals out of a high EMI environment, >>over fiber. The customer wants at least 500 MHz bandwidth. Think >>fiber-optic scope probe. >> >>I got some fiber-coupled LEDs, which will modulate (baseband, pure >>analog) nicely without mode jumps or interference wobbles, but they >>are too slow. I was thinking PWM or FM with a laser, but the FM thing >>will be difficult, as you note. Blindingly fast PWM is a possibility. >>The budget doesn't include lithium niobate modulators. > >What did they used to send analog CATV signals over long distances >using fibre ? Apparently they just amplitude modulated the light using >the full CATV downlink frequency range. >I think they generally use lithium niobate Mach-Zender (interferance based) modulators, which tend to be expensive. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
