These look great, very high Q. I'm thinking about a 50 or maybe 100 MHz Colpitts oscillator PLL. http://www.coilcraft.com/1515sq.cfm I wonder if the thermal expansion tempco of the FR4 board will stretch the coil and change its native tempco. FR4 is variously cited as being around +5 to +17 ppm/K, which isn't bad. That might even reduce the tempco of the inductor. I also wonder how a PCB ground plane effects L and Q. I guess I'll order a kit and try it.
inductor tempco
Started by ●December 13, 2015
Reply by ●December 13, 20152015-12-13
On Sun, 13 Dec 2015 17:26:05 -0800, John Larkin <jjlarkin@highlandtechnology.com> wrote:> >These look great, very high Q. I'm thinking about a 50 or maybe 100 >MHz Colpitts oscillator PLL. > >http://www.coilcraft.com/1515sq.cfm > >I wonder if the thermal expansion tempco of the FR4 board will stretch >the coil and change its native tempco. FR4 is variously cited as being >around +5 to +17 ppm/K, which isn't bad. That might even reduce the >tempco of the inductor. > >I also wonder how a PCB ground plane effects L and Q. I guess I'll >order a kit and try it. > >I meant "affects" of course.
Reply by ●December 13, 20152015-12-13
On 12/13/2015 7:26 PM, John Larkin wrote:> > These look great, very high Q. I'm thinking about a 50 or maybe 100 > MHz Colpitts oscillator PLL. > > http://www.coilcraft.com/1515sq.cfm > > I wonder if the thermal expansion tempco of the FR4 board will stretch > the coil and change its native tempco. FR4 is variously cited as being > around +5 to +17 ppm/K, which isn't bad. That might even reduce the > tempco of the inductor. > > I also wonder how a PCB ground plane effects L and Q. I guess I'll > order a kit and try it.If you do, please post your findings. Colpitts with a JFET is a good way to go. Low distortion.
Reply by ●December 13, 20152015-12-13
On Monday, 14 December 2015 12:26:17 UTC+11, John Larkin wrote:> These look great, very high Q. I'm thinking about a 50 or maybe 100 > MHz Colpitts oscillator PLL. > > http://www.coilcraft.com/1515sq.cfm > > I wonder if the thermal expansion tempco of the FR4 board will stretch > the coil and change its native tempco. FR4 is variously cited as being > around +5 to +17 ppm/K, which isn't bad. That might even reduce the > tempco of the inductor.It depends on whether the individual turns of the coils are stuck together. If they were wound with self-bonding wire, movement in the FR4 would bend the mounting leads rather than changing the length of the coil. To get a maximum SRF in a cylindrical air coil, you are supposed to gap the turns by their thickness, which isn't the way the data sheet shows them http://www.coilcraft.com/pdfs/1515sq.pdf but the wires may have a thicker layer of insulation than you might expect. If the wire's wasn't circular to start with, life could get even more interesting. http://www.coilws.com/index.php?main_page=page&id=102> I also wonder how a PCB ground plane effects L and Q. I guess I'll > order a kit and try it.The stray field from a cylindrical coil mounted parallel to a ground plane would induce some current in the ground plane, and lower the inductance and the Q, but if you think about the currents induced in the ground plane, it can't be much. -- Bill Sloman, Sydney
Reply by ●December 13, 20152015-12-13
On Sun, 13 Dec 2015 19:56:02 -0600, John S <Sophi.2@invalid.org> wrote:>On 12/13/2015 7:26 PM, John Larkin wrote: >> >> These look great, very high Q. I'm thinking about a 50 or maybe 100 >> MHz Colpitts oscillator PLL. >> >> http://www.coilcraft.com/1515sq.cfm >> >> I wonder if the thermal expansion tempco of the FR4 board will stretch >> the coil and change its native tempco. FR4 is variously cited as being >> around +5 to +17 ppm/K, which isn't bad. That might even reduce the >> tempco of the inductor. >> >> I also wonder how a PCB ground plane effects L and Q. I guess I'll >> order a kit and try it. > >If you do, please post your findings. > >Colpitts with a JFET is a good way to go. Low distortion. > >I'm thinking about... https://dl.dropboxusercontent.com/u/53724080/Circuits/Oscillators/Colpitts.JPG It's a clock generator, so distortion doesn't matter much. Low tempco and low phase noise are good. I guess I'll build one and try it.
Reply by ●December 13, 20152015-12-13
On 12/13/2015 8:38 PM, John Larkin wrote:> On Sun, 13 Dec 2015 19:56:02 -0600, John S <Sophi.2@invalid.org> > wrote: > >> On 12/13/2015 7:26 PM, John Larkin wrote: >>> >>> These look great, very high Q. I'm thinking about a 50 or maybe 100 >>> MHz Colpitts oscillator PLL. >>> >>> http://www.coilcraft.com/1515sq.cfm >>> >>> I wonder if the thermal expansion tempco of the FR4 board will stretch >>> the coil and change its native tempco. FR4 is variously cited as being >>> around +5 to +17 ppm/K, which isn't bad. That might even reduce the >>> tempco of the inductor. >>> >>> I also wonder how a PCB ground plane effects L and Q. I guess I'll >>> order a kit and try it. >> >> If you do, please post your findings. >> >> Colpitts with a JFET is a good way to go. Low distortion. >> >> > > I'm thinking about... > > https://dl.dropboxusercontent.com/u/53724080/Circuits/Oscillators/Colpitts.JPG > > It's a clock generator, so distortion doesn't matter much. Low tempco > and low phase noise are good. > > I guess I'll build one and try it.Good idea. I LTSpiced it and it looks okay, but the amplitude is only +/- .8V. Is that enough?
Reply by ●December 14, 20152015-12-14
On Sun, 13 Dec 2015 21:15:27 -0600, John S <Sophi.2@invalid.org> wrote:>On 12/13/2015 8:38 PM, John Larkin wrote: >> On Sun, 13 Dec 2015 19:56:02 -0600, John S <Sophi.2@invalid.org> >> wrote: >> >>> On 12/13/2015 7:26 PM, John Larkin wrote: >>>> >>>> These look great, very high Q. I'm thinking about a 50 or maybe 100 >>>> MHz Colpitts oscillator PLL. >>>> >>>> http://www.coilcraft.com/1515sq.cfm >>>> >>>> I wonder if the thermal expansion tempco of the FR4 board will stretch >>>> the coil and change its native tempco. FR4 is variously cited as being >>>> around +5 to +17 ppm/K, which isn't bad. That might even reduce the >>>> tempco of the inductor. >>>> >>>> I also wonder how a PCB ground plane effects L and Q. I guess I'll >>>> order a kit and try it. >>> >>> If you do, please post your findings. >>> >>> Colpitts with a JFET is a good way to go. Low distortion. >>> >>> >> >> I'm thinking about... >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Oscillators/Colpitts.JPG >> >> It's a clock generator, so distortion doesn't matter much. Low tempco >> and low phase noise are good. >> >> I guess I'll build one and try it. > >Good idea. I LTSpiced it and it looks okay, but the amplitude is only >+/- .8V. Is that enough?Yup. It will drive a comparator to become a clock. The amplitude is limited by the c-b conduction of the transistor. With a high-Q inductor, it takes surprisingly little emitter current, like 100 uA, to oscillate. We'll coarse tune it with a digital capacitor, and fine-tune/phaselock it with a varicap. Varicaps are awful, so it's best to minimize the varicap pull range.
Reply by ●December 14, 20152015-12-14
On 14/12/2015 13:38, John Larkin wrote:> On Sun, 13 Dec 2015 19:56:02 -0600, John S <Sophi.2@invalid.org> > wrote: > >> On 12/13/2015 7:26 PM, John Larkin wrote: >>> >>> These look great, very high Q. I'm thinking about a 50 or maybe 100 >>> MHz Colpitts oscillator PLL. >>> >>> http://www.coilcraft.com/1515sq.cfm >>> >>> I wonder if the thermal expansion tempco of the FR4 board will stretch >>> the coil and change its native tempco. FR4 is variously cited as being >>> around +5 to +17 ppm/K, which isn't bad. That might even reduce the >>> tempco of the inductor. >>> >>> I also wonder how a PCB ground plane effects L and Q. I guess I'll >>> order a kit and try it. >> >> If you do, please post your findings. >> >> Colpitts with a JFET is a good way to go. Low distortion. >> >> > > I'm thinking about... > > https://dl.dropboxusercontent.com/u/53724080/Circuits/Oscillators/Colpitts.JPG > > It's a clock generator, so distortion doesn't matter much. Low tempco > and low phase noise are good. > > I guess I'll build one and try it. >It will be slightly microphonic though not as much as if the turns of the coil were not bonded together. Still an inductor deposited on ceramic or silicon would be more mechanically stable and have less thermal expansion. I suspect that if you are building a VCO, the varactors and other semiconductor junctions will dominate the temperature coefficient of frequency. It is difficult to get consistently low phase noise (and reliable oscillation, and predictable tuning sensitivity) if the amplitude is not controlled by some sort of feedback system. On cellphone chips there is pretty much always some sort of amplitude control. This has to be done carefully as it may introduce its own noise, but is worthwhile. If you need good phase noise then you would also want a quiet supply regulator. On cellphone chips it is common use one substantially better than 20nV/root-Hz. (For some reason people don't sell those as stand-alone regulators, you have to buy a cellphone radio with it.) At high frequencies you can get similar noise by low-pass filtering, but then the cost, size and startup time are orders of magnitude worse. If you want low phase noise close to the carrier (e.g. at 1kHz offset) then the supply needs to be quiet at frequencies similarly far from DC (e.g. 1kHz). It is also much less practical to design quiet oscillators without something like SpectreRF which can not only simulate phase noise of free-running oscillators (which you could also measure on a prototype) but also tell you which components the noise is coming from (which a prototype is not able to tell you). You might find that an integrated circuit microwave VCO plus a divider is better than building one from scratch, though I don't know if you need any special features that those would not offer as standard products. If you do build your own oscillator, and if you find that the inductor is the main cause of temperature coefficient, then you could use a ceramic coil former (or buy a coilcraft inductor that is wound on a block of ceramic). Provided the wire is tightly wound, I expect that its diameter will vary as does that of the ceramic, which would be more stable than copper or FR4. Chris
Reply by ●December 14, 20152015-12-14
On Mon, 14 Dec 2015 15:16:27 +1100, Chris Jones <lugnut808@spam.yahoo.com> wrote:>On 14/12/2015 13:38, John Larkin wrote: >> On Sun, 13 Dec 2015 19:56:02 -0600, John S <Sophi.2@invalid.org> >> wrote: >> >>> On 12/13/2015 7:26 PM, John Larkin wrote: >>>> >>>> These look great, very high Q. I'm thinking about a 50 or maybe 100 >>>> MHz Colpitts oscillator PLL. >>>> >>>> http://www.coilcraft.com/1515sq.cfm >>>> >>>> I wonder if the thermal expansion tempco of the FR4 board will stretch >>>> the coil and change its native tempco. FR4 is variously cited as being >>>> around +5 to +17 ppm/K, which isn't bad. That might even reduce the >>>> tempco of the inductor. >>>> >>>> I also wonder how a PCB ground plane effects L and Q. I guess I'll >>>> order a kit and try it. >>> >>> If you do, please post your findings. >>> >>> Colpitts with a JFET is a good way to go. Low distortion. >>> >>> >> >> I'm thinking about... >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Oscillators/Colpitts.JPG >> >> It's a clock generator, so distortion doesn't matter much. Low tempco >> and low phase noise are good. >> >> I guess I'll build one and try it. >> > >It will be slightly microphonic though not as much as if the turns of >the coil were not bonded together. Still an inductor deposited on >ceramic or silicon would be more mechanically stable and have less >thermal expansion. I suspect that if you are building a VCO, the >varactors and other semiconductor junctions will dominate the >temperature coefficient of frequency. > >It is difficult to get consistently low phase noise (and reliable >oscillation, and predictable tuning sensitivity) if the amplitude is not >controlled by some sort of feedback system. On cellphone chips there is >pretty much always some sort of amplitude control. This has to be done >carefully as it may introduce its own noise, but is worthwhile.In the circuit that I posted, amplitude is limited by conduction of the transistor c-b junction. That probably has costs.> >If you need good phase noise then you would also want a quiet supply >regulator. On cellphone chips it is common use one substantially better >than 20nV/root-Hz. (For some reason people don't sell those as >stand-alone regulators, you have to buy a cellphone radio with it.) At >high frequencies you can get similar noise by low-pass filtering, but >then the cost, size and startup time are orders of magnitude worse. If >you want low phase noise close to the carrier (e.g. at 1kHz offset) then >the supply needs to be quiet at frequencies similarly far from DC (e.g. >1kHz). > >It is also much less practical to design quiet oscillators without >something like SpectreRF which can not only simulate phase noise of >free-running oscillators (which you could also measure on a prototype) >but also tell you which components the noise is coming from (which a >prototype is not able to tell you). > >You might find that an integrated circuit microwave VCO plus a divider >is better than building one from scratch, though I don't know if you >need any special features that those would not offer as standard products.It's kind of a weird application, so I have to build my own oscillator.> >If you do build your own oscillator, and if you find that the inductor >is the main cause of temperature coefficient, then you could use a >ceramic coil former (or buy a coilcraft inductor that is wound on a >block of ceramic). Provided the wire is tightly wound, I expect that its >diameter will vary as does that of the ceramic, which would be more >stable than copper or FR4. > >Chris > > >On a previous generation of this sort of PLL, I used a Coilcraft inductor that's wound on a plastic form. They must wind it tight, because the plastic cold flows and the inductance creeps over time. We fixed that by baking them before assembly, to relieve the stress. Standard surface-mount inductors, 1206's and such, are handy and stable, but Q tends to be low. Coilcraft has an 1812 part with Q=80 at 50 MHz, which is fair. I wonder if paralleling them improves Q. I haven't used these springform inductors before, so I don't know if they will be stable. I'll try it I guess.
Reply by ●December 14, 20152015-12-14
On Sun, 13 Dec 2015 23:22:51 +1100, Clifford Heath <no.spam@please.net> wrote:>On 14/12/15 14:15, John S wrote: >> On 12/13/2015 8:38 PM, John Larkin wrote: >>> On Sun, 13 Dec 2015 19:56:02 -0600, John S <Sophi.2@invalid.org> >>>> On 12/13/2015 7:26 PM, John Larkin wrote: >>>>> These look great, very high Q. I'm thinking about a 50 or maybe 100 >>>>> MHz Colpitts oscillator PLL. >>>>> http://www.coilcraft.com/1515sq.cfm >>> I'm thinking about... >>> https://dl.dropboxusercontent.com/u/53724080/Circuits/Oscillators/Colpitts.JPG >>> It's a clock generator, so distortion doesn't matter much. Low tempco >>> and low phase noise are good. >>> I guess I'll build one and try it. >> Good idea. I LTSpiced it and it looks okay, but the amplitude is only >> +/- .8V. Is that enough? > >The amplitude at the BFT25A's base is about 40mV, based on the >capacitive divider ratio. Without having Spice'd it, there should be >enough gain headroom to double the output by increasing the divider >ratio, which will probably increase the signal purity also (more tank >energy, same base bleed). > >Clifford Heath.I'm simulating 1.7 volts p-p at the tank, and 170 mV p-p at the emitter. I'm not sure what might be the optimum capacitor ratio. 10:1 sounds unusually high, but it seems to work fine. I'm not an RF guy! Sinewaves are my least favorite waveform.
