On Wed, 28 Sep 2011 06:26:05 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Sep 27, 6:27=A0pm, John S <soph...@invalid.org> wrote: >> On 9/27/2011 3:43 PM, Tim Wescott wrote: >> >> >> >> > The nonlinearity that you're battling isn't the diode conduction -- =it's>> > the diode's varying capacitance as its reverse voltage changes. ==A0It's>> > going to be severe at low voltage/high capacitance. =A0I'm _not_ the >> > world's smartest SPICE guy, but I do know (more or less) that you =want to>> > simulate the varactor with a really large-area diode. =A0I _think_ =that>> > there's an area parameter that you can dink with, if you copy a =diode>> > model out of the LTSpice library and paste it into your schematic. ==A0Or go>> > out on the World of the Wide Web and find a varactor model. >> >> > I'm thinking that if you want to avoid seeing major distortion you =want>> > to have the AC voltage be more like tens of millivolts, which will =leave>> > you open to noise problems. >> >> Tim - >> >> I agree. I do not know how to dink with a diode parameter, but I =managed>> to make a device to simulate a VVC in LTSpice. I no longer have that >> file, but I remember using either an arbitrary function generator or a >> voltage controlled current to simulate a VVC. It was then that I saw =the>> sine distortion. >> >> Sorry I don't have specifics. > >No problem, I was thinking that if I reversed the DC voltage across >one of the vari-caps then during the amplitude max of the sine wave >one vari-cap would be going up and the other down...that might help >reduce the distortion. It'll be a fun circuit to play with.. and I'll >use the vari-caps in a real LC oscillator. > >George H.=46or your stated task this site may give you a few useful ideas: http://midnightdesignsolutions.com/dds60/index.html ?-)
50 kHz VCO w/sine output
Started by ●September 27, 2011
Reply by ●October 1, 20112011-10-01
Reply by ●October 1, 20112011-10-01
On Wed, 28 Sep 2011 14:40:01 -0400, Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:>On Wed, 28 Sep 2011 08:57:52 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >> >> >>Another way to smooth a triangle onto a sine is with linear segment >>breakpoints. One opamp and a mess of resistors and diodes can do that, >>but there are several ways. >> >>Two breaks works pretty well. It can be done at high level, which is >>convenient. >> >>I wonder how you'd find the optimum break formula. Or, in general, how >>one best approximates a function, over some range, with N linear >>segments. >> >>John > >Many years ago I wrote an optimization program for doing this for a >nonlinear function with 'n' breakpoints. It would slide around the >points from reasonable initial positions and optimize whatever error >function you wanted (absolute error, error squared or whatever). These >days you could probably do it with Excel's solver. I used it for >fitting thermocouple curves.=20Gee, i currently have a real good use for something like that. I would like to see your answer. ?-)
Reply by ●October 1, 20112011-10-01
>> > If you=92ve never done this before I would discourage you:>> > I=92m just trying to be helpful and I wish you the best of luck. > > Thanks again. =A0At the moment I'm just going to order some VCO's from > minicirciuts... (throw some money at the problem.) and see what the > output looks like. =A0I'll also push forward on the triangle wave/ sine > shaper idea.Download the tek FG02 to see how to do it. http://bama.edebris.com/manuals/= tek/fg502/> > This is a nice app note,http://www.national.com/an/AN/AN-263.pdf > > George H. > > > > > > > Btw, you do want to go with frequencies up over 10MHz. =A0Get far away > > from your baseband, just makes the IF filtering all that much > > easier.- Hide quoted text - > > > - Show quoted text -- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text -
Reply by ●October 1, 20112011-10-01
On 10/01/2011 12:24 PM, cbarn24050@aol.com wrote:> > >>> If you�ve never done this before I would discourage you: > > >>> I�m just trying to be helpful and I wish you the best of luck. >> >> Thanks again. At the moment I'm just going to order some VCO's from >> minicirciuts... (throw some money at the problem.) and see what the >> output looks like. I'll also push forward on the triangle wave/ sine >> shaper idea. > > > Download the tek FG02 to see how to do it. http://bama.edebris.com/manuals/tek/fg502/ >>Wow, that's really rococo. Four diode bridges to make a breakpoint amp! I like the diode bridge tri wave generator. And the real problem with those 500-series plugins is that they die if you hot-plug them. 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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●October 4, 20112011-10-04
On Sep 30, 2:01=A0pm, me0...@yahoo.com wrote:> On Sep 30, 10:32=A0am, George Herold <gher...@teachspin.com> wrote: > > > > > > > On Sep 30, 1:10=A0pm, me0...@yahoo.com wrote: > > > > On Sep 30, 8:57=A0am, George Herold <gher...@teachspin.com> wrote: > > > > > On Sep 29, 2:25=A0pm, me0...@yahoo.com wrote: > > > > > > On Sep 29, 9:11=A0am, George Herold <gher...@teachspin.com> wrote=:> > > > > > > On Sep 28, 5:38=A0pm, Bill Sloman <bill.slo...@ieee.org> wrote: > > > > > > > > On Sep 28, 9:41=A0pm, George Herold <gher...@teachspin.com> w=rote:> > > > > > > > > On Sep 28, 1:41=A0pm, "Michael A. Terrell" <mike.terr...@ea=rthlink.net>> > > > > > > > wrote: > > > > > > > > > > George Herold wrote: > > > > > > > > > > > Yeah DDS from a micro looks like the 'best' path. =A0It='s just outside> > > > > > > > > > my present comfort zone and so hard to predict how long=it will> > > > > > > > > > take. > > > > > > > > > > =A0 =A0You can get a "AD9850 Module DDS Signal Generator =with Circuit> > > > > > > > > Diagram" in ebay for about $13 and demo software on the A=nalog Devices> > > > > > > > > website. =A0There is also VB source code & 80*51 source c=ode availible for> > > > > > > > > free. > > > > > > > > > > =A0 =A0I just bought a couple for a project I'm working o=n.> > > > > > > > > > -- > > > > > > > > > You can't have a sense of humor, if you have no sense. > > > > > > > > > Thanks Michael, =A0there's so many DDS chips. =A0Which ones=will still be> > > > > > > > around in ten years? > > > > > > > > The first mention of the AD9850 I can find here dates back to=2000, so> > > > > > > it has been around for more than ten years already - always a=good> > > > > > > sign - and I think that it had been around for a while then. =IIRR it> > > > > > > was the first of Analog Devices DDS chips, and it seems to ha=ve been> > > > > > > designed into a lot of applications, so it may be around for =a while> > > > > > > yet. > > > > > > > > -- > > > > > > > Bill Sloman, Nijmegen- Hide quoted text - > > > > > > > > - Show quoted text - > > > > > > > Thanks Bill, that's good to know. > > > > > > > George H.- Hide quoted text - > > > > > > > - Show quoted text - > > > > > > George, > > > > > > What kind of tuning resolution do you need?- Hide quoted text - > > > > > > - Show quoted text - > > > > > Hi MeO, =A0The product will be doing all sorts of Fourier analysis.==A0At> > > > the moment I think the reason for the VCO is only to look at > > > > sidebands*. =A0And mostly sidebnads at a high modulation index... S=o I'd> > > > like to modulate it with a (say) 1kHz sine wave and see sidebands o=ut> > > > to maybe the 7th to 10th harmonic. =A0I'm not sure how to turn this=into> > > > a resolution spec. > > > > > George H. > > > > > *It would be nice if we had some other 'physics' to do with the VCO=.> > > > Then I might have some 'real' specs.- Hide quoted text - > > > > > - Show quoted text - > > > > George, > > > > So do I understand correctly, you need a variable sine source that > > > tunes in frequency from 30Khz to 100KHz; this will serve as the LO in > > > a system that generates a modulated output? > > > > So my question re resolution, how course a step size do you need the > > > variable sine source to move in? =A0Is it 1Hz, 100Hz, 1KHz, 10KHz, et=c?> > > > Then of course the next question is, what is your requirement on > > > frequency accuracy? =A0For example say you want to set the system to > > > 30KHz, if it=92s off by 60KHz is that an issue? =A0If it wonders arou=nd by> > > +/- 30KHz once set, is that an issue? > > > > Regards- Hide quoted text - > > > > - Show quoted text - > > > Hey MeO, thanks for all your effort here! =A0(I'm feeling guilty, > > wasting too much of everyones time already.) =A0I've got a minicircuits > > order in, and I'll play around with the pieces and report back. =A0Afte=r> > that I'll have a better idea of how this approach will work. > > > George H.- Hide quoted text - > > > - Show quoted text - > > Good luck George, > > I won't beat a dead horse, but the DDS really sounds like it was > designed for your app. > > Have a look at there app notes if you already haven't. =A0Depending on > the part, great freq resolution, modulation capability and crystal > stability. > > Regards- Hide quoted text - > > - Show quoted text -No problem MeO, A DDS design ....someday... George H.
Reply by ●October 4, 20112011-10-04
On Sep 30, 7:50=A0pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote:> On 09/30/2011 12:01 PM, George Herold wrote: > > > > > > > On Sep 29, 4:05 pm, Phil Hobbs > > <pcdhSpamMeSensel...@electrooptical.net> =A0wrote: > >> On 09/29/2011 12:11 PM, George Herold wrote: > > >>> On Sep 28, 4:33 pm, me0...@yahoo.com wrote: > >>>> On Sep 28, 12:34 pm, George Herold<gher...@teachspin.com> =A0 =A0wro=te:> > >>>>> On Sep 28, 1:38 pm, Tim Wescott<t...@seemywebsite.com> =A0 =A0wrote=:> > >>> <big snip> > >>>>> - Show quoted text - > > >>>> Hey George, > > >>>> Here s some more food for thought re the downconverter approach: > > >>> Hi MeO > > >>>> (you ve never said what you re looking for in Frequency stability, b=ut> >>>> I m assuming you want accuracy set by your XOSC and phase noise is > >>>> really not an issue) > > >>> Hmm, this is a small part of a new project. =A0I'm still trying to ge=t a> >>> handle on the specs I need. > > >>>> If you ve never done this before I would discourage you: > > >>>> 1) =A0 =A0 =A0 Unless you ve got the time to put into the learning c=urve, not> >>>> only RF but the Freq Synth part > >>>> 2) =A0 =A0 =A0You have access to a decent SA or FFT analyzer that ca=n cover your> >>>> frequency of interest > >>>> 3) =A0 =A0 =A0A low frequency network analyzer would also make the j=ob a lot> >>>> easier > > >>> Well, an RF amplifier design ~20 years ago. =A0(would have been nice =to> >>> have a spectrum analyzer at the time, I had some ring down issues in > >>> one of the amp stages.) =A0The only RF recently is a few watt oscilla=tor> >>> for a Rubidium discharge lamp. =A0(Where frequency stability is not m=uch> >>> of an issue.) =A0I've got a 'new' SRS770 spectrum analyzer, (DC-100kH=z)> >>> which should tell me all I need to know about what's happening after > >>> the mixer. > > >>>> You ve got another potential problem when you attempt this kind of > >>>> frequency translation. =A0The low Q VCO is going to have a tendency =to> >>>> be pulled around by the high Q XOSC. =A0They sniff each other when > >>>> they get close frequency wise. > > >>> Hmm a bit of padding should take care of that... I'll just burn > >>> power! > > >>>> So you have to make sure your system has enough isolation between th=e> >>>> two oscillators. =A0You need to take into account things like LO to =RF> >>>> isolation when looking at the mixer. =A0In S parameter terms it will > >>>> be the S12 term of the VCO chain. > > >>>> Again I will state, I really believe your VCO will have to be phased > >>>> locked. =A0Otherwise it ll be hit and miss when trying to set it s > >>>> freq. =A0Each one of those gain curves will be slightly different fr=om> >>>> part to part. > > >>>> I don t mean to discourage you. =A0In fact, for somebody that does s=ynth> >>>> design this is pretty trivial. > > >>>> But for a first timer it will be daunting, and could really end up > >>>> being a big waste of time. =A0The digital approach is just a matter =of> >>>> ramping up and not equipment intensive. =A0And software is comparati=vely> >>>> easier to change then hardware. > > >>> Thanks, =A0I'd like to do a DDS design sometime, but right now that > >>> looks like a longer learning curve/ time sink. =A0(strange as that ma=y> >>> sound.) > > >>>> I m just trying to be helpful and I wish you the best of luck. > > >>> Thanks again. =A0At the moment I'm just going to order some VCO's fro=m> >>> minicirciuts... (throw some money at the problem.) and see what the > >>> output looks like. =A0I'll also push forward on the triangle wave/ si=ne> >>> shaper idea. > > >>> This is a nice app note, > >>>http://www.national.com/an/AN/AN-263.pdf > > >>> George H. > > >>>> Btw, you do want to go with frequencies up over 10MHz. =A0Get far aw=ay> >>>> from your baseband, just makes the IF filtering all that much > >>>> easier.- Hide quoted text - > > >>>> - Show quoted text - > > >> As I posted to the sample-hold thread by mistake: > > >> Using a CD4017 and some resistors will get rid of all harmonics up to > >> the ninth, so a fixed LPF will make a nice sine wave with 3:1 tunabili=ty.> > >> 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 > >> 845-480-2058 > > >> hobbs at electrooptical dot nethttp://electrooptical.net-Hide quoted t=ext -> > >> - Show quoted text - > > > Hi Phil, =A0CD4017? =A0A decade counter. =A0Summing different 'fraction=s'> > into an opamp to get a 'stepped' sine wave? > > Color me confused. > > > George H. > > It's not just a decade counter--it's a decade counter > _with_one-of-10_outputs_--the poor man's sequence generator. =A0Wire the > outputs together via weighted resistors. =A0You have 10 samples per cycle=,> so you can get rid of the second through ninth harmonics. =A0If you don't > mind adding a couple of open-collector or tri-state buffer parts(*), the > resistors just go as > > R_N =3D 1/sin((N+1/2) pi/10). > > Pretty simple. =A0If you want elegance, you can save the buffers and just > form a weighted sum of all of the conductances--the math isn't hard, > because it's one-of-10.Thanks Phil, James A. set me straight. Sitting in the hospital yesterday, waiting for my Mum to get out from knee surgery, I realized you can do a simple little 90 degree phase shift too.> > 4017s are wonderful for bodging together some little piece of test gear > that needs good timing accuracy. > > Cheers > > Phil Hobbs > > (*) You know the trick for getting open-collector behaviour out of > tri-state parts, right? =A0You ground the inputs, and put the logic signa=l> into tri-state line. =A0You don't get as many per package, but they come > in faster logic families.Nah, I hardly know any good tricks. But thanks, I looked it up in AoE. I was thinking of just summing current at the inverting input of an opamp, so I wouldn't need the open collectro trick... That's assuming that a digital 'zero' isn't too far from analog ground. George H.> > -- > 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 > 845-480-2058 > > hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted tex=t -> > - Show quoted text -
Reply by ●October 4, 20112011-10-04
On Tue, 4 Oct 2011 06:47:21 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Sep 30, 2:01�pm, me0...@yahoo.com wrote: >> Good luck George, >> >> I won't beat a dead horse, but the DDS really sounds like it was >> designed for your app. >> >> Have a look at there app notes if you already haven't. �Depending on >> the part, great freq resolution, modulation capability and crystal >> stability. >> >> Regards- Hide quoted text - >> >> - Show quoted text - > >No problem MeO, A DDS design ....someday...You really don't need a purpose-built DDS chip to do direct digital synthesis. This project constructed around a (relatively) ancient AT90S2313 chip <http://www.myplace.nu/avr/minidds/index.htm> is a good example. It can be expanded to fractional-N and more bells and whistles (literally, I'd imagine ;-) but the basic principal is pretty straightforward. -- Rich Webb Norfolk, VA
Reply by ●October 4, 20112011-10-04
On Oct 1, 12:24=A0pm, "cbarn24...@aol.com" <cbarn24...@aol.com> wrote:> =A0> > > > > If you=92ve never done this before I would discourage you: > > > > I=92m just trying to be helpful and I wish you the best of luck. > > > Thanks again. =A0At the moment I'm just going to order some VCO's from > > minicirciuts... (throw some money at the problem.) and see what the > > output looks like. =A0I'll also push forward on the triangle wave/ sine > > shaper idea. > > Download the tek FG02 to see how to do it.http://bama.edebris.com/manuals=/tek/fg502/> > > > > > > This is a nice app note,http://www.national.com/an/AN/AN-263.pdf > > > George H. > > > > Btw, you do want to go with frequencies up over 10MHz. =A0Get far awa=y> > > from your baseband, just makes the IF filtering all that much > > > easier.- Hide quoted text - > > > > - Show quoted text -- Hide quoted text - > > > - Show quoted text -- Hide quoted text - > > > - Show quoted text -- Hide quoted text - > > - Show quoted text -Thanks, I don't quite get the diode bridges. I was going to try Phil's tanh shaper idea. (logarthimic shaper in the AN263 app note.) George H.
Reply by ●October 4, 20112011-10-04
On 10/4/2011 8:58 AM, Rich Webb wrote:> On Tue, 4 Oct 2011 06:47:21 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > >> On Sep 30, 2:01 pm, me0...@yahoo.com wrote: >>> Good luck George, >>> >>> I won't beat a dead horse, but the DDS really sounds like it was >>> designed for your app. >>> >>> Have a look at there app notes if you already haven't. Depending on >>> the part, great freq resolution, modulation capability and crystal >>> stability. >>> >>> Regards- Hide quoted text - >>> >>> - Show quoted text - >> >> No problem MeO, A DDS design ....someday... > > You really don't need a purpose-built DDS chip to do direct digital > synthesis. This project constructed around a (relatively) ancient > AT90S2313 chip<http://www.myplace.nu/avr/minidds/index.htm> is a good > example. It can be expanded to fractional-N and more bells and whistles > (literally, I'd imagine ;-) but the basic principal is pretty > straightforward. >Nice! I think I'd like to try it using a Cypress PSoC to make a tiny DDS for the range of about .1 Hz to 10 kHz or so. Thanks. John S
Reply by ●October 4, 20112011-10-04
John S wrote:> > On 10/4/2011 8:58 AM, Rich Webb wrote: > > On Tue, 4 Oct 2011 06:47:21 -0700 (PDT), George Herold > > <gherold@teachspin.com> wrote: > > > >> On Sep 30, 2:01 pm, me0...@yahoo.com wrote: > >>> Good luck George, > >>> > >>> I won't beat a dead horse, but the DDS really sounds like it was > >>> designed for your app. > >>> > >>> Have a look at there app notes if you already haven't. Depending on > >>> the part, great freq resolution, modulation capability and crystal > >>> stability. > >>> > >>> Regards- Hide quoted text - > >>> > >>> - Show quoted text - > >> > >> No problem MeO, A DDS design ....someday... > > > > You really don't need a purpose-built DDS chip to do direct digital > > synthesis. This project constructed around a (relatively) ancient > > AT90S2313 chip<http://www.myplace.nu/avr/minidds/index.htm> is a good > > example. It can be expanded to fractional-N and more bells and whistles > > (literally, I'd imagine ;-) but the basic principal is pretty > > straightforward. > > > > Nice! I think I'd like to try it using a Cypress PSoC to make a tiny DDS > for the range of about .1 Hz to 10 kHz or so.Tiny? look at the Analog Devices AD9833. It's a 10 pin SOIC. -- You can't have a sense of humor, if you have no sense.
