George Herold wrote:> On Sep 27, 2:35 pm, Joerg <inva...@invalid.invalid> wrote: >> George Herold wrote: >>> 30kHz to 100kHz VCO with sine wave output. >>> I know this has been covered before in previous threads. >>> The obvious way to do this is a digital signal generator (DSG) IC with >>> maybe an A-D to measure the voltage. I ve not done much digital stuff >>> in a while, so this has a bit of a learning curve. But if anyone has >>> some good chips to look at or app notes then please do share. I've >>> been looking at IC's on Analogs web site, but there are a lot to >>> choose from! >>> I was also thinking I could do this with a ~20MHz varactor controlled >>> VCO as a variable clock into a DSG chip. This looks easier to my >>> unsophisticated digital mind. >>> Finally I offer the following, a Wien bridge oscillator with >>> varactor diodes as the capacitors. I was also thinking I could use >>> crappy Z5U ceramic caps instead of the varactors. >>> http://imageshack.us/photo/my-images/88/wien1.jpg/ >>> (Excuse my chicken scratching.) >>> Comments welcome. >> This is the TV dinner version, costs a bit more but you just rip open, >> microwave, and eat: >> >> http://www.analog.com/static/imported-files/data_sheets/AD9837.PDF > > Thanks, I only did a quick scan... To use it as a VCO I'd have to > write new frequencies via the SPI interface.Yep, it's gonna be digital. Eeuw, yuck ...>> Z5U as a VCO works but it's tough to get the distortion low enough, >> since you want a nice sine wave and not a crummy one I assume. > > Hmm, I'm not so sure. What we really want to do. (I think.....) is > drive the piss out of a VCO so we can see all the various side bands. > Crank it up to where you see the fundamental dissapear...(The first > zero of the Bessel function) And even higher... watch the first > sideband go away... Anyway I think this is what we want, I'll have to > check. So I'm not sure I care too much about a crummy sine wave. > Maybe the first few harmonics down by 40dB is OK. >Then by all means. But then you could also use a NE555 :-)>> If it has to be cheap there's this secret trick: >> >> Let the VCO run on a much higher and more practical frequency. Let's say >> 2030kHz to 2100kHz, for example. That can easily be done with a >> varicap, a transistor, some R, C, and an inductor. Very cheap. Then mix >> that with a 2MHz clock. The result will be these three spectra: >> >> 30kHz-100kHZ >> 2030kHz-2100kHz (40dB down or better if using a DBM) >> 4030kHz-4100kHz >> plus some harmonics. >> >> A very simple lowpass that cuts off at a few hundred kHZ can fish out >> 30kHz-100kHz and ... bingo. > > Yeah I thought of that too. Cheap is not really important. But I'd > really like parts that are still going to be around in ten years. >> If you search around you might find a radio chip that does most of >> nearly all of this. Depends a bit on what supply voltages you have >> available. Something like this maybe: >> >> http://www.nxp.com/documents/data_sheet/SA612A.pdf > Nice, thanks. I usually just go to minicircuits for this type of > stuff. >That chip should be around for a while because it's used in so many gizmos. Arrow has 60,000 in stock, those are usually the signs I look for before deciding. They wouldn't do this if it had become a dud. -- Regards, Joerg http://www.analogconsultants.com/
50 kHz VCO w/sine output
Started by ●September 27, 2011
Reply by ●September 27, 20112011-09-27
Reply by ●September 27, 20112011-09-27
On Tue, 27 Sep 2011 12:44:44 -0700, George Herold wrote:> On Sep 27, 2:17 pm, Tim Wescott <t...@seemywebsite.com> wrote: >> On Tue, 27 Sep 2011 09:54:36 -0700, George Herold wrote: >> > 30kHz to 100kHz VCO with sine wave output. >> >> > I know this has been covered before in previous threads. >> >> > The obvious way to do this is a digital signal generator (DSG) IC >> > with maybe an A-D to measure the voltage. I’ve not done much digital >> > stuff in a while, so this has a bit of a learning curve. But if >> > anyone has some good chips to look at or app notes then please do >> > share. I've been looking at IC's on Analogs web site, but there are >> > a lot to choose from! >> >> > I was also thinking I could do this with a ~20MHz varactor controlled >> > VCO as a variable clock into a DSG chip. This looks easier to my >> > unsophisticated digital mind. >> >> > Finally I offer the following, a Wien bridge oscillator with >> > varactor diodes as the capacitors. I was also thinking I could use >> > crappy Z5U ceramic caps instead of the varactors. >> >> It'll be rough to get the high impedances and low voltage swings you >> need for the Wien bridge oscillator to work either with varactors or >> with a suggestion that I've made previously of using JFETs as variable >> resistors. > > Hi Tim, I've got a spice simulation with just diode limiting for the > varible gain. ~1.2 Vp-p on the output. I couldn't find any varactors > in LTspice so I just used a diode and added some more C in parallel. The > AC voltage across the 'varactor' is less than a volt. By playing around > with the gain control I can probably get reduce that by a factor of two. > Is that too much?The nonlinearity that you're battling isn't the diode conduction -- it's the diode's varying capacitance as its reverse voltage changes. It's going to be severe at low voltage/high capacitance. I'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. I _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. Or 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.>> So: a high frequency VCO to a DDS chip, or a fixed-frequency reference >> to a DDS that you write a frequency to with a micro, or (if you are for >> some reason insisting on "analog only") a VCO feeding a mixer, beating >> against a crystal oscillator. > > Yeah DDS from a micro looks like the 'best' path. It's just outside my > present comfort zone and so hard to predict how long it will take.It occurs to me that if you're just bound and determined to avoid microprocessors that you could do this with switched capacitor techniques. This should give you an oscillation frequency that's proportional to your switching frequency, and let you make a cruddy old VCO out of a 4046 or something. You may even be able to find a switched capacitor filter that can be made to oscillate, and save having to roll your own. An alternative would be a 30-100kHz square wave generator, followed by a few really good bandpass filters, in bands to cover the frequency range (this would be a lot easier if you didn't want that 3:10 ratio). I've also seen a counter fed by a higher-frequency clock, and driving resistors that have been selected give a sine-wave approximation. If you select your resistors right you can weigh things so that the harmonics up to (IIRC) the number of resistors can be suppressed, leaving the fundamental unscathed (and leaving you with the problem of where to find such precise resistors). This significantly eases your filtering requirements. Of course, you may well be able to spend your time learning your way around PIC programming and get a result in the time you could have spent doing either of the above. But if you just _have_ to avoid a microprocessor, it's a way forward. -- www.wescottdesign.com
Reply by ●September 27, 20112011-09-27
On Tue, 27 Sep 2011 13:11:20 -0700, George Herold wrote:> On Sep 27, 2:35 pm, Joerg <inva...@invalid.invalid> wrote: >> George Herold wrote: >> > 30kHz to 100kHz VCO with sine wave output. >> >> > I know this has been covered before in previous threads. >> >> > The obvious way to do this is a digital signal generator (DSG) IC >> > with maybe an A-D to measure the voltage. I ve not done much digital >> > stuff in a while, so this has a bit of a learning curve. But if >> > anyone has some good chips to look at or app notes then please do >> > share. I've been looking at IC's on Analogs web site, but there are >> > a lot to choose from! >> >> > I was also thinking I could do this with a ~20MHz varactor controlled >> > VCO as a variable clock into a DSG chip. This looks easier to my >> > unsophisticated digital mind. >> >> > Finally I offer the following, a Wien bridge oscillator with >> > varactor diodes as the capacitors. I was also thinking I could use >> > crappy Z5U ceramic caps instead of the varactors. >> >> >http://imageshack.us/photo/my-images/88/wien1.jpg/ >> >> > (Excuse my chicken scratching.) >> > Comments welcome. >> >> This is the TV dinner version, costs a bit more but you just rip open, >> microwave, and eat: >> >> http://www.analog.com/static/imported-files/data_sheets/AD9837.PDF > > Thanks, I only did a quick scan... To use it as a VCO I'd have to write > new frequencies via the SPI interface. >> >> Z5U as a VCO works but it's tough to get the distortion low enough, >> since you want a nice sine wave and not a crummy one I assume. > > Hmm, I'm not so sure. What we really want to do. (I think.....) is > drive the piss out of a VCO so we can see all the various side bands. > Crank it up to where you see the fundamental dissapear...(The first zero > of the Bessel function) And even higher... watch the first sideband go > away... Anyway I think this is what we want, I'll have to check. So I'm > not sure I care too much about a crummy sine wave. Maybe the first few > harmonics down by 40dB is OK. >The 3rd harmonic of 30kHz is lower than 100kHz. I think you need a clean sine wave.> >> If it has to be cheap there's this secret trick: >> >> Let the VCO run on a much higher and more practical frequency. Let's >> say >> 2030kHz to 2100kHz, for example. That can easily be done with a >> varicap, a transistor, some R, C, and an inductor. Very cheap. Then mix >> that with a 2MHz clock. The result will be these three spectra: >> >> 30kHz-100kHZ >> 2030kHz-2100kHz (40dB down or better if using a DBM) 4030kHz-4100kHz >> plus some harmonics. >> >> A very simple lowpass that cuts off at a few hundred kHZ can fish out >> 30kHz-100kHz and ... bingo. > > Yeah I thought of that too. Cheap is not really important. But I'd > really like parts that are still going to be around in ten years. >> >> If you search around you might find a radio chip that does most of >> nearly all of this. Depends a bit on what supply voltages you have >> available. Something like this maybe: >> >> http://www.nxp.com/documents/data_sheet/SA612A.pdf > Nice, thanks. I usually just go to minicircuits for this type of stuff.Well, heck -- if you're going that route just get a crystal oscillator, a regular VCO, and a mixer. Filter the piss (well, the harmonics) out of the oscillator that you feed into the "RF" side of the mixer (so you get a clean sine wave out), and go. -- www.wescottdesign.com
Reply by ●September 27, 20112011-09-27
On Tue, 27 Sep 2011 13:18:06 -0700, George Herold wrote:> On Sep 27, 3:40 pm, whit3rd <whit...@gmail.com> wrote: >> On Tuesday, September 27, 2011 9:54:36 AM UTC-7, George Herold wrote: >> > 30kHz to 100kHz VCO with sine wave output. I was also thinking I >> > could do this with a ~20MHz varactor controlled VCO as a variable >> > clock into a DSG chip. This looks easier to my unsophisticated >> > digital mind. >> >> Workable, of course, but not exactly a VCO anymore... and there's the >> Nyquist filtering to do afterward. >> >> > Finally I offer the following, a Wien bridge oscillator with >> > varactor diodes as the capacitors. >> >> Not a great choice, because the Wein bridge requires MATCHED >> capacitors, tuned together, for best performance. It can be done with >> a few trimmers (probably easiest to trim the offset + gain of the >> control voltages). Then, there's the level translation problem (the >> capacitors in a Wien bridge don't have one end grounded). > > Hi Whit3rd, > Did you look at my schematic? I seem to have it running in LTspice. >> >> Good sinewave LC VCOs in narrow ranges are easier, and a 4.0 MHz sine >> source and 4.030 to 4.100 MHz sine VCO can be mixed down to get you >> what you want. > > Ah, good. This was my first suggestion on how to get what was wanted. > It was rejected. But perhaps I should push it a bit more!It's what I'd push, if I couldn't convince people to use a DDS or if there were some overriding systems reason why a DDS wouldn't do. You probably don't want two LC VCO's -- one crystal oscillator and one VCO is probably the way to go. If your frequency range were narrower you could use one crystal oscillator and one VXCO -- but it isn't, so you have to do with what you have. -- www.wescottdesign.com
Reply by ●September 27, 20112011-09-27
Tim Wescott wrote:> On Tue, 27 Sep 2011 13:18:06 -0700, George Herold wrote: > >> On Sep 27, 3:40 pm, whit3rd <whit...@gmail.com> wrote: >>> On Tuesday, September 27, 2011 9:54:36 AM UTC-7, George Herold wrote: >>>> 30kHz to 100kHz VCO with sine wave output. I was also thinking I >>>> could do this with a ~20MHz varactor controlled VCO as a variable >>>> clock into a DSG chip. This looks easier to my unsophisticated >>>> digital mind. >>> Workable, of course, but not exactly a VCO anymore... and there's the >>> Nyquist filtering to do afterward. >>> >>>> Finally I offer the following, a Wien bridge oscillator with >>>> varactor diodes as the capacitors. >>> Not a great choice, because the Wein bridge requires MATCHED >>> capacitors, tuned together, for best performance. It can be done with >>> a few trimmers (probably easiest to trim the offset + gain of the >>> control voltages). Then, there's the level translation problem (the >>> capacitors in a Wien bridge don't have one end grounded). >> Hi Whit3rd, >> Did you look at my schematic? I seem to have it running in LTspice. >>> Good sinewave LC VCOs in narrow ranges are easier, and a 4.0 MHz sine >>> source and 4.030 to 4.100 MHz sine VCO can be mixed down to get you >>> what you want. >> Ah, good. This was my first suggestion on how to get what was wanted. >> It was rejected. But perhaps I should push it a bit more! > > It's what I'd push, if I couldn't convince people to use a DDS or if > there were some overriding systems reason why a DDS wouldn't do. ...I'll second that. Maybe we should make some signs and start a virtual picket line. Need a nice chant though. "Rah-rah-rah, os-ci-llate, down-con-vert, rah-rah-rah". Oh, and drums, of course :-) [...] -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●September 27, 20112011-09-27
On Tue, 27 Sep 2011 13:27:16 -0700, Joerg wrote:> George Herold wrote: >> On Sep 27, 2:35 pm, Joerg <inva...@invalid.invalid> wrote: >>> George Herold wrote: >>>> 30kHz to 100kHz VCO with sine wave output. I know this has been>>> snip <<<>>> http://www.nxp.com/documents/data_sheet/SA612A.pdf >> Nice, thanks. I usually just go to minicircuits for this type of >> stuff. >> >> > That chip should be around for a while because it's used in so many > gizmos. Arrow has 60,000 in stock, those are usually the signs I look > for before deciding. They wouldn't do this if it had become a dud.It's on its way to jelly-bean status. The 602 is also available, and it's been around since at least the 1980's. It's just a really useful, easy to use building block if you don't need "mini-circuits" quality. -- www.wescottdesign.com
Reply by ●September 27, 20112011-09-27
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. It's > going to be severe at low voltage/high capacitance. I'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. I _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. Or 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.
Reply by ●September 27, 20112011-09-27
On Tue, 27 Sep 2011 14:00:41 -0700, Joerg <invalid@invalid.invalid> wrote:> >I'll second that. Maybe we should make some signs and start a virtual >picket line. Need a nice chant though. "Rah-rah-rah, os-ci-llate, >down-con-vert, rah-rah-rah". Oh, and drums, of course :-) > >[...]Sort of like our college math club football cheer: Secant tangent Cosine Sine 3.14159 e to the x y to the rho Beer in the end zone Go team go. Jim
Reply by ●September 27, 20112011-09-27
RST Engineering wrote:> On Tue, 27 Sep 2011 14:00:41 -0700, Joerg <invalid@invalid.invalid> > wrote: > > >> I'll second that. Maybe we should make some signs and start a virtual >> picket line. Need a nice chant though. "Rah-rah-rah, os-ci-llate, >> down-con-vert, rah-rah-rah". Oh, and drums, of course :-) >> >> [...] > > Sort of like our college math club football cheer: > > Secant tangent > Cosine Sine > 3.14159 > e to the x > y to the rho > Beer in the end zone > Go team go. >Cool! -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●September 27, 20112011-09-27
On 09/27/2011 02:17 PM, Tim Wescott wrote:> On Tue, 27 Sep 2011 09:54:36 -0700, George Herold wrote: > >> 30kHz to 100kHz VCO with sine wave output. >> >> I know this has been covered before in previous threads. >> >> The obvious way to do this is a digital signal generator (DSG) IC with >> maybe an A-D to measure the voltage. I’ve not done much digital stuff >> in a while, so this has a bit of a learning curve. But if anyone has >> some good chips to look at or app notes then please do share. I've been >> looking at IC's on Analogs web site, but there are a lot to choose from! >> >> I was also thinking I could do this with a ~20MHz varactor controlled >> VCO as a variable clock into a DSG chip. This looks easier to my >> unsophisticated digital mind. >> >> Finally I offer the following, a Wien bridge oscillator with varactor >> diodes as the capacitors. I was also thinking I could use crappy Z5U >> ceramic caps instead of the varactors. > > It'll be rough to get the high impedances and low voltage swings you need > for the Wien bridge oscillator to work either with varactors or with a > suggestion that I've made previously of using JFETs as variable resistors. > > So: a high frequency VCO to a DDS chip, or a fixed-frequency reference to > a DDS that you write a frequency to with a micro, or (if you are for some > reason insisting on "analog only") a VCO feeding a mixer, beating against > a crystal oscillator. > > I'd look to the DDS solution, or see ifyou can make a suitable block> with a PIC or similar, a good DAC, and some code. >You can still get low-nanofarad Y5V caps in small sizes, and those plus a boost converter are about the best things going right now for high kilohertz stuff. All the MVAM-style varactors are long gone. I'd probably be wanting to use a tri-wave oscillator with a sine shaper. (You don't have to use the crappy diode ones--you can use a tanh to roll over the peaks and then subtract a bit of the original sine, as we discussed a year or so back. I posted a suggestion at http://electrooptical.net/www/sed/sed.html , but it's a much older technique. 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
