HCT4046 VCO noise
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From: Jim Thompson on 18 Jun 2010 12:28 On Fri, 18 Jun 2010 09:22:32 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 17 Jun 2010 05:19:47 -0700, "GBaars" <g.baars13(a)Chello.nl> >wrote: > >>For narrow (fdev ~ 0.05 %) FM detection the phase noise of the CD74HCT4046 >>PLL's VCO >>causes poor S+N/N ratio. Has a smaller / larger oscillator C any influence >>and also the values of R1 and R2 of the VCO. >>Do the 74HC(T)7046 or 74HC(T)9046 perform any better? >> > >A 4046 will always be noisy here. > >Consider buying a narrowband packaged VCO; they cost a few dollars and >will have far less phase noise. An XOR phase detector will be nice and >linear. > >Or consider some other kind of non-PLL FM discriminator, like a high-Q >phase shifter (technically a very peaky lowpass filter) and an XOR. > >Or heterodyne it low enough to get, say, a 2:1 frequency swing and use >a tachometer discriminator. > >ftp://jjlarkin.lmi.net/DoubleTach.jpg > >John > > Sure looks like the right solution for "narrow (fdev ~ 0.05 %) FM detection" :-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: Tim Wescott on 18 Jun 2010 12:32 On 06/18/2010 08:53 AM, GBaars wrote: (top posting fixed) > "Tim Wescott"<tim(a)seemywebsite.now> wrote in message > news:WZWdnU8vyJPsG4bRnZ2dnUVZ_j6dnZ2d(a)web-ster.com... >> On 06/17/2010 05:19 AM, GBaars wrote: >>> For narrow (fdev ~ 0.05 %) FM detection the phase noise of the >>> CD74HCT4046 >>> PLL's VCO >>> causes poor S+N/N ratio. Has a smaller / larger oscillator C any >>> influence >>> and also the values of R1 and R2 of the VCO. >>> Do the 74HC(T)7046 or 74HC(T)9046 perform any better? >> >> You've had all the comments that stay in the box you've jumped into. >> >> Here's mine: >> >> Just use an LC oscillator! Unless you work hard to screw it up, the phase >> noise is going to be miles lower than anything you can do with a >> multivibrator. >> >> If oscillators were hammers, most multivibrators would make very nice >> rocks. Sometimes that's all you need. >> >> -- >> Tim Wescott >> Control system and signal processing consulting >> www.wescottdesign.com > Plan B was a quadrature detector which works with LC phase shifter > where S+N/N is proportional to the Q of the LC. > An LC VCO is a good option indeed but then a 74HCT4046 for just an EXOR > is not. A small signal multiplier could be driven directly by a colpitts. > I'll try this first, and see what it does. Why not a single gate? http://www.onsemi.com/pub/Collateral/NL17SZ86-D.PDF There are others. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: GBaars on 18 Jun 2010 14:52 "Tim Wescott" <tim(a)seemywebsite.now> wrote in message news:gISdnUtW1_IkAobRnZ2dnUVZ_t6dnZ2d(a)web-ster.com... > On 06/18/2010 08:53 AM, GBaars wrote: > (top posting fixed) >> "Tim Wescott"<tim(a)seemywebsite.now> wrote in message >> news:WZWdnU8vyJPsG4bRnZ2dnUVZ_j6dnZ2d(a)web-ster.com... >>> On 06/17/2010 05:19 AM, GBaars wrote: >>>> For narrow (fdev ~ 0.05 %) FM detection the phase noise of the >>>> CD74HCT4046 >>>> PLL's VCO >>>> causes poor S+N/N ratio. Has a smaller / larger oscillator C any >>>> influence >>>> and also the values of R1 and R2 of the VCO. >>>> Do the 74HC(T)7046 or 74HC(T)9046 perform any better? >>> >>> You've had all the comments that stay in the box you've jumped into. >>> >>> Here's mine: >>> >>> Just use an LC oscillator! Unless you work hard to screw it up, the >>> phase >>> noise is going to be miles lower than anything you can do with a >>> multivibrator. >>> >>> If oscillators were hammers, most multivibrators would make very nice >>> rocks. Sometimes that's all you need. >>> >>> -- >>> Tim Wescott >>> Control system and signal processing consulting >>> www.wescottdesign.com > > Plan B was a quadrature detector which works with LC phase shifter > > where S+N/N is proportional to the Q of the LC. > > An LC VCO is a good option indeed but then a 74HCT4046 for just an EXOR > > is not. A small signal multiplier could be driven directly by a > > colpitts. > > I'll try this first, and see what it does. > > Why not a single gate? > http://www.onsemi.com/pub/Collateral/NL17SZ86-D.PDF > > There are others. > > -- > Tim Wescott > Control system and signal processing consulting > www.wescottdesign.com Of course it is logical to work with TTL levels in a small signal design. Just set the inputs at Vcc/2 and couple AC don';t you agree.
From: Joerg on 18 Jun 2010 15:36 GBaars wrote: > "Tim Wescott" <tim(a)seemywebsite.now> wrote in message > news:gISdnUtW1_IkAobRnZ2dnUVZ_t6dnZ2d(a)web-ster.com... >> On 06/18/2010 08:53 AM, GBaars wrote: >> (top posting fixed) >>> "Tim Wescott"<tim(a)seemywebsite.now> wrote in message >>> news:WZWdnU8vyJPsG4bRnZ2dnUVZ_j6dnZ2d(a)web-ster.com... >>>> On 06/17/2010 05:19 AM, GBaars wrote: >>>>> For narrow (fdev ~ 0.05 %) FM detection the phase noise of the >>>>> CD74HCT4046 >>>>> PLL's VCO >>>>> causes poor S+N/N ratio. Has a smaller / larger oscillator C any >>>>> influence >>>>> and also the values of R1 and R2 of the VCO. >>>>> Do the 74HC(T)7046 or 74HC(T)9046 perform any better? >>>> You've had all the comments that stay in the box you've jumped into. >>>> >>>> Here's mine: >>>> >>>> Just use an LC oscillator! Unless you work hard to screw it up, the >>>> phase >>>> noise is going to be miles lower than anything you can do with a >>>> multivibrator. >>>> >>>> If oscillators were hammers, most multivibrators would make very nice >>>> rocks. Sometimes that's all you need. >>>> >>>> -- >>>> Tim Wescott >>>> Control system and signal processing consulting >>>> www.wescottdesign.com >>> Plan B was a quadrature detector which works with LC phase shifter >>> where S+N/N is proportional to the Q of the LC. >>> An LC VCO is a good option indeed but then a 74HCT4046 for just an EXOR >>> is not. A small signal multiplier could be driven directly by a >>> colpitts. >>> I'll try this first, and see what it does. >> Why not a single gate? >> http://www.onsemi.com/pub/Collateral/NL17SZ86-D.PDF >> >> There are others. >> >> -- >> Tim Wescott >> Control system and signal processing consulting >> www.wescottdesign.com > > > Of course it is logical to work with TTL levels in a small > signal design. Just set the inputs at Vcc/2 and couple AC > don';t you agree. > Can be risky if the device does have Schmitt inputs. Else, if the AC-coupled input signal stops or becomes too slow, a CMOS device biased at VCC/2 can draw a lot of current and also oscillate ... phsss ... *POOF* Also, logic stuff is going more towards 3.3V supplies these days. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Tim Wescott on 18 Jun 2010 16:50 On 06/18/2010 11:52 AM, GBaars wrote: > "Tim Wescott"<tim(a)seemywebsite.now> wrote in message > news:gISdnUtW1_IkAobRnZ2dnUVZ_t6dnZ2d(a)web-ster.com... >> On 06/18/2010 08:53 AM, GBaars wrote: >> (top posting fixed) >>> "Tim Wescott"<tim(a)seemywebsite.now> wrote in message >>> news:WZWdnU8vyJPsG4bRnZ2dnUVZ_j6dnZ2d(a)web-ster.com... >>>> On 06/17/2010 05:19 AM, GBaars wrote: >>>>> For narrow (fdev ~ 0.05 %) FM detection the phase noise of the >>>>> CD74HCT4046 >>>>> PLL's VCO >>>>> causes poor S+N/N ratio. Has a smaller / larger oscillator C any >>>>> influence >>>>> and also the values of R1 and R2 of the VCO. >>>>> Do the 74HC(T)7046 or 74HC(T)9046 perform any better? >>>> >>>> You've had all the comments that stay in the box you've jumped into. >>>> >>>> Here's mine: >>>> >>>> Just use an LC oscillator! Unless you work hard to screw it up, the >>>> phase >>>> noise is going to be miles lower than anything you can do with a >>>> multivibrator. >>>> >>>> If oscillators were hammers, most multivibrators would make very nice >>>> rocks. Sometimes that's all you need. >>>> >>>> -- >>>> Tim Wescott >>>> Control system and signal processing consulting >>>> www.wescottdesign.com >>> Plan B was a quadrature detector which works with LC phase shifter >>> where S+N/N is proportional to the Q of the LC. >>> An LC VCO is a good option indeed but then a 74HCT4046 for just an EXOR >>> is not. A small signal multiplier could be driven directly by a >>> colpitts. >>> I'll try this first, and see what it does. >> >> Why not a single gate? >> http://www.onsemi.com/pub/Collateral/NL17SZ86-D.PDF >> >> There are others. >> >> -- >> Tim Wescott >> Control system and signal processing consulting >> www.wescottdesign.com > > > Of course it is logical to work with TTL levels in a small > signal design. Just set the inputs at Vcc/2 and couple AC > don';t you agree. Well... TTL levels aren't 0V and 5V -- it's more like 0.2V and 2.7V on the output side. 5V CMOS levels are 0 and 5V, with all sorts of caveats. Using 0 and 5V _levels_ isn't a bad idea, but if you're concerned about phase noise then trying to use a TTL gate to square up a sine wave could be -- I'd use some good ol' analog circuitry to square things up _then_ run it all through the TTL (or CMOS) gates. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
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