Prev: CA3240 Advantage over TL082?
Next: Selecting photodiode for Tranimpedance amplifier
From: Phil Hobbs on 18 Jun 2010 09:14
TheM wrote:
>> On Jun 17, 3:11 pm, Bill Sloman
>>
>>> The only useful way of reducing this noise is to run the VCO faster
>>> and use a counter to divide the output frequency down to actual
>>> frequency that you need.
>>> The phase noise is still there, but it is reduced in proportion to the
>>> divide ratio, because the voltage noise in the transistors is
>>> unchanged but the voltage ramp that is driving the switching is
>>> steeper in proportion to the divide ratio.
>
> 3dB improvement in S/N per divide by 2?
>
> M
>
>

6 dB. Phase is like amplitude.

To the OP:

[Obligatory 7 AM rant: the HC versions of the 4046 are quicker, but
whoever designed it did a rotten job. The oscillator in the metal gate
version is good for 1000:1 frequency range, but the HC one quits when
the control voltage is below about VDD/10. Pathetic.]

However, the leading possibility for the origin of your nasty phase
noise is the dead band in the phase-frequency detector (PD II, I think).
If you're using that one, try sticking a large value resistor from the
PD output to ground to pull it out of the dead band--your phase noise
should improve remarkably.

If that isn't it, there are a few possibilities, one of which is
frequency division as Bill suggested.

Besides that, you could run an offset loop by mixing the oscillator
output with a crystal-controlled clock, and running the PD at the
difference frequency. If the VCO range is too wide, and the phase
detector is symmetrical (e.g. an XOR) you can lock up on the wrong
sideband. Either restrict the tuning range (not that easy with a 4046
since the frequency tolerances are so sloppy), or use the
phase-frequency detector.

Locking up on the lower sideband requires the phase detector to run 180
degrees instead of 0 (or -90 instead of +90, depending). XORs and diode
mixers work fine like that, but PFDs don't--the loop has to try servoing
on the big cliff instead of the nice gentle slope. The effective loop
gain is different by about 500X, so the loop won't lock there--it'll
bounce around until it finds the stable null, which forces it to pick
the right sideband.

All that is a lot more work than adding one resistor!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: Tim Wescott on 18 Jun 2010 10:44
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
From: GBaars on 18 Jun 2010 11:53

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.

G.Baars

"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


From: Jim Thompson on 18 Jun 2010 12:13
On Fri, 18 Jun 2010 08:53:49 -0700, "GBaars" <g.baars13(a)Chello.nl>
wrote:

>
>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.
>
>G.Baars
>
[snip]

Use an ECL/PECL XOR (plus an OpAmp) as an analog PD, as in...

Patent 3,644,835 Phase Detector

(Copy available on my website.)

...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: John Larkin on 18 Jun 2010 12:22
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



First  |  Prev  |  Next  |  Last
Pages: 1 2 3 4
Prev: CA3240 Advantage over TL082?
Next: Selecting photodiode for Tranimpedance amplifier