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From: John Fields on 12 May 2010 20:09
On Tue, 11 May 2010 20:39:37 -0700 (PDT), Daku <dakupoto(a)gmail.com>
wrote:

>Dear Sir,
>I do not intend to use a ring oscillator based VCO.
>However, as you have said below, literature search
>shows a lot of people using the ring oscillator based
>voltage controlled oscillator in phase locked loop
>designs, which is sounds strange to me, since in a
>phase locked loop, the VCO oscillation frequency
>must be very sensitive to the input voltage level.
>Maybe the ring oscillator is designed to oscillate
>at the central frequency, but frequency variation is
>difficult and tricky.

---
Please bottom post, or inline post when it's necessary for clarity.
Thank you. :-)

As others have noted, your original post seems to indicate that you
don't have a really good grasp on what's required to change the
frequency of an oscillator as a function of an applied voltage.

For an LC tank, where:

1
f = -------------,
2pi sqrt LC

then either L or C must be varied as a function of voltage in order to
make f change.

In the old days it was done with a saturable reactor, where the DC
voltage on the control winding and the attendant current through it
changed the reluctance of the core and, thererfore, the inductance
described by the secondary.

Today, afaik, it's done with varactors.

Comment?

From: Daku on 12 May 2010 23:07
On May 13, 5:09 am, John Fields <jfie...(a)austininstruments.com> wrote:
>
> ---
> Please bottom post, or inline post when it's necessary for clarity.
> Thank you. :-)
>
> As others have noted, your original post seems to indicate that you
> don't have a really good grasp on what's required to change the
> frequency of an oscillator as a function of an applied voltage.
>
> For an LC tank, where:
>
> 1
> f = -------------,
> 2pi sqrt LC
>
> then either L or C must be varied as a function of voltage in order to
> make f change.
>
> In the old days it was done with a saturable reactor, where the DC
> voltage on the control winding and the attendant current through it
> changed the reluctance of the core and, thererfore, the inductance
> described by the secondary.
>
> Today, afaik, it's done with varactors.
>
> Comment?

Dear Sir,
I am fully aware of the LC-tank circuit, and that a varactor needs to
be
added in the tank to get the voltage control. However, a simple
literature
search on voltage controlled oscillators indicates that a large number
of
designs are based on the CMOS ring oscillator. As one of other posters
noted before (and I fully agree with him) that it is difficult to
dynamically
control the oscillation frequency of the CMOS ring oscillator - this
frequency
is based on the characteristics of the MOSFETs in the inverters of
the
ring oscillator. Given that, how are these ring oscillator based
designs
being labelled "voltage controlled" since the voltage control merely
switches the oscillations on and off, NOT control the frequency as in
a
LC tank.
From: krw on 12 May 2010 23:07
On Wed, 12 May 2010 19:09:42 -0500, John Fields
<jfields(a)austininstruments.com> wrote:

>On Tue, 11 May 2010 20:39:37 -0700 (PDT), Daku <dakupoto(a)gmail.com>
>wrote:
>
>>Dear Sir,
>>I do not intend to use a ring oscillator based VCO.
>>However, as you have said below, literature search
>>shows a lot of people using the ring oscillator based
>>voltage controlled oscillator in phase locked loop
>>designs, which is sounds strange to me, since in a
>>phase locked loop, the VCO oscillation frequency
>>must be very sensitive to the input voltage level.
>>Maybe the ring oscillator is designed to oscillate
>>at the central frequency, but frequency variation is
>>difficult and tricky.
>
>---
>Please bottom post, or inline post when it's necessary for clarity.
>Thank you. :-)
>
>As others have noted, your original post seems to indicate that you
>don't have a really good grasp on what's required to change the
>frequency of an oscillator as a function of an applied voltage.
>
>For an LC tank, where:
>
> 1
> f = -------------,
> 2pi sqrt LC
>
>then either L or C must be varied as a function of voltage in order to
>make f change.
>
>In the old days it was done with a saturable reactor, where the DC
>voltage on the control winding and the attendant current through it
>changed the reluctance of the core and, thererfore, the inductance
>described by the secondary.
>
>Today, afaik, it's done with varactors.
>
>Comment?

It can be done with ring oscillators, too. Gate delay isn't a strong function
of Vcc, though so it's not done often. DLLs generally use a mux to select the
number of gates in the ring.
From: Michael Black on 12 May 2010 23:20
On Wed, 12 May 2010, Daku wrote:

> On May 13, 5:09 am, John Fields <jfie...(a)austininstruments.com> wrote:
>>
>> ---
>> Please bottom post, or inline post when it's necessary for clarity.
>> Thank you. :-)
>>
>> As others have noted, your original post seems to indicate that you
>> don't have a really good grasp on what's required to change the
>> frequency of an oscillator as a function of an applied voltage.
>>
>> For an LC tank, where:
>>
>> 1
>> f = -------------,
>> 2pi sqrt LC
>>
>> then either L or C must be varied as a function of voltage in order to
>> make f change.
>>
>> In the old days it was done with a saturable reactor, where the DC
>> voltage on the control winding and the attendant current through it
>> changed the reluctance of the core and, thererfore, the inductance
>> described by the secondary.
>>
>> Today, afaik, it's done with varactors.
>>
>> Comment?
>
> Dear Sir,
> I am fully aware of the LC-tank circuit, and that a varactor needs to
> be
> added in the tank to get the voltage control. However, a simple
> literature
> search on voltage controlled oscillators indicates that a large number
> of
> designs are based on the CMOS ring oscillator. As one of other posters
> noted before (and I fully agree with him) that it is difficult to
> dynamically
> control the oscillation frequency of the CMOS ring oscillator - this
> frequency
> is based on the characteristics of the MOSFETs in the inverters of
> the
> ring oscillator. Given that, how are these ring oscillator based
> designs
> being labelled "voltage controlled" since the voltage control merely
> switches the oscillations on and off, NOT control the frequency as in
> a
> LC tank.
>
Show us a sample circuit, and we'll explain it. Until we actually
see what you are talking about, then "how" can only be conjecture.
Show us a sample circuit, and then that circuit can be explained.

Michael

From: Jasen Betts on 13 May 2010 03:45
On 2010-05-13, Daku <dakupoto(a)gmail.com> wrote:
> However, a simple literature
> search on voltage controlled oscillators indicates that a large number
> of designs are based on the CMOS ring oscillator. As one of other posters
> noted before (and I fully agree with him) that it is difficult to
> dynamically control the oscillation frequency of the CMOS ring oscillator
> - this frequency is based on the characteristics of the MOSFETs in the
> inverters of
> the ring oscillator. Given that, how are these ring oscillator based
> designs
> being labelled "voltage controlled" since the voltage control merely
> switches the oscillations on and off,

No, it doesn't


also feedback helps things.


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