Linearizing VCO transfer function

[John Wilkinson]

Hi,
I am designing a PLL and have found that the gain of the VCO is very
non-linear, and causes all my carefull calculations to fall over and the
loop to become unstable at the lower end.
I read in Wez Hawards "Radio Frequency Design" that quite often designer
uses diode resistor networks to piecewise linearize the gain of a VCO.
Does anyone have experience of this method or have any circuits I may
look at for clues.

Best regards,
John.

 

[Jim Thompson]

On Sat, 09 Apr 2005 21:53:01 +0100, John Wilkinson
<johnwilkinson@clara.co.uk> wrote:

Hi,
I am designing a PLL and have found that the gain of the VCO is very
non-linear, and causes all my carefull calculations to fall over and the
loop to become unstable at the lower end.
I read in Wez Hawards "Radio Frequency Design" that quite often designer
uses diode resistor networks to piecewise linearize the gain of a VCO.
Does anyone have experience of this method

Yes.

or have any circuits I may
look at for clues.

Best regards,
John.

Surf on piecewise-linear synthesis. Depending on the curve, it can be
rather painful, particularly if you need temperature compensation as
well.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[John Fields]

On Sat, 09 Apr 2005 21:53:01 +0100, John Wilkinson
<johnwilkinson@clara.co.uk> wrote:

Hi,
I am designing a PLL and have found that the gain of the VCO is very
non-linear, and causes all my carefull calculations to fall over and the
loop to become unstable at the lower end.
I read in Wez Hawards "Radio Frequency Design" that quite often designer
uses diode resistor networks to piecewise linearize the gain of a VCO.
Does anyone have experience of this method or have any circuits I may
look at for clues.

---
If you know the transfer function of the VCO, you can do this:


V(f)>---[ADC]---[LUT]---[DAC]---[VCO]-->fOUT

and force

d (fOUT)
---------- ~ 1
d (V(f))

if that's what you need.

--
John Fields
Professional Circuit Designer

 

[Joerg]

Hello Jim,

Surf on piecewise-linear synthesis. Depending on the curve, it can be
rather painful, particularly if you need temperature compensation as
well.

Run another diode in a servo loop, a diode that is located close to the
others. Best to use doubles with a common cathode like the BAV70 where
the servo diode would be in the same SOT23 package. Has worked well for me.

Regards, Joerg

http://www.analogconsultants.com

 

[John Miles]

In article <1113079633.16271.0@damia.uk.clara.net>,
johnwilkinson@clara.co.uk says...

Hi,
I am designing a PLL and have found that the gain of the VCO is very
non-linear, and causes all my carefull calculations to fall over and the
loop to become unstable at the lower end.
I read in Wez Hawards "Radio Frequency Design" that quite often designer
uses diode resistor networks to piecewise linearize the gain of a VCO.
Does anyone have experience of this method or have any circuits I may
look at for clues.

Best regards,
John.

IMHO you don't want to design a PLL that's that twitchy. Break it into
two loops with separate VCOs if you can.

Failing that, you can design for worst-case phase margin. You may have
to deal with increased lock times, but it should definitely be possible
to make the loop stable everywhere.

ADISimPLL is a great tool for this stuff, if you haven't tried it yet.

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------

 

[Mike Monett]

John Wilkinson wrote:

Hi,
I am designing a PLL and have found that the gain of the VCO is very
non-linear, and causes all my carefull calculations to fall over and the
loop to become unstable at the lower end.
I read in Wez Hawards "Radio Frequency Design" that quite often designer
uses diode resistor networks to piecewise linearize the gain of a VCO.
Does anyone have experience of this method or have any circuits I may
look at for clues.

Best regards,
John.

Don't worry about it. The loop response varies as the square root of the vco gain,
so it is not really critical. You can optimize the response at one point and let it
vary above and below this point. As long as you are not trying to operate at very
high reverse bias on the varicap, you usually can get quite acceptable response over
the entire range.

If your tuning ratio is too high and you are using a regular abrupt junction
varicap, try going to a hyperabrupt. If you are already using one, figure out how to
reduce the required tuning range.

Mike Monett

 

[Jim Thompson]

On Sat, 09 Apr 2005 22:55:07 +0100, John Wilkinson
<johnwilkinson@clara.co.uk> wrote:

Joerg wrote:
Hello Jim,

Surf on piecewise-linear synthesis. Depending on the curve, it can be
rather painful, particularly if you need temperature compensation as
well.


Run another diode in a servo loop, a diode that is located close to the
others. Best to use doubles with a common cathode like the BAV70 where
the servo diode would be in the same SOT23 package. Has worked well for me.

Regards, Joerg

http://www.analogconsultants.com
Hi Joerg,
This sound simple and efficient, can you send me an example schematic,
so I can get my head round it.

Thanks,
John.

Joerg is just addressing the TC of the break-point system. I was
commenting that the tuning curve may have a TC as well (varicap, for
instance).

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[John Wilkinson]

Joerg wrote:

Hello Jim,

Surf on piecewise-linear synthesis. Depending on the curve, it can be
rather painful, particularly if you need temperature compensation as
well.


Run another diode in a servo loop, a diode that is located close to the
others. Best to use doubles with a common cathode like the BAV70 where
the servo diode would be in the same SOT23 package. Has worked well for me.

Regards, Joerg

http://www.analogconsultants.com
Hi Joerg,

This sound simple and efficient, can you send me an example schematic,
so I can get my head round it.

Thanks,
John.

 

[Jim Thompson]

On Sat, 9 Apr 2005 14:43:06 -0700, John Miles
<jmiles@pop.removethistomailme.net> wrote:

In article <1113079633.16271.0@damia.uk.clara.net>,
johnwilkinson@clara.co.uk says...
Hi,
I am designing a PLL and have found that the gain of the VCO is very
non-linear, and causes all my carefull calculations to fall over and the
loop to become unstable at the lower end.
I read in Wez Hawards "Radio Frequency Design" that quite often designer
uses diode resistor networks to piecewise linearize the gain of a VCO.
Does anyone have experience of this method or have any circuits I may
look at for clues.

Best regards,
John.



IMHO you don't want to design a PLL that's that twitchy. Break it into
two loops with separate VCOs if you can.

It's not "twitchy", it's just the variation in loop gain that results
from a wide tuning range using a non-linear element, such as with a
varicap.

Failing that, you can design for worst-case phase margin. You may have
to deal with increased lock times, but it should definitely be possible
to make the loop stable everywhere.

ADISimPLL is a great tool for this stuff, if you haven't tried it yet.

-- jm

True, Just design for critical or slightly over-damped, at the
worst-case... than all other cases stay over-damped (and slow
closing).

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[colin]

"John Wilkinson" <johnwilkinson@clara.co.uk> wrote in message
news:1113079633.16271.0@damia.uk.clara.net...

Hi,
I am designing a PLL and have found that the gain of the VCO is very
non-linear, and causes all my carefull calculations to fall over and the
loop to become unstable at the lower end.
I read in Wez Hawards "Radio Frequency Design" that quite often designer
uses diode resistor networks to piecewise linearize the gain of a VCO.
Does anyone have experience of this method or have any circuits I may
look at for clues.

Best regards,
John.

yes a simple way to give some level of improvement is to have a voltage
divider with the botom end conected to a reference voltage through a diode.
this reduces the maximum gain when the control voltage reaches the ref
voltage + diode drop and the diode conducts. the reference voltage can be
acheived with voltage divider too and the efective resistance of this can
function as the botom resitor in the first divider.

of course if your maximum gain is at the low voltage end you can turn the
diode around. you would need to carefuly analyze your vco charecterisitc to
get it right. this efectivly counteracts a single kink in the
characteristic, if this isnt enough you can have two such dividers but this
is best done using an op amp as well, but might be more trouble than its
worth as temp efects of the diodes means you wil stil have to give a
reasonable margin for loop stability.

Colin.

 

[Joerg]

Hello John,

Run another diode in a servo loop, a diode that is located close to
the others. Best to use doubles with a common cathode like the BAV70
where the servo diode would be in the same SOT23 package. Has worked
well for me.

This sound simple and efficient, can you send me an example schematic,
so I can get my head round it.

I can't send you one of the actual designs I did because it is
confidential information. But there is pretty good educational material
on the web about this topic, for example this one from Ireland:

http://www.electronics.dit.ie/staff/ypanarin/Lecture%20Notes/FT221-4/5%20Signal%20Shaping.pdf

As to Jim's point, of course this does not take care of other
non-linearities such as that of the varicap etc. They can be compensated
for as well but this gets a bit more intricate. One could, for example,
run a separate oscillator with another varicap of same type and measure
its frequency with a uC, possibly divided down. But it may be tough to
find a dual varicap in the same package. Also, the loop could become
kind of slow.

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

Hello John,

Come to think of it, if you just want to linearize the transfer function
somewhat to make you loop calculation easier, or your loop faster, you
probably don't need to tempcomp a piecewise linear control even with
diodes. At least not as long as you have a healthy voltage swing in the
diode section so your control doesn't linger in any transfer area for
too much of the total range.

Regards, Joerg

http://www.analogconsultants.com

 

[Marc H.Popek]

Back to back varicaps in a culprits oscillator configuration wide band and
very nearly linear. One of my first patented design at Motorola many if
times ago... public domain now! enjoy!

Marc Popek
"John Wilkinson" <johnwilkinson@clara.co.uk> wrote in message
news:1113079633.16271.0@damia.uk.clara.net...

Hi,
I am designing a PLL and have found that the gain of the VCO is very
non-linear, and causes all my carefull calculations to fall over and the
loop to become unstable at the lower end.
I read in Wez Hawards "Radio Frequency Design" that quite often designer
uses diode resistor networks to piecewise linearize the gain of a VCO.
Does anyone have experience of this method or have any circuits I may
look at for clues.

Best regards,
John.

 

[Winfield Hill]

Marc H. Popek wrote...

Back to back varicaps in a culprits oscillator configuration wide band
and very nearly linear. One of my first patented design at Motorola
many if times ago... public domain now! enjoy!

Shame on you, top poster! Anyway, Marc, very nice, how does that work,
I thought the back-back connection merely lowered distortion? Also,
can you tell us the patent number?

"John Wilkinson" <johnwilkinson@clara.co.uk> wrote in message
news:1113079633.16271.0@damia.uk.clara.net...
Hi,
I am designing a PLL and have found that the gain of the VCO is very
non-linear, and causes all my carefull calculations to fall over and the
loop to become unstable at the lower end.
I read in Wez Hawards "Radio Frequency Design" that quite often designer
uses diode resistor networks to piecewise linearize the gain of a VCO.
Does anyone have experience of this method or have any circuits I may
look at for clues.

Best regards,
John.

--
Thanks,
- Win