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VCO basics

I'm using an ICS AC9173-01 to recover a video dot clock. The datasheet
offers no insights as to what's in there and what kind of power supply
it wants. I have 5mV p-p noise as measured across the device's power
pins. Now that's pretty good for a purely digital supply, but I think
it's not good for a VCO that can cover 10-75MHz off a 5V supply.

Supposing that a 0-5V swing covers 65MHz (guessing that the VCO uses
0-5V), that means 1mV corresponds to a 13KHz variation. So the 5mV
noise could modulate the VCO by +\- 33KHz from the center frequency.

Anyone have any clues how a VCO is made inside an IC and how sensitive
they are to power supply noise? I'm thinking of regulating a local 5V
just for this guy, what's a good linear regulator for the job? LT1963
comes to mind, any others?

BTW, ICS/IDT is the worst company I've ever dealt with to try and get
the application notes they suggest you read in the datasheet...
 
J

Jim Thompson

Jan 1, 1970
0
I'm using an ICS AC9173-01 to recover a video dot clock. The datasheet
offers no insights as to what's in there and what kind of power supply
it wants. I have 5mV p-p noise as measured across the device's power
pins. Now that's pretty good for a purely digital supply, but I think
it's not good for a VCO that can cover 10-75MHz off a 5V supply.

Supposing that a 0-5V swing covers 65MHz (guessing that the VCO uses
0-5V), that means 1mV corresponds to a 13KHz variation. So the 5mV
noise could modulate the VCO by +\- 33KHz from the center frequency.

Anyone have any clues how a VCO is made inside an IC and how sensitive
they are to power supply noise? I'm thinking of regulating a local 5V
just for this guy, what's a good linear regulator for the job? LT1963
comes to mind, any others?

BTW, ICS/IDT is the worst company I've ever dealt with to try and get
the application notes they suggest you read in the datasheet...

Can you provide links? I can't even find a data sheet.

...Jim Thompson
 
B

Bob

Jan 1, 1970
0
I'm using an ICS AC9173-01 to recover a video dot clock. The datasheet
offers no insights as to what's in there and what kind of power supply
it wants. I have 5mV p-p noise as measured across the device's power
pins. Now that's pretty good for a purely digital supply, but I think
it's not good for a VCO that can cover 10-75MHz off a 5V supply.

Supposing that a 0-5V swing covers 65MHz (guessing that the VCO uses
0-5V), that means 1mV corresponds to a 13KHz variation. So the 5mV
noise could modulate the VCO by +\- 33KHz from the center frequency.

Anyone have any clues how a VCO is made inside an IC and how sensitive
they are to power supply noise? I'm thinking of regulating a local 5V
just for this guy, what's a good linear regulator for the job? LT1963
comes to mind, any others?

BTW, ICS/IDT is the worst company I've ever dealt with to try and get
the application notes they suggest you read in the datasheet...

ICS used to be very good before IDT took them over. It was easy to contact
people that REALLY knew the product. Good luck, now.

It can be very important to keep the VCO's supply super clean. It's
difficult to know what a given part's supply rejection is, so I always
include extra filtering.

The LT1963 is a superb low-noise LDO. It's one of the ones that Xilinx
recommends for their FPGA serdes supplies. It may be overkill, however.

I've had good luck, with sensitive ICS (pll-based) clock multipliers chips,
by simply using a resistor between the supply and the chip and a parallel
cap (chip side to gnd, of course). You have to keep the R small enough so as
not to violate the chip's VDD min, and keep the C large enough to keep the
noise low at low frequencies. Be sure to add smaller parallel caps so the
noise filtering is effective even at the higher frequencies (hundreds of
MHzs).

We had one design that used an ICS8xxxx part. Its output jitter was terrible
(way over spec) even though we used a series ferrite bead and small parallel
cap. We pulled out the ferrite bead and replaced it with a 4.7ohm R, and we
increased the cap (10uF iirc). It solved the problem.

Bob
 
Thanks Bob, I was starting to think I was losing it. The data sheet
calls out twice to read AB01 if I'm using Hsync below 25KHz... can't
find AB01.

This is for a hobby project, so I just sort of tossed things that
looked good together. I can recover my dot clock, but my initial
measurements with my sampling scope show me some pretty poor jitter
performance. I mean I can see on my target LCD monitor the pixels
flashing on and off as the scan doubler's setup and hold time is
violated, the scope just brings it home.

I'm gonna cut and jump a heavy ferrite/RLC filter on the power supply.
Good thing I have a quiet analog scope otherwise I'd never see that 5mV
pp noise on the supply.
 
C

colin

Jan 1, 1970
0
Thanks Bob, I was starting to think I was losing it. The data sheet
calls out twice to read AB01 if I'm using Hsync below 25KHz... can't
find AB01.

This is for a hobby project, so I just sort of tossed things that
looked good together. I can recover my dot clock, but my initial
measurements with my sampling scope show me some pretty poor jitter
performance. I mean I can see on my target LCD monitor the pixels
flashing on and off as the scan doubler's setup and hold time is
violated, the scope just brings it home.

I'm gonna cut and jump a heavy ferrite/RLC filter on the power supply.
Good thing I have a quiet analog scope otherwise I'd never see that 5mV
pp noise on the supply.

Regulators are good for rejecting low frequency supply noise but not so good
at high frequency,
as Bob said a resistor followed by a capacitor or 2 is often a good idea,
ferrite beads work at high frequency but you may find dont work at the
frequency you want,
choose R just high enough so it only cuases a slight v drop.

The ground and control voltage are important of course, you need to isolate
any ground noise that will superimpose itself on the control voltage, ie
make sure there is absolutly no digital ground currents flowing between the
trace conecting vco gnd and PLL gnd or whatever generates the control.


Colin =^.^=
 
J

John Popelish

Jan 1, 1970
0
I'm using an ICS AC9173-01 to recover a video dot clock. The datasheet
offers no insights as to what's in there and what kind of power supply
it wants. I have 5mV p-p noise as measured across the device's power
pins. Now that's pretty good for a purely digital supply, but I think
it's not good for a VCO that can cover 10-75MHz off a 5V supply.

Do you really need a 7.5 to one VCO range?
Supposing that a 0-5V swing covers 65MHz (guessing that the VCO uses
0-5V), that means 1mV corresponds to a 13KHz variation. So the 5mV
noise could modulate the VCO by +\- 33KHz from the center frequency.

Anyone have any clues how a VCO is made inside an IC and how sensitive
they are to power supply noise? I'm thinking of regulating a local 5V
just for this guy, what's a good linear regulator for the job? LT1963
comes to mind, any others?

I am assuming the VCO runs in a phase locked loop. So the
stability of the supply is not very important for
frequencies well below the phase detector sampling
frequency, since the negative feedback will correct errors
that are changing slowly. I would first try an LC supply
filter for the phase detector and oscillator.
 
I'm using an ICS AC9173-01 to recover a video dot clock. The datasheet
offers no insights as to what's in there and what kind of power supply
it wants. I have 5mV p-p noise as measured across the device's power
pins. Now that's pretty good for a purely digital supply, but I think
it's not good for a VCO that can cover 10-75MHz off a 5V supply.

Supposing that a 0-5V swing covers 65MHz (guessing that the VCO uses
0-5V), that means 1mV corresponds to a 13KHz variation. So the 5mV
noise could modulate the VCO by +\- 33KHz from the center frequency.

Anyone have any clues how a VCO is made inside an IC and how sensitive
they are to power supply noise? I'm thinking of regulating a local 5V
just for this guy, what's a good linear regulator for the job? LT1963
comes to mind, any others?

BTW, ICS/IDT is the worst company I've ever dealt with to try and get
the application notes they suggest you read in the datasheet...

Do you really need that much range in the VCO? If not, then use a
different chip. The lowest noise VCOs just "pull" a crystal or LC tank.


BTW, IDT was easily the worse place I ever worked. Len Perham rose to
the top by the vacuum principle, i.e. someone either died or quit, and
he got sucked up into the next level. Before you knew it, has was CEO.
It's been some time since he was gone, but it looks like it was too
late to fix the mess he left.
 
Do you really need that much range in the VCO? If not, then use a
different chip. The lowest noise VCOs just "pull" a crystal or LC tank.

It's a genlock chip that has 1x and 2x outputs that covers my frequency
of interest. There aren't many. Some have pre-set internal dividers
that I'd have to bypass anyways.
Since the divider is 1016, the feedback happens slowly enough that all
kinds of stuff can happen in the meantime.
There are other, newer video dot clock generators that cover even
higher ranges, it's worse.
I'm gonna go to a VCXO for the next version...
BTW, IDT was easily the worse place I ever worked. Len Perham rose to
the top by the vacuum principle, i.e. someone either died or quit, and
he got sucked up into the next level. Before you knew it, has was CEO.
It's been some time since he was gone, but it looks like it was too
late to fix the mess he left.

I'm on my 4th week of sending out emails to get this AB01, still no
reply.
 
colin said:
Regulators are good for rejecting low frequency supply noise but not so good
at high frequency,

I'll take everything I can get. I've seen VCOs have their own local
boost converter followed by a reference chip (!) for power. Something
like 5 to 6V, followed by a REF195 padded with tons of caps and
ferrites. But that was for high speed serdes stuff, where jitter is
measured in picoseconds.
as Bob said a resistor followed by a capacitor or 2 is often a good idea,
ferrite beads work at high frequency but you may find dont work at the
frequency you want,

I'll take everything I can get. A ferrite with 1000ohms at 100MHz is 9
cents, that still buys me 20dB attenuation at 20MHz. Thing with the RC
filter is that the 7805 is kind of sloppy in its output specs, mine is
running at 4.9V. Toss in the RI loss and I might create more problems
than I solve. If I release my project for others to build, I can't
control the voltage the regulator outputs. Maybe it's time to spec
another regulator for the board.

I'm also a bit miffed that I placed the regulator as far away as
possible from the genlock chip... Means the 5V that reaches the chip
has all the noise of the scan converter, microcontroller, and input
buffer, all 5 mV of it.
choose R just high enough so it only cuases a slight v drop.

The ground and control voltage are important of course, you need to isolate
any ground noise that will superimpose itself on the control voltage, ie
make sure there is absolutly no digital ground currents flowing between the
trace conecting vco gnd and PLL gnd or whatever generates the control.

The phase detector, filter and VCO are in the same chip. The divider is
external. The genlock chip is about 200 mils away from the clock inputs
on the main chip. There's little else on the board. No currents except
the return current flows underneath. (knock on wood)

Whatever the filter is, if it's working with noisy digital signals from
the phase comp, that's probably the real problem. Maybe I can schmitt
the inputs too.
 
C

colin

Jan 1, 1970
0
I'll take everything I can get. I've seen VCOs have their own local
boost converter followed by a reference chip (!) for power. Something
like 5 to 6V, followed by a REF195 padded with tons of caps and
ferrites. But that was for high speed serdes stuff, where jitter is
measured in picoseconds.


I'll take everything I can get. A ferrite with 1000ohms at 100MHz is 9
cents, that still buys me 20dB attenuation at 20MHz. Thing with the RC
filter is that the 7805 is kind of sloppy in its output specs, mine is
running at 4.9V. Toss in the RI loss and I might create more problems
than I solve. If I release my project for others to build, I can't
control the voltage the regulator outputs. Maybe it's time to spec
another regulator for the board.

I'm also a bit miffed that I placed the regulator as far away as
possible from the genlock chip... Means the 5V that reaches the chip
has all the noise of the scan converter, microcontroller, and input
buffer, all 5 mV of it.


The phase detector, filter and VCO are in the same chip. The divider is
external. The genlock chip is about 200 mils away from the clock inputs
on the main chip. There's little else on the board. No currents except
the return current flows underneath. (knock on wood)

Whatever the filter is, if it's working with noisy digital signals from
the phase comp, that's probably the real problem. Maybe I can schmitt
the inputs too.

Well it wouldnt hurt to use a regulator ferrite bead and resistor,
The noise you realy need to worry about is that wich is above your pll
filter bandwidth,
If this is about 100khz then a ferrite bead isnt going to cut it and a
regulator might just let this through,
however you could put a rc filter before the regulator this will solve both
problems.


Colin =^.^=
 
John said:
Do you really need a 7.5 to one VCO range?

Nope, but the other genlocks have even wider ranges. I don't know what
LCD monitors use to recover the clock on the VGA side but they have to
use something similar to what I'm doing, so it's possible.

I am assuming the VCO runs in a phase locked loop. So the
stability of the supply is not very important for
frequencies well below the phase detector sampling
frequency, since the negative feedback will correct errors
that are changing slowly. I would first try an LC supply
filter for the phase detector and oscillator.

When I get my 10Hz - 1MHz spec an working I'll poke around the supply
more but I will add the 1$ of parts to try to clean up the genlock's
supply first.
 
J

Joel Kolstad

Jan 1, 1970
0
BTW, IDT was easily the worse place I ever worked. Len Perham rose to
the top by the vacuum principle, i.e. someone either died or quit, and
he got sucked up into the next level. Before you knew it, has was CEO.

Was it during that period that IDT performed such bone-headed manuevers such
as purchasing Quality Semiconductor and -- within months -- discontinuing a
bunch of their products that were better than what IDT themselves offered?
How naive...

I would say that I've used plenty of IDT FIFOs and multi-port RAMs and have
always been happy with them.
 
Joel said:
Was it during that period that IDT performed such bone-headed manuevers such
as purchasing Quality Semiconductor and -- within months -- discontinuing a
bunch of their products that were better than what IDT themselves offered?
How naive...

I would say that I've used plenty of IDT FIFOs and multi-port RAMs and have
always been happy with them.

I was at IDT in the late 80's. The Quality buy out was a decade later.
Cheun Der Lien is the only name I now recognize when I look at their
website.

Nothing wrong with the bread and butter IDT parts (FIFOs), though they
fell behind on processing yield and speed due to not taking care of the
talent. Of course, not taking care of the talent is the classic way
Silicon Valley companies self destruct.
 
It's a genlock chip that has 1x and 2x outputs that covers my frequency
of interest. There aren't many. Some have pre-set internal dividers
that I'd have to bypass anyways.
Since the divider is 1016, the feedback happens slowly enough that all
kinds of stuff can happen in the meantime.
There are other, newer video dot clock generators that cover even
higher ranges, it's worse.
I'm gonna go to a VCXO for the next version...

VXCO is the way to go for stability, but they are slow to lock. There
is also a slight microphonic issue.
 
VXCO is the way to go for stability, but they are slow to lock. There
is also a slight microphonic issue.

Could you offer some hints as to how to select a pullable crystal at
32MHz? I think crystals over 27MHz are 3rd overtone, perhaps I'm out of
date? It is my understanding that a 3rd overtone crystal is 3 times
less pullable. Or is it 9? Are there perhaps resonators that can be
used in this role? I think a resonator's Q would be still be higher
than the apparent Q of a wideband VCO.
 
Could you offer some hints as to how to select a pullable crystal at
32MHz? I think crystals over 27MHz are 3rd overtone, perhaps I'm out of
date? It is my understanding that a 3rd overtone crystal is 3 times
less pullable. Or is it 9? Are there perhaps resonators that can be
used in this role? I think a resonator's Q would be still be higher
than the apparent Q of a wideband VCO.

I did some research on this kind of project years ago, but never built
the hardware. {cancelled project.] From memory (so use this in research
rather than accept it to be gospel), there are two basic ways to pull a
crystal. One is with a varactor, much like you would use if tuning the
osc by hand. The other is a more integrated approach used in a Philips
chip. I'm drawing a blank on how they did it, but no varactor was
required. However, if you building a board level product, I don't see
an issue with a varactor. There are a few that are dirt cheap since
they are used in AM/FM radios.

My recollection is you can only pull a crystall about 1000ppm. Remember
the low jitter is related to the high-Q, which in turn means it takes a
long time to move the frequency.

There is an old book on crystal oscillators by a guy named Ferking. I
recall using a copy from Stanford. I dug this up on Amazon:
-----------
"... co and go to application notes. Ferking M. E. (1978): Cristal
Oscillator Design and Temperature Compensation, New York: Van Nostrand
Reinhold. Hambley A. R. (1994): Electronics. ..
--------------

For something on crystals, a 1978 reference is actually pretty recent.
;-) I'm not sure if the 1994 reference is to a more recent book.

The other source I found was from the Swiss Federal Institute, though
the author doesn't come to mind.

This is the company that at the time I did the research had the lowest
"noise" crystals:
http://www.crovencrystals.com/
 
C

colin

Jan 1, 1970
0
Could you offer some hints as to how to select a pullable crystal at
32MHz? I think crystals over 27MHz are 3rd overtone, perhaps I'm out of
date? It is my understanding that a 3rd overtone crystal is 3 times
less pullable. Or is it 9? Are there perhaps resonators that can be
used in this role? I think a resonator's Q would be still be higher
than the apparent Q of a wideband VCO.

I did some research on this kind of project years ago, but never built
the hardware. {cancelled project.] From memory (so use this in research
rather than accept it to be gospel), there are two basic ways to pull a
crystal. One is with a varactor, much like you would use if tuning the
osc by hand. The other is a more integrated approach used in a Philips
chip. I'm drawing a blank on how they did it, but no varactor was
required. However, if you building a board level product, I don't see
an issue with a varactor. There are a few that are dirt cheap since
they are used in AM/FM radios.

My recollection is you can only pull a crystall about 1000ppm. Remember
the low jitter is related to the high-Q, which in turn means it takes a
long time to move the frequency.

You could always use a DDC, they are a bit expensive though, and use a fair
amount of power.

If im not mistaken pulling a crystal can be done as quickly as you want,
the rate of pulling is not limited by the Q of the crystal just the extent.

Once the energy is built up in the circuit it is bound to oscillate at the
resonant frequency.
But its not something ive looked into.

A ceramic resonator however makes a pretty good VCO.

Colin =^.^=
 
L

lemonjuice

Jan 1, 1970
0
Could you offer some hints as to how to select a pullable crystal at
32MHz?
If you are not using a fixed overtone crystal oscillator 32 MHz should
be a frequency lying between the resonant and antiresonant frequency
of the crystal. You would have to add a capacitor in series(assuming
the crystal is looking into a low impedance load in the VCXO circuit)
to make it pullable. That will only raise the frequency. An inductor
in series will change that. Or you can add a phase lag around the
oscillator circuit to lower the frequency, best if you are interested
in setting your oscillator to a fixed frequency.

An LC selector tank substituting one of the capacitors in the typical
capacitor stack (of your VCXO) fed into your amplifying unit should
fix that. At low frequencies, this circuit appears inductive, while
above resonance,its capacitance dominates. By setting the resonant
frequency to bemidway between the highest unwanted overtone and the
next desiredovertone, C and L will be of opposite sign at all
lower-frequencyunwanted overtones, but of the same sign (capacitive
reactance) at the desired overtone above the selector resonant
frequency.
I think crystals over 27MHz are 3rd overtone, perhaps I'm out of
date?
Yes ... bout 20MHz right up to 60MHz then 5th overtone take over,
though above 50MHz you would need to add a shunt inductance to make
the crystal resonate. As at such frequencies the parallel motional
capacitance shorts out the series motional inductance and capacitance
in the crystal.
It is my understanding that a 3rd overtone crystal is 3 times
less pullable. Or is it 9?

From what I heart the pullability is limited by about 0.15% the
Crystal's fundamental frequency, though units specifically designed
to resonate at an overtone frequency resist being pulled at that
frequency but have good frequency variability at their fundamental
value.

That shouldn't be your worry ... by varying the external capacitance
as I suggested above should fix things if you are using a crystal not
resonating at a given overtone frequency.

lemonjuice
 
M

Michael A. Terrell

Jan 1, 1970
0
lemonjuice said:
If you are not using a fixed overtone crystal oscillator 32 MHz should
be a frequency lying between the resonant and antiresonant frequency
of the crystal. You would have to add a capacitor in series(assuming
the crystal is looking into a low impedance load in the VCXO circuit)
to make it pullable. That will only raise the frequency. An inductor
in series will change that. Or you can add a phase lag around the
oscillator circuit to lower the frequency, best if you are interested
in setting your oscillator to a fixed frequency.

An LC selector tank substituting one of the capacitors in the typical
capacitor stack (of your VCXO) fed into your amplifying unit should
fix that. At low frequencies, this circuit appears inductive, while
above resonance,its capacitance dominates. By setting the resonant
frequency to bemidway between the highest unwanted overtone and the
next desiredovertone, C and L will be of opposite sign at all
lower-frequencyunwanted overtones, but of the same sign (capacitive
reactance) at the desired overtone above the selector resonant
frequency.
Yes ... bout 20MHz right up to 60MHz then 5th overtone take over,
though above 50MHz you would need to add a shunt inductance to make
the crystal resonate. As at such frequencies the parallel motional
capacitance shorts out the series motional inductance and capacitance
in the crystal.

From what I heart the pullability is limited by about 0.15% the
Crystal's fundamental frequency, though units specifically designed
to resonate at an overtone frequency resist being pulled at that
frequency but have good frequency variability at their fundamental
value.

That shouldn't be your worry ... by varying the external capacitance
as I suggested above should fix things if you are using a crystal not
resonating at a given overtone frequency.

lemonjuice


A few companies make fundamental crystals to 125 MHz. We used them
at Microdyne. They were in welded metal TO-5 cans, and custom made.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
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