Re: An All Digital Phase Lock Loop

[Ray Andraka]

Yes, we have. One approach we had success with was one that used a DDS type
setup where the increment value got bumped up/down depending on the phase of the
accumulator vs the reference. Works quite well as long as the reference is
fairly stable. In our case we needed to track a mechanical system which had a
fairly high variance on the reference, so we wound up putting a lot of extra
crap on the design to improve the transient response without it going into
oscillation. Still lots smaller than doing a digital equivalent of an analog
PLL though.

Hello guru's

I was wondering if anyone has ever attempted a phase lock loop in digital
before (specifically VHDL). I'm looking for some examples or pointers on
trying to build one for a low frequency range of 200 Hz to 200 kHz. I would
appreciate any comments or suggestions. Google didn't get me very far, so if
you know of any app notes, etc. please let me know.

Thanks,

Jason

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email (e-mail address removed)
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

 

[Kevin Neilson]

My all-digital PLL was similar and worked very well. My reference clock and
system clock were pretty stable so I didn't do a lot of analysis. I just
designed the loop with a small gain, so it took a bit to lock on (which
wasn't a problem in my application) but once it was locked it was very
stable and didn't oscillate. The only problem with the low gain is that it
took forever to lock in the simulation, so I made an adjustable loop gain so
I could kick it up for simulations and see it lock faster (although it rung
a bit before locking.)
-Kevin

 

[Ray Andraka]

Mine used a barrel shift in the feedback to get a gain that increased with the
size of the error. Had to do that to get a quick lock and still be able to
chase the reference. The reference was derived from a quad encoder on the
mechanical media path. The PLL had to adjust a process to keep a certain number
of events between encoder pulses. All in all, it was a pretty nasty problem
because of the dynamics and limited resolution of the encoder.

My all-digital PLL was similar and worked very well. My reference clock and
system clock were pretty stable so I didn't do a lot of analysis. I just
designed the loop with a small gain, so it took a bit to lock on (which
wasn't a problem in my application) but once it was locked it was very
stable and didn't oscillate. The only problem with the low gain is that it
took forever to lock in the simulation, so I made an adjustable loop gain so
I could kick it up for simulations and see it lock faster (although it rung
a bit before locking.)
-Kevin

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email (e-mail address removed)
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

 

[Jay]

Hi all.

I was actually thinking about digital PLLs recently when I was doing
investigation on analog PLLs.

Can someone describe the basic parts and operation of a DPLL?
Essentially these are the components I figured would be needed and this
is how the operation might work, I'd appreciate comments....

1. Phase / Frequency Detector

Standard two-DFF version, UP/DOWN outputs are used to control a
Numerically Controlled Oscillator block.

Input one to the PFD is the signal to be locked onto and input two is
the current output(or an ouput controlled by the DPLL), thus UP/DOWN
tries to drive the error down. UP/DOWN may have to be sychronized,
comments?

A lock signal could be the NAND(UP,DOWN) signals. Thus the output is
true when UP and DOWN are both zero.

Though I wonder, since this is a digital system, will 100% lock ever be
achieved, thus will the lock signal ever light?.

2. NCO

This block is essentially a mod-N counter which, at the end of it's
count, re-loads a count value from a register (perhaps double-buffered).
The register("error counter") is setup to increment or decrement it's
current value depending on the UP/DOWN signal from the Phase/Frequency
detector block.

The mod-N counter is set to count down from the current value, thus, a
smaller value in the counter means a higher frequency. This operation
(should?) emulate a VCO.

If the counter is configured to count down, I would guess that the
"error counter" would have to numerically increment the value it stores
when DOWN is true and numerically decrement with UP is true. I reason
this because there is an inverse relationship between the value stored
in the register and the output frequency (since we're controlling
"time" in terms of clock cycles).

3. Gain

The "error" register could be setup to increment by more than decimal 1,
perhaps by a power of 2 or something. Maybe even a "coarse" and fine
UP/DOWN could be used to get a faster lock.

My guess is that the setup I described emulates a P/I controller since
we could control how much the "error register" changes for an UP/DOWN
"event", thus this control the "P". And since the "error register" has
memory and stores a value even when the system is locked, we have "I".

I'd appreciate any comments on my scheme, what kinds of input clocks I
might need, if there are pitfalls and so on. I haven't actually built
this, but it's just an idea I've been toying with.

Thanks everyone.

-- Jay.

 

[Kevin Neilson]

Mine used a barrel shift in the feedback to get a gain that increased with the
size of the error. Had to do that to get a quick lock and still be able to
chase the reference. The reference was derived from a quad encoder on the
mechanical media path. The PLL had to adjust a process to keep a certain number
of events between encoder pulses. All in all, it was a pretty nasty problem
because of the dynamics and limited resolution of the encoder.

Kevin Neilson wrote:

I like that idea. Actually, I recalled that mine had a time-varying gain,
but it was much simpler. The gain was high before lock, and after locking,
the gain switched to something lower. The gain was just implemented by
left-shifting the output from the loop filter (which was just a comb or
moving-average filter), so the gain could only be powers of two. Does the
barrel shifter you describe increase the order of the loop? That probably
makes it a lot harder to describe mathematically. I would have liked to do
an analysis of mine, but of course I didn't have time, and for my
application stability was much more important than lock time so I didn't
really have to optimize it.
-Kevin

 

[Kevin Neilson]

Mine used a barrel shift in the feedback to get a gain that increased with the
size of the error. Had to do that to get a quick lock and still be able to
chase the reference. The reference was derived from a quad encoder on the
mechanical media path. The PLL had to adjust a process to keep a certain number
of events between encoder pulses. All in all, it was a pretty nasty problem
because of the dynamics and limited resolution of the encoder.

Kevin Neilson wrote:

I like that idea. Actually, I recalled that mine had a time-varying gain,
but it was much simpler. The gain was high before lock, and after locking,
the gain switched to something lower. The gain was just implemented by
left-shifting the output from the loop filter (which was just a comb or
moving-average filter), so the gain could only be powers of two. Does the
barrel shifter you describe increase the order of the loop? That probably
makes it a lot harder to describe mathematically. I would have liked to do
an analysis of mine, but of course I didn't have time, and for my
application stability was much more important than lock time so I didn't
really have to optimize it.
-Kevin

 

[Kevin Neilson]

Mine used a barrel shift in the feedback to get a gain that increased with the
size of the error. Had to do that to get a quick lock and still be able to
chase the reference. The reference was derived from a quad encoder on the
mechanical media path. The PLL had to adjust a process to keep a certain number
of events between encoder pulses. All in all, it was a pretty nasty problem
because of the dynamics and limited resolution of the encoder.

Kevin Neilson wrote:

I like that idea. Actually, I recalled that mine had a time-varying gain,
but it was much simpler. The gain was high before lock, and after locking,
the gain switched to something lower. The gain was just implemented by
left-shifting the output from the loop filter (which was just a comb or
moving-average filter), so the gain could only be powers of two. Does the
barrel shifter you describe increase the order of the loop? That probably
makes it a lot harder to describe mathematically. I would have liked to do
an analysis of mine, but of course I didn't have time, and for my
application stability was much more important than lock time so I didn't
really have to optimize it.
-Kevin

 

[Kevin Neilson]

Mine used a barrel shift in the feedback to get a gain that increased with the
size of the error. Had to do that to get a quick lock and still be able to
chase the reference. The reference was derived from a quad encoder on the
mechanical media path. The PLL had to adjust a process to keep a certain number
of events between encoder pulses. All in all, it was a pretty nasty problem
because of the dynamics and limited resolution of the encoder.

Kevin Neilson wrote:

I like that idea. Actually, I recalled that mine had a time-varying gain,
but it was much simpler. The gain was high before lock, and after locking,
the gain switched to something lower. The gain was just implemented by
left-shifting the output from the loop filter (which was just a comb or
moving-average filter), so the gain could only be powers of two. Does the
barrel shifter you describe increase the order of the loop? That probably
makes it a lot harder to describe mathematically. I would have liked to do
an analysis of mine, but of course I didn't have time, and for my
application stability was much more important than lock time so I didn't
really have to optimize it.
-Kevin

 

[Kevin Neilson]

Mine used a barrel shift in the feedback to get a gain that increased with the
size of the error. Had to do that to get a quick lock and still be able to
chase the reference. The reference was derived from a quad encoder on the
mechanical media path. The PLL had to adjust a process to keep a certain number
of events between encoder pulses. All in all, it was a pretty nasty problem
because of the dynamics and limited resolution of the encoder.

Kevin Neilson wrote:

I like that idea. Actually, I recalled that mine had a time-varying gain,
but it was much simpler. The gain was high before lock, and after locking,
the gain switched to something lower. The gain was just implemented by
left-shifting the output from the loop filter (which was just a comb or
moving-average filter), so the gain could only be powers of two. Does the
barrel shifter you describe increase the order of the loop? That probably
makes it a lot harder to describe mathematically. I would have liked to do
an analysis of mine, but of course I didn't have time, and for my
application stability was much more important than lock time so I didn't
really have to optimize it.
-Kevin

 

[Kevin Neilson]

Mine used a barrel shift in the feedback to get a gain that increased with the
size of the error. Had to do that to get a quick lock and still be able to
chase the reference. The reference was derived from a quad encoder on the
mechanical media path. The PLL had to adjust a process to keep a certain number
of events between encoder pulses. All in all, it was a pretty nasty problem
because of the dynamics and limited resolution of the encoder.

Kevin Neilson wrote:

I like that idea. Actually, I recalled that mine had a time-varying gain,
but it was much simpler. The gain was high before lock, and after locking,
the gain switched to something lower. The gain was just implemented by
left-shifting the output from the loop filter (which was just a comb or
moving-average filter), so the gain could only be powers of two. Does the
barrel shifter you describe increase the order of the loop? That probably
makes it a lot harder to describe mathematically. I would have liked to do
an analysis of mine, but of course I didn't have time, and for my
application stability was much more important than lock time so I didn't
really have to optimize it.
-Kevin

 

[Jay]

Hi all.

I was actually thinking about digital PLLs recently when I was doing
investigation on analog PLLs.

Can someone describe the basic parts and operation of a DPLL?
Essentially these are the components I figured would be needed and this
is how the operation might work, I'd appreciate comments....[/QUOTE]

Just wondering, I never got any responses to my original (and slightly
off-topic) post. Was I off in my design insights? I'm actually thinking
about using my aforementioned design in a project, I would like to know
if it's a good starting point or not...

One thing I was particularly curious about, when thinking about my own
design, do I have to worry about synchronizing / clock domain problems
between the two reference inputs to the loop or does the standard two-
DFF design actually take care of that?

I'm guessing that the *outputs* (Up/Down) from the DFF loop should be
put through a de-metastablizer (two DFF in series or something) since
those outputs could change irrespective of my system clock. Comments?

Oh, finally, is there a better frequency/phase detector than the
standard two-DFF one? I know that the two-DFF one hunts when the
phase/frequency are close to being locked.

Thanks!

-- Jay.

 

[Rudolf Usselmann]

Jasson,

take a look at out free USB 1.1 PHY IP core. It includes
a very simple DPLL. It is written in Verilog however.

Best Regards,
rudi

 

[Ray Andraka]

No it was a fixed second order loop. The barrel shifts just increased the
gain.

I like that idea. Actually, I recalled that mine had a time-varying gain,
but it was much simpler. The gain was high before lock, and after locking,
the gain switched to something lower. The gain was just implemented by
left-shifting the output from the loop filter (which was just a comb or
moving-average filter), so the gain could only be powers of two. Does the
barrel shifter you describe increase the order of the loop? That probably
makes it a lot harder to describe mathematically. I would have liked to do
an analysis of mine, but of course I didn't have time, and for my
application stability was much more important than lock time so I didn't
really have to optimize it.
-Kevin

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email (e-mail address removed)
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

 

[sridevi123]

Hi
I am designing ADPLL for clock recovery using vhdl ...Can some one please tell me how the DCO works... and also loop filter ... I am using JKFF for Phase and frequency detector...which gives up and down output...
I am held up with loop filter... I dont know the condition when carry and borrow should go high and when it should go low... Any help is welcome..