[time-nuts] time-nuts Frequency Divider

David C. Partridge david.partridge at dsl.pipex.com
Sun Apr 5 15:56:01 UTC 2009


Just another reminder - this job has been done, at least no-one has come
back to me and said "Dave, your divider isn't up to scratch".

Cheers
Dave
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of EWKehren at aol.com
Sent: 05 April 2009 15:26
To: time-nuts at febo.com
Subject: Re: [time-nuts] time-nuts Frequency Divider

Thank you Bruce for your valuable insight. In the past I did spend time on
Cesium Standards and with valuable inputs from Corby concerning the FTS tube
interface I converted a HP 5062C to a Standard using a FTS tube. I also made
other changes like changing to a 10 MHz 10811, the Master Clock 1 PPS
assembly and the 1 PPS Advance Generator assembly. It has served me well
over the last  ten years. Only recently did I get back into the game with
more focus on oscillators, mainly because of the vast accumulation of units
and with Corby's help am looking at oscillators and try to get good short
term as well as long term performance. This is all part of a downsizing
effort. Way to much  stuff.
I only recently joined time-nuts and follow the dialog. It is clear that
there is a vast knowledge base that could and probably is used to develop
solutions in this arena and a candidate could be a  general purpose  divider
chain that addresses most if not all the issues.
Over forty years ago in my college years I did a counter using Motorola MC
790 series and presented a student paper on that subject at IEEE. Did some
more  work at home at TI that you can find in old TI Opto Electronics
Catalogs. In  this case my contribution would be limited to laying out a PC
board. 
Bert
 
 
 
In a message dated 4/4/2009 7:09:00 P.M. Eastern Daylight Time,
bruce.griffiths at xtra.co.nz writes:

Bert

Unless you can provide a set of specifications it isn't  possible to
recommend any particular design as being fit for the  application.

For the purposes of comparing a divider generated PPS  signal against a GPS
derived PPS output almost any input clock shaper  should have adequate
performance.
For such applications achieving a  sufficiently low jitter PPS output can be
achieved using almost any divider  chain.
However ensuring a low clock to output propagation delay tempco  is entirely
another matter and either requires a fully synchronous  divider or using an
output retiming synchroniser.
If multiple ripple  clocking is used in the divider then a 3 stage
synchroniser that first  resynchronises to 1MHz, then 2MHz and finally to
10MHz may be  required.
If the lower frequency dividers in the cascade use something as  slow as
4000 series CMOS then even more synchroniser stages will be  required.

If we consider minimising the trigger jitter of an SR620  driven from say a
2V pp 10MHz  sine wave, then a single stage limiter  with a slope gain of
about 5x and a bandwidth of about 50MHz will ensure  that the trigger
circuit jitter due to its linter and trigger circuit input  noise is reduced
to below 2ps rms with a low noise input source. If a 2  stage limiter
cascade with the same slope gain is used the jitter due  to limiter and
trigger circuit input noise can be reduced to below  1.3ps rms with a low
noise source if the low pass filter time constant at  the output of each
stage is chosen appropriately.

If the output of a  divider is to be used as a low phase noise source then
picosecond or even  subpicosecond jitter is desirable.
For the ultimate performance in such  applications a regenerative divider
can have a much lower output phase  noise than a digital divider.
Injection locked dividers are one form of  regenerative divider complete
with integrated mixer and filter, however  conjugate regenerative dividers
using a diode mixer and low phase noise  amplifier etc will have lower phase
noise.

Bruce

EWKehren at aol.com wrote:
> Bruce, thank you  for the info. I have never had the need or desire to 
> get
1  
> ps  accuracy however in designing low noise signal sources I have 
> always
had to  
>  battle  reference oscillator noise and was often nor sure  if it was 
> the oscillator or the input circuit. However I would  like to see a
recommendation as  
> to an attainable design. Thanks  again Bert
>  
>  
> In a message dated 4/4/2009  4:35:09 P.M. Eastern Daylight Time, 
> bruce.griffiths at xtra.co.nz  writes:
>
> Bert
>
> Neither the HP5370 nor the SR620  have low enough internal  jitter to 
> accurately characterise the  intrinisic output jitter of either a  
> 74HC04
> (~4ps) or a 74AC04  (~1ps).
> Rather than just tossing together a  divider from various  parts 
> though to produce an output with low jitter its  better to  be able to
characterise
> the jitter properties (intrinsic as well   as that due to logic device
> input noise with a finite input signal  slew  rate)  of various logic 
> families.
> It is then  possible to actually  design a sine to logic level 
> converter that  achieves the lowest possible  output jitter for a given
complexity
> and specified input frequency and   amplitude.
>
> The real problem is that one needs to accurately  measure  jitter of 
> 1ps or so.
> There are few time interval  counters that allow  this.
> One can also measure the change in  noise floor when such a device  is 
> placed in the clock input path  of a high frequency ADC and thence  
> derive the jitter.
> In  principle, the output jitter of a divider can also  be calculated  
> from the phase noise spectrum of its  output.
>
>  Bruce
>
>
>
> EWKehren at aol.com  wrote:
>   
>> Having  built eight of Brooks  units, my experience was that the 
>> problem
was 
>>      
>   
>   
>> not with the amplifier  but the way the RS F/F in the phase 
>> comparator  II
>>   
> was
>   
>> working in  some of the devices. For me they all worked  in the 
>> oscillator
 
>>     
> input
>    
>> but some brands did not work properly  with the GPS input.  With all 
>> the
>>     
> dialog
>   
>> on  the divider subject,  is it  not time to develop one design that
>>     
>  combines
>   
>> KISS and  all  the  collective know how? Bert Kehren WB5MZJ
>>   
>>   
>> In a message dated 4/3/2009 5:17:18 P.M.  Eastern Daylight Time,   
>> bruce.griffiths at xtra.co.nz  writes:
>>
>>  Correction:
>>
>> I  forgot to include the intrinsic jitter of the  gate  in the
calculations.
>> See underlined corrections    below.
>>
>>
>> Bruce
>>
>> Bruce  Griffiths  wrote:
>>
>>
>>    
>>     
>>>    Magnus
>>>
>>> The input noise of a logic inverter or  other  trigger  device used 
>>> as a clock shaper  is  important.
>>> If we have a logic  inverter device  with the  following
characteristics:
>>>
>>>  Input  noise: 100uV  rms
>>> Intrinsic jitter: 1ps  rms
>>>
>>> Then  the  input signal slew  rate at the threshold crossing has to  be
>>> greater   than
>>>
>>> 3x1E-4/1E-12 = 3E8  V/s or 300  V/us
>>>
>>> to  ensure that the output  jitter  isnt increased by more than 5% 
>>> from
the
>>>   intrinsic  jitter.
>>>
>>> With a 1.4V pk 10MHz  sinewave input the   maximum slew rate is ~89V/us

(at
>>> the zero crossing).
>>>  For such   an input signal the output jitter will be about _1.5   ps_.
>>> This  increases to about _1.72ps_ if there is  a  threshold offset of
1V.
>>> This  can be reduced to  about 1.05ps  by amplifying the slope of 
>>> the
input
>>>   signal by ~  3.4x.
>>>
>>> The intrinsic jitter  (RJ. DDJ isn't   important when the input signal
is 
 a
>>> low distortion sinewave) of  a  74AC04 inverter is  about 1ps.
>>> However the equivalent input  noise is   unknown.
>>> The noise could, in principle, be  determined by  measuring the  
>>> output jitter as a function of  the  input signal slew  rate.
>>>
>>> Whilst AM and  other  noise associated with the source can  be 
>>> reduced  by  filtering, the input noise of a trigger circuit  cannot
(except  perhaps
>>> for the trigger circuits  input current   noise).
>>>
>>> Magnus  Danielson wrote:
>>>   
>>>   
>>>       
>>>>  Bruce Griffiths  skrev:
>>>>    
>>>>    
>>>>     
>>>>          
>>>>>   Ulrich
>>>>>
>>>>> Your  experience  with the  SR620 illustrates the point I was making

>>>>>           
>  quite
>   
>>>>>   well.
>>>>> It really does matter what you do  in front  of  the limiter 
>>>>> circuit
built
>>>>> into  the   counter.
>>>>> A bandpass or any other  filter by itself is   ineffective unless the
>>>>>  signal is exceptionally    noisy.
>>>>>
>>>>> By using the inverter  in the  74HCT4046  you have added a low 
>>>>> gain
>>>>>           
>  limiter
>   
>>>>> stage  the bandwidth  of which  is smaller than that of the SR620   
input
>>>>>  circuit.
>>>>> This has  the  effect of increasing the slew rate of  the input
signal
>>>>> whilst producing an output with less  jitter   than the SR620 input 
>>>>>     
> circuit
>    
>>>>> would without this low  pass  filtered  limiter circuit (the 
>>>>> inverter
from
>>>>> the   74HCT4046).  The slew rate at the 74HCT4046 inverter output   is
>>>>> greater than  that of the input signal which  means  that the jitter
due
>>>>> the  counter  input circuit noise  is smaller than when this low gain

low
>>>>> bandwidth  limiter isn't  used.
>>>>> The input  circuit of the SR620  has a  wide noise bandwidth (~ 470MHz
>>>>>  assuming a   single pole response with a 300MHz 3dB high frequency

>>>>>           
>  cutoff)
>   
>>>>> and a correspondingly  high total input noise  (~350uV  rms).
>>>>> If  the slew rate of the SR 620 input  signal at the  trigger point
the
>>>>> jitter due to this  noise dominates the   trigger circuit output
jitter.
>>>>>  The HP5370 time  interval  counter input circuit has a lower   noise
>>>>> bandwidth (~160MHz??)  and is quieter (~  100uV  rms) than the input
>>>>> circuit of the   SR620 and thus  the HP5370 jitter (without the  
74HCT4046
>>>>>  limiter)  for the same 10MHz  signal should be less than that of the

 
>>>>>           
>  SR620
>   
>>>>> (without the  74HCT4046  limiter).
>>>>>    
>>>>>     
>>>>>   
>>>>>       
>>>> As a  curiosity, there are  various variants of the  original 4046
which 
>>>>   has different sensitivity on the  input side... one of them has
several  
>>>> inverters in a  row to get the needed  gain where as the other  variant

>>>>  does not.  This difference made a huge difference in some    
applications.
>>>>
>>>>     
>>>>   
>>>>       
>>>>         
>>>  The appropriate device (one that will have the  least output   jitter)
to
>>> use will vary with the input signal zero   crossing  slew rate.
>>> That is it depends on both the input  signal   frequency and  amplitude.
>>>
>>>    
>>>   
>>>        
>>>>> If one  uses a state  of the art trigger  circuit with a noise
bandwidth  
>>>>>     
> of
>    
>>>>>  1GHz or more then the total input noise will be  even larger  so  
>>>>> it becomes even more  important to use an optimised  cascade  of 
>>>>> limiter+
low
>>>>> output pass filter stages to  increase the  slew  rate of the counter
>>>>> input    trigger circuit at the trigger  threshold.
>>>>>  Careful  optimisation of the gain of each stage and  the   
corresponding
>>>>> output filter cutoff frequency for   each  stage is necessary to 
minimise
>>>>> the  output  jitter of the  counter trigger  circuit.
>>>>> There is also  an optimum number  of  such stages that minimises the
>>>>>   trigger  jitter.
>>>>>
>>>>> The  optimisation  problem for Limiter  stages with gaussian  
>>>>> wideband
>>>>>         
> input
>   
>>>>> noise  was solved in the   1990's.
>>>>> Unfortunately  the optimum number of stages,  associated  gains 
>>>>> and
output
>>>>> filter bandwidths  depends on the input  signal  frequency and
amplitude  
>>>>>   
> so
>    
>>>>> that in general it isn't possible  to use the  same  limiter 
>>>>> cascade
for a
>>>>> wide range of  signal   amplitudes and frequencies and minimise the  
jitter
>>>>> for  each  frequency and  amplitude.
>>>>>        
>>>>>        
>>>>>   
>>>>>     
>>>> Actually, you can make   a  cascade setup which is approaching optimum 
and
>>>>  insert signal  at  the stage where the signals slewrate matches 
>>>> the
range 
>>>>         
>   
>   
>>>> for each  stage. Since the gain  steps is larger later in  a slew rate 
>>>>   amplifier chain, the last stages may have a  little coarse slew  rate

>>>> range, but additional mid-range   amplifiers that can act as  
alternative 
>>>> input amps  could  curcumvent that such that a wide  range but and
fairly  
>>>>  good trigger jitter could be   achieved.
>>>>
>>>>  The comparator level  is fed to whatever  stage is the first   stage.
>>>>
>>>> Such an approach could  lead  to  much improved jitter values for lower

>>>>  frequency signals  with  associated gain in measurement   accuracy.
>>>>
>>>> It is easy to  make  a  pre-amplifier set that achieves this, but 
>>>> you
want  
>>>> to   integrate the control algorithms for  automatic   use.
>>>>
>>>>    
>>>>     
>>>>     
>>>>          
>>> That would  constitute an interesting design   challenge.
>>>    
>>>      
>>>       
>>>>> Thus such  circuits  aren't  usually employed in general purpose
>>>>>           
>  frequency
>   
>>>>>     
>>>>>         
>>  counters.
>>    
>>     
>>>>>     
>>>>>        
>>>>>   
>>>>>     
>>>> Certainly  true. A  generic  counter is usually equipped with 
>>>> triggers
>>>>         
> such
>   
>>>>  that  they can measure  slewrate without too much    difficulty.
>>>>
>>>>     
>>>>   
>>>>       
>>>>          
>>>>> However if the input signal frequency and   amplitude  are known and 
>>>>>       
> stable
>    
>>>>> then using such a limiter   filter cascade  is feasible.
>>>>>        
>>>>>        
>>>>>   
>>>>>     
>>>>    Indeed.
>>>>
>>>>  Cheers,
>>>>    Magnus
>>>>
>>>>    _______________________________________________
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>>>> to
>>>>     
>>>>          
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>>   
>>     
>>>> and follow   the instructions  there.
>>>>
>>>>   
>>>>   
>>>>     
>>>>          
>>> Bruce
>>>
>>>    _______________________________________________
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>>>   
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