# [time-nuts] time-nuts Frequency Divider

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Apr 3 01:14:35 UTC 2009

```Correction:

For those who didn't spot it, the formula should have been:

delta(t) ~ (1/(2*PI*f))*((Vt/A)/(1 + (Vt/A)*(Vt/A)))*SQRT(BW)*10^(Am/20));

Bruce

Bruce Griffiths wrote:
> Mike
>
> Its well worthwhile estimating the additional jitter due to this effect
> when using such a circuit to square up the output of an OCXO:
>
> If the input signal characteristics are:
>
> Frequency 10MHz
> Amplitude at the gate input:  A = 1.4V pk
> Threshold mismatch             Vt  =   1V
> AM noise:                             Am = -120dBc/Hz
> Input signal AM noise bandwidth: BW = 1MHz (eg a low Q bandpass filter).
>
> Rms Output jitter due to AM noise is given by
>
> delta(t) ~ (1/(2*PI*f))*((Vt/A)/(1 + (Vt/A)*(Vt/A)))*(BW*1)^(Am/20))
>
> i.e.
> delta(t) ~ 0.5*1.6E-8 *(1E-3) sec
> ~ 8ps rms.
>
> Wideband AM noise as high as -120dBc/Hz is somewhat higher than is
> typical for a good OCXO.
>
> Thus in applications such as a PPS divider this effect is probably
> insignificant.
> However it may be useful to use a low Q bandpass filter to limit the
> integrated AM and PM noise seen at the gate input.
>
> Bruce
>
>
> Mike Monett wrote:
>
>>   > Message: 3
>>   > Date: Fri, 03 Apr 2009 09:04:59 +1300
>>   > From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>>   > Subject: Re: [time-nuts] Frequency Divider
>>
>>   > Hal Murray wrote:
>>
>>   >>> A large  resistor connected between the input  and  output would
>>   >>> accommodate threshold variations better. Even better would  be a
>>   >>> feedback loop that adjusts the input bias point to  maintain the
>>   >>> output duty cycle at 50%.
>>
>>   >> Isn't that resistor a feedback loop?
>>
>>   >> I played  with  that setup in the lab many years  ago.  It didn't
>>   >> work as  well  as  I was expecting. I didn't  figure  out  why it
>>   >> didn't work better.
>>
>>   >> Maybe some gain in the feedback path would help. Then we  have to
>>   >> consider stability. Ugh.
>>
>>   > Hal
>>
>>   > Yes, a  resistor  connected  between the input  and  output  of an
>>   > inverter is a feedback loop but the loop gain is relatively low.
>>
>>   > With a high amplitude input threshold variations from  the nominal
>>   > can cause the input protection diodes to conduct.
>>
>>   > Once these  diodes  conduct   the  output  jitter  may deteriorate
>>   > significantly (it does for HCMOS inverters).
>>
>>   > Using a  non  inverting   integrator   in  the  feedback  path can
>>   > accurately stabilise the duty cycle.
>>
>>   > Bruce
>>
>>   The 74HC  and 74AC input threshold tolerance is +/- 30%.  This means
>>   the threshold can vary from 1.5V to 3.5V with a Vcc of 5V.
>>
>>   This limits  the  maximum input signal to 3V  p-p  or  +13.5dBm, and
>>   leads to  a very subtle flaw discovered in some  amazing engineering
>>   work by Martein Bakker, PA3AKE.
>>
>>   If the threshold is not controlled, it can cause AM noise to convert
>>   to PM  noise  and  degrade the jitter.  This  occurs  in  the Analog
>>   Devices AD9910 1GHz DDS chip.
>>
>>   Martein Bakker  discovered  this in his  noise  analysis,  and Kevin
>>   Wheatly gave a nice entry in his blog on how to fix it:
>>
>>   http://www.m0khz.com/?p=589
>>
>>   Mike
>>
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>>
>>
>
>
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```