[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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