[time-nuts] Frequency divider design critique request

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jul 12 05:06:33 EDT 2008


Magnus
>> Magnus
>>
>> A minimalist approach for the 5MHz to 10MHz doubler could use a full 
>> wave (diode, BJT or JFET) doubler followed by a series tuned 5MHz 
>> shunt trap to minimise the 5MHz content in the output.
>>     
>
> Actually, it depends on weither you would like to get a 10 MHz or not.
> Another solution would be to run the first divider to /5 rather than /10
> and only use the doubler for the 10 MHz output. Ah well.
>
>   
>
>> The other harmonics are of little concern as the comparator output is 
>> a square wave and the rectified sinewave waveform would produce a 
>> duty cycle of around 44% at the comparator output.
>>     
>
> Unless the duty cycle is important, the overtones help to keep the 
> slew rate
> high and this avoids adding too much jitter.
>
>   
Yes, its only necessary to ensure that the odd harmonics of the 5MHz 
input are low the even harmonics need not be removed.
>
> Good thought. Should be a trivial thing.
>
>   
Except that with a Schmitt trigger device in the loop the duty 
stabiliser will oscillate at a low frequency in the absence of an input 
signal.
This may or may not be a problem.
In fact if one uses a comparator with a totem pole output the loop is 
practically guaranteed to oscillate at low frequency in the absence of 
an input signal.
One could always disable the duty cycle stabiliser unless the input 
signal is large enough (requires an RF detector and another comparator).
>> However such a duty cycle stabiliser only works when the input signal 
>> waveform is sinusoidal, rectified sinewave or any other signal with a 
>> slow enough slew rate.
>>     
>
> Indeed.
>
> I was only thinking that maybe there ought to be a buffer from the input
> to the rectifier, or else higher frequency energy will escape out 
> towards the
> source. At least some isolation should be there.
>
>   
For suitable designs see:
http://www.ko4bb.com/~bruce/IsolationAmplifiers.html 
<http://www.ko4bb.com/%7Ebruce/IsolationAmplifiers.html>

If the phase noise performance isnt too critical a low noise wideband 
opamp could be used.
Alternatively if 30 dB or so reverse isolation is OK use an emitter 
follower (if the relatively high collector current required is 
acceptable) or better still cascade a couple for improved isolation.
> Cheers,
> Magnus
>
>   

Bruce




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