[time-nuts] Basic question regarding comparing two frequencies

Tue Jul 27 03:36:07 UTC 2010

Understood.

Regards.

Max.  K 4 O D S.

Email: max at maxsmusicplace.com

Transistor site http://www.funwithtransistors.net
Vacuum tube site: http://www.funwithtubes.net
Music site: http://www.maxsmusicplace.com

To subscribe to the fun with transistors group send an email to.
funwithtransistors-subscribe at yahoogroups.com

To subscribe to the fun with tubes group send an email to,
funwithtubes-subscribe at yahoogroups.com

----- Original Message ----- 
From: "Neville Michie" <namichie at gmail.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Monday, July 26, 2010 5:48 PM
Subject: Re: [time-nuts] Basic question regarding comparing two frequencies

> The reason to divide was that the signal from the phase detector  "folds 
> back" as the phase shift gets to 360*.
> At 10Mhz the fold back occurs every 100ns. At 100kHz it is every  10usec. 
> As the fold back (359.9 - 0.1degree) zone may have false  triggering or 
> other noise
> it made sense for it to be made a less frequent event. Also I did not 
> have faith in the CMOS output giving a true PWM average when clocking  so 
> fast. Chip capacitance produces a more significant amount of  current at 
> the higher clock rate.
> It may well work OK at the 10MHz rate. I also needed to divide to 
> increase the full scale time to account for large time jitter of 
> mechanical clocks so I set it up to divide at any of a wide range of 
> frequencies.
> Cheers, Neville Michie
>
> On 27/07/2010, at 3:12 AM, Max Robinson wrote:
>
>> Hal Murray wrote:
>>
>>>> There is another way to compare two frequencies, relevant when   they 
>>>> are
>>>> very close together. I divide a reference down to 100KHz and use   it 
>>>> to clock
>>>> a phase detector made of a pair of D flip flops. The unknown   (divided 
>>>> to
>>>> 100KHz) is fed into the circuit and an output   that is   proportional 
>>>> to the
>>>> phase difference appears on the output as a changing mark-space 
>>>> ratio.
>>
>> I'm wondering why divide the frequency at all.  Seems to me you  would 
>> get much greater resolution if you did the phase comparison  at the 
>> native frequency.
>>
>> Regards.
>>
>> Max.  K 4 O D S.
>>
>> Email: max at maxsmusicplace.com
>>
>> Transistor site http://www.funwithtransistors.net
>> Vacuum tube site: http://www.funwithtubes.net
>> Music site: http://www.maxsmusicplace.com
>>
>> To subscribe to the fun with transistors group send an email to.
>> funwithtransistors-subscribe at yahoogroups.com
>>
>> To subscribe to the fun with tubes group send an email to,
>> funwithtubes-subscribe at yahoogroups.com
>>
>> ----- Original Message ----- From: "Neville Michie"  <namichie at gmail.com>
>> To: "Discussion of precise time and frequency measurement" <time- 
>> nuts at febo.com>
>> Sent: Monday, July 26, 2010 1:19 AM
>> Subject: Re: [time-nuts] Basic question regarding comparing two 
>> frequencies
>>
>>
>>> Hi,
>>> the original was built using a HP10811 oscillator and a Garmin 17  GPS 
>>> that delivered PPS.
>>> The HP10811 ran a divider by 10 by 10 by 10 down to 1 hz.
>>> I was the servo that adjusted the EFC of the OCXO so that the PPS 
>>> matched the 1Hz.
>>> The divider clocked a counter of three decades of BCD, with  latches 
>>> driving a 3 decade DAC. (about 12 bits of modified R-2R  chain)
>>> The latches were triggered by a pendulum clock being observed, or  the 
>>> PPS of the Garmin GPS receiver.
>>> That delivered a DC signal that could be logged to observe phase   drift 
>>> on a chart recorder or data logger.
>>> For higher frequencies, I used the D FF phase detector, which  could  be 
>>> used at 1MHz, 100kHZ, 10kHz, 1kHz or 100Hz,
>>> depending on how sensitive I wanted the frequency (phase)  comparison. 
>>> The test was that the phase noise must be less than  one tenth
>>> of a period, so the automatic regeneration of the more  significant 
>>> digits in XL afterwards did not have ambiguities.
>>> For any oscillator under examination I used a 4046 PLL to generate  a 
>>> high enough frequency to drive the phase detector.
>>> My 1 Hz pendulum clock generated a 1kHz signal via the 4046 so  the 
>>> phase detector gave 1ms full scale on the chart recorder,
>>> with a resolution of 1 microsecond. The low pass filtering  inherent  in 
>>> the PLL was not a worry as I was concerned with  longer term drift.
>>>
>>> It all avoids using digital processing and other instruments, the   main 
>>> reason for that was to be able to leave it running for weeks   with only 
>>> low
>>> battery backup power required.
>>>
>>> cheers, Neville Michie
>>>
>>> On 26/07/2010, at 3:12 PM, Hal Murray wrote:
>>>
>>>>
>>>>> There is another way to compare two frequencies, relevant when   they 
>>>>> are
>>>>> very close together. I divide a reference down to 100KHz and  use  it 
>>>>> to clock
>>>>> a phase detector made of a pair of D flip flops. The unknown 
>>>>> (divided to
>>>>> 100KHz) is fed into the circuit and an output   that is   proportional 
>>>>> to the
>>>>> phase difference appears on the output as a changing mark-space 
>>>>> ratio.
>>>>
>>>> I like it.  Thanks.
>>>>
>>>> How did you pick 100 KHz?
>>>>
>>>>> Using CMOS and a precise power supply (because under no load, CMOS
>>>>> output is precisely rail to rail), the averaged output (100ms RC 
>>>>> filter) is
>>>>> fed to a strip chart recorder.
>>>>
>>>> Has anybody checked the edge cases and/or linearity of a setup  like 
>>>> this?
>>>>
>>>>> The recorder shows the changing phase difference and folds back   each 
>>>>> time
>>>>> a whole cycle passes. A 12 bit analog data logger resolves  2.5ns  of 
>>>>> phase
>>>>> and gives data for further analysis.
>>>>
>>>> Is 2.5 ns good enough?  What would you gain by using a 16 bit DAC?
>>>>
>>>>
>>>>
>>>> If 2.5 ns is good enough, I'll bet you can do the whole thing in 
>>>> digital
>>>> logic.  Just get a fast FPGA/CPLD.  I haven't done a serious   design, 
>>>> but a
>>>> quick check at some old data sheets shows it's not silly.  You   could 
>>>> probably
>>>> bump it up by another factor of 2 with some external (p)ECL chips.
>>>>
>>>>
>>>>
>>>> -- 
>>>> These are my opinions, not necessarily my employer's.  I hate spam.
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> time-nuts mailing list -- time-nuts at febo.com
>>>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/ listinfo/ 
>>>> time-nuts
>>>> and follow the instructions there.
>>>
>>>
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com
>>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/ 
>>> listinfo/time-nuts
>>> and follow the instructions there.
>>
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ 
>> time-nuts
>> and follow the instructions there.
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to 
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
> 