[time-nuts] Basic question regarding comparing two frequencies

Max Robinson max at maxsmusicplace.com
Mon Jul 26 13:12:18 EDT 2010


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

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----- 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.
>>
>>
>>
>>
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