[time-nuts] Homebrew frequency counter, need help
Bob Camp
kb8tq at n1k.org
Mon Dec 1 18:41:42 EST 2014
Hi
Looks like something came loose when you swapped around the parts. That or the regulator is very noisy.
Bob
> On Dec 1, 2014, at 11:39 AM, Li Ang <lllaaa at gmail.com> wrote:
>
> New tests today. It's strange that the data is not so stable as before.
>
> I've already add a 22uF Ceramic Cap and a 100nF. 10ohm resistor is removed.
>
>
>
>
> First:
> reg [15:0] shift;
> always @(posedge refclk10M) begin
> shift <= {shift[14:0], sw_gate};
> end
> assign tdc_start = shift[3];
> assign tdc_stop1 = shift[4];
> assign tdc_stop2 = shift[5];
>
> stop1-start= 935, stop2-stop1= 1049
> stop1-start= 957, stop2-stop1= 1047
> stop1-start= 961, stop2-stop1= 1046
> stop1-start= 1005, stop2-stop1= 1049
> stop1-start= 930, stop2-stop1= 1048
> stop1-start= 911, stop2-stop1= 1046
> stop1-start= 1005, stop2-stop1= 1049
> stop1-start= 936, stop2-stop1= 1048
> stop1-start= 930, stop2-stop1= 1047
> stop1-start= 1006, stop2-stop1= 1048
> stop1-start= 955, stop2-stop1= 1047
> stop1-start= 976, stop2-stop1= 1047
> stop1-start= 932, stop2-stop1= 1047
> stop1-start= 965, stop2-stop1= 1048
> stop1-start= 913, stop2-stop1= 1048
> stop1-start= 955, stop2-stop1= 1048
> stop1-start= 953, stop2-stop1= 1049
> stop1-start= 993, stop2-stop1= 1049
> stop1-start= 993, stop2-stop1= 1049
> stop1-start= 982, stop2-stop1= 1048
> stop1-start= 918, stop2-stop1= 1047
> stop1-start= 961, stop2-stop1= 1046
> stop1-start= 1007, stop2-stop1= 1047
> stop1-start= 912, stop2-stop1= 1046
> stop1-start= 928, stop2-stop1= 1049
> stop1-start= 995, stop2-stop1= 1048
> stop1-start= 1003, stop2-stop1= 1047
> stop1-start= 966, stop2-stop1= 1046
> stop1-start= 953, stop2-stop1= 1050
> stop1-start= 971, stop2-stop1= 1047
> stop1-start= 995, stop2-stop1= 1045
> stop1-start= 959, stop2-stop1= 1048
> stop1-start= 914, stop2-stop1= 1047
> stop1-start= 1002, stop2-stop1= 1047
> stop1-start= 1004, stop2-stop1= 1047
>
>
> =========================================
> Second:
> assign tdc_start = shift[3];
> assign tdc_stop1 = shift[5];
> assign tdc_stop2 = shift[7];
>
> //stop2-stop1 is not exact 2 times as test 1.
> stop1-start= 2024, stop2-stop1= 2104
> stop1-start= 2034, stop2-stop1= 2106
> stop1-start= 1991, stop2-stop1= 2104
> stop1-start= 2016, stop2-stop1= 2105
> stop1-start= 2019, stop2-stop1= 2105
> stop1-start= 2005, stop2-stop1= 2106
> stop1-start= 2006, stop2-stop1= 2104
> stop1-start= 1962, stop2-stop1= 2104
> stop1-start= 2056, stop2-stop1= 2106
> stop1-start= 2020, stop2-stop1= 2105
> stop1-start= 1961, stop2-stop1= 2107
> stop1-start= 2011, stop2-stop1= 2105
> stop1-start= 2005, stop2-stop1= 2106
> stop1-start= 2035, stop2-stop1= 2106
> stop1-start= 2011, stop2-stop1= 2105
> stop1-start= 2006, stop2-stop1= 2104
> stop1-start= 2013, stop2-stop1= 2104
> stop1-start= 1964, stop2-stop1= 2104
> stop1-start= 2054, stop2-stop1= 2105
> stop1-start= 2056, stop2-stop1= 2106
> stop1-start= 2019, stop2-stop1= 2106
> stop1-start= 1982, stop2-stop1= 2105
> stop1-start= 2010, stop2-stop1= 2105
> stop1-start= 1964, stop2-stop1= 2106
> stop1-start= 1989, stop2-stop1= 2106
> stop1-start= 2008, stop2-stop1= 2107
> stop1-start= 2059, stop2-stop1= 2106
> stop1-start= 1985, stop2-stop1= 2104
> stop1-start= 2011, stop2-stop1= 2105
> stop1-start= 1962, stop2-stop1= 2105
> stop1-start= 2012, stop2-stop1= 2104
> stop1-start= 2008, stop2-stop1= 2105
> stop1-start= 2048, stop2-stop1= 2104
> stop1-start= 2009, stop2-stop1= 2106
> stop1-start= 2009, stop2-stop1= 2105
> stop1-start= 2006, stop2-stop1= 2105
> stop1-start= 2009, stop2-stop1= 2105
> stop1-start= 2009, stop2-stop1= 2104
> stop1-start= 2005, stop2-stop1= 2105
> stop1-start= 2049, stop2-stop1= 2105
> stop1-start= 2023, stop2-stop1= 2106
>
> ++++++++++++++++++++++++++++++++++++++++++++++++++
> Third:
> //This test shows that stop2 path is shorter than stop1 path. I need to
> consider it when doing the calc.
> assign tdc_start = shift[3];
> assign tdc_stop1 = shift[5];
> assign tdc_stop2 = shift[5];
>
> stop1-start= 2059, stop2-stop1= -5
> stop1-start= 2065, stop2-stop1= -6
> stop1-start= 1989, stop2-stop1= -2
> stop1-start= 2013, stop2-stop1= -2
> stop1-start= 2011, stop2-stop1= -5
> stop1-start= 2019, stop2-stop1= -6
> stop1-start= 1987, stop2-stop1= -6
> stop1-start= 2065, stop2-stop1= -2
> stop1-start= 1974, stop2-stop1= -6
> stop1-start= 1968, stop2-stop1= -2
> stop1-start= 2055, stop2-stop1= -5
> stop1-start= 2065, stop2-stop1= -5
> stop1-start= 2012, stop2-stop1= -2
> stop1-start= 2035, stop2-stop1= -2
> stop1-start= 2014, stop2-stop1= -2
> stop1-start= 2014, stop2-stop1= -6
> stop1-start= 2028, stop2-stop1= -1
> stop1-start= 2034, stop2-stop1= -6
> stop1-start= 2013, stop2-stop1= -2
> stop1-start= 1974, stop2-stop1= -5
> stop1-start= 2014, stop2-stop1= -2
> stop1-start= 2009, stop2-stop1= -1
> stop1-start= 2029, stop2-stop1= -5
> stop1-start= 2053, stop2-stop1= -2
> stop1-start= 2017, stop2-stop1= -5
> stop1-start= 2013, stop2-stop1= -2
> stop1-start= 1992, stop2-stop1= -1
> stop1-start= 2016, stop2-stop1= -2
> stop1-start= 1970, stop2-stop1= -2
> stop1-start= 1973, stop2-stop1= -2
> stop1-start= 2065, stop2-stop1= -5
> stop1-start= 1975, stop2-stop1= -5
> stop1-start= 2017, stop2-stop1= -6
> stop1-start= 2013, stop2-stop1= -6
> stop1-start= 2016, stop2-stop1= -2
> stop1-start= 1987, stop2-stop1= -2
> stop1-start= 2062, stop2-stop1= -6
> stop1-start= 2021, stop2-stop1= -5
>
>
>
>
> 2014-11-29 1:41 GMT+08:00 Bob Camp <kb8tq at n1k.org>:
>
>> Hi
>>
>>> On Nov 28, 2014, at 10:14 AM, Li Ang <lllaaa at gmail.com> wrote:
>>>
>>>>
>>>> 1) What frequency is the crystal at? (can you drive the chip from an
>> OCXO?)
>>>
>>> I'm clking this chip with the refclk/2=5Mhz, which is done by cpld.
>>> something like always @(posedge refclk) tdc_clk <= ~tdc_clk; Not sure if
>>> it's noisy.
>>
>> If the clock source into the CPLD is clean, the output should not be to
>> bad.
>>
>>>
>>> 2) Is there more bypassing on the circuit than shown? (If not, add some
>>>> more).
>>>
>>> Since there is only few power supply pins, I just put 100uF + 100nF
>> there.
>>> I can try put more to see if helps. And I'm going to use dedicated LDO
>> for
>>> digital part and analog part next time to make a new pcb.
>>
>> I suspect that a few more 100nF caps might be a good idea. They may not
>> help, but it would rule out the supply as an issue.
>>
>>>
>>> 3) How confident are you of your input signal? (can you check it with a
>>>> “known good” counter?)
>>>
>>> The best counter I get is the RACAL DANA 1992. That is not good enough.
>> I
>>> think the accurarcy is not a problem, as if the reference is good enough,
>>
>> That’s correct. I was hoping you might be able to borrow a SR620 or 53132
>> for a few minutes to check your input signal.
>>
>>> it's only the matter of math. I prefer to enhance the resolution and
>> reduce
>>> noise.
>>
>> If the noise is on your test signal, it can be very frustrating to chase
>> it with a lot of software work.
>>
>>> I'm using Trimble GPSDO(NTPX26AB) as the signal source
>>
>> The GPSDO should be fairly quiet, but it has an ADEV that’s a bit high.
>> The outputs also can have noise on them.
>>
>>> and FE5650 Rb
>>
>> The FE Rb’s tend to have a lot of spurs on the output. In some cases that
>> can get you in trouble.
>>
>>> as the ref. I've tried Rb+MV89A
>>
>> If the MV89A is working properly, it should be a pretty good source. Based
>> on some of the prices I’ve seen on the internal China market, you might get
>> a couple of them as sources.
>>
>>> and Rb + SMY01 signal generator, same
>>> performance. The 2nd-hand 53132A,PM6690,SR620 about 600~800$ here in
>> China.
>>> I'm trying not to get one unless necessary. If I have get one to compare
>>> the performance, what's your suggested model?
>>
>> The SR620 is a good counter, so is the 53132. They both can have problems.
>> The 53132 display wears out and it’s power supply can fail. The 620 can run
>> a bit hot, which kills a variety of parts in it. It’s better to pay a bit
>> more for one you can actually check out before you buy than to get one
>> shipped in.
>>
>>>
>>>
>>>> 4) Have you tried jumping the 10 ohm resistor on the regulator output?
>> (it
>>>> may not be helping things …)
>>>
>>> I'll try to remove that tomorrow
>>
>> It may be allowing the supply to drop a bit when the chip goes into some
>> sort of computation. Often these things happen at just the wrong time …
>>
>>
>> ————————
>>
>> If you have a CPLD and a MV89:
>>
>> 1) With a 10 MHz sine wave out of the OCXO, you need to convert it to
>> logic first. A biased input is a pretty good way to do this.
>>
>> 2) Generate 200 or 400 ns wide pulses out of the CPLD for testing. That
>> will eliminate any issues from the 5 MHz crystal in the MV-89.
>>
>> 3) Keep the PCB as simple as you can. You need at least a double sided
>> board (one side ground plane). If you can get a cheap 4 layer board, go for
>> it. A full internal ground plane is a good thing.
>>
>> 4) Route the high speed signals (like the OCXO output) through solid
>> connections. Flying wire leads are not a good idea. Mounting a MV89 direct
>> to the PCB is a good way to do things. SMA connectors are also good.
>>
>> 5) If you have an oscilloscope or can borrow one, take a look at the
>> signals on your board. Even a quick check can tell you a lot about signals
>> that are not what they should be.
>>
>> 6) Be careful of ground loops and power supply issues. I’ve spent a *lot*
>> of time on breadboards that didn’t work because I had power line noise
>> running around.
>>
>> Good luck !!
>>
>> Bob
>>
>>
>>>
>>>
>>> All of which is *plenty* good enough to make a decent counter. That
>>>> assumes that they are talking about accuracy (even 1 sigma) rather than
>>>> just the resolution of the LSB. Specs are often confusing on parts like
>>>> this.
>>>
>>> I guess the accuracy is not important in the interpolator scenario. It
>> has
>>> a feature to output the result of delta_time / ref_cycle_time(it will
>>> measure the 2cycle_time - 1cycle_time after the delta_time measurement
>> and
>>> do the float calculation). All I need is this part(it's the fraction
>> part
>>> of refcnt). So if I can reach 90ps resolution they claim, the counter can
>>> tell 1/1000 of one reference cycle. That's 3 digits.
>>>
>>>
>>>
>>> Thanks.
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