[time-nuts] Wavecrest DTS-2077 Teardown

Ed Palmer ed_palmer at sasktel.net
Wed Aug 21 22:36:17 EDT 2013


On 8/21/2013 5:52 PM, Magnus Danielson wrote:
> On 08/21/2013 03:51 AM, Ed Palmer wrote:
>> Adrian,
>>
>> I used Timelab to assess the reaction of the DTS-2077 to different
>> sine wave inputs.  The differences in the noise floor are surprising.
>> The attached picture was made by taking the output of an HP 8647A
>> Synthesized Generator through a splitter, and then through different
>> lengths of cables to the inputs of the DTS-2077.  The combination of
>> splitter and cable loss meant I couldn't get +7 dbm @ 1 GHz.  If I
>> could have, the 1 GHz line might have been lower than it was.
> This is not very surprising. As you increase frequency, the slew-rate
> changes, and as slewrate increases, it convert noise to jitter to a
> lesser degree. Formula for slew-rate:
>
> S = A*2*pi*f
>
> where A is the amplitude and f is the frequency
>
> Formula for jitter
>
> T = e_n / S
>
> where e_n is the noise RMS amplitude, S the slew-rate and T the timing
> jitter RMS.
>
> As you get closer to the instruments internal jitter, which forms a
> floor, the increased slew-rate does not improve as quickly as you would
> think.
>
> Cheers,
> Magnus

Yes, I realize all that.  Since I couldn't vary the slew rate of a 
pulse, I used sine waves to 'stand in' for varying slew rates to find 
the value that didn't degrade the results.  5 or 10 MHz is a high enough 
frequency that most equipment won't have a problem with it.  But the DTS 
has such a high level of performance that you need to pay special 
attention to the quality of the input signal.

It would have been nice if Wavecrest had at least mentioned it in the 
manual as something to be considered.  Not doing so can result in misuse 
by the operator that makes their equipment look bad.  I had to think 
about the poor results I was seeing.  At first I wondered if my unit was 
defective.  I haven't read through all their app notes so there could be 
something buried in there.  I know that other vendors do discuss this 
topic in either manuals or app notes related to their counters.  HP App 
Note 200 is a good example of this.  But it's worth noting that in a 
table of Trigger Error vs. Slew Rate, the lowest trigger error listed is 
10ns - not even remotely close to the performance level of the DTS even 
though the copyright date is similar to the DTS's production date.

We tend to fall into a rut when "it's never been a problem before". 
Equipment vendors need to warn us when their equipment makes our 
previous assumptions invalid.

Ed



More information about the time-nuts mailing list