[SI-LIST] Re: f vs. Tf vs. jitter
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Steven Kan <steven@xxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 18 Jun 2007 13:31:47 -0700
Steven you're welcome. If the AM is done with very linear
multiplication, the slew rate will not be that of the modulating signal,
but that of the product of the carrier and the modulating signal.
Still, that seven orders of magnitude will indeed be very very difficult
to manage in analog land. Even with exceptionally small non-linearities
and very low noise floors that span results in a huge dynamic range.
Regards,
Steve.
Steven Kan wrote:
> ----- Original Message ----- From: "steve weir" <weirsi@xxxxxxxxxx>
> Cc: "SI-List" <si-list@xxxxxxxxxxxxx>
> Sent: Monday, June 18, 2007 12:08 PM
> Subject: Re: [SI-LIST] f vs. Tf vs. jitter
>
>> *Counters-
>> Fixed divisors ( limitation )
>> Wide frequency range
>> Simple
>>
>> *AM
>> Arbitrary divisors ( strength )
>> Need filters tuned to remove the unwanted side-band ( limitation )
>> Non-linearity results in unwanted spectral lines ( limitation )
>>
>> I disagree on the slew-rate issue. In an ideal AM implementation,
>> the slew-rate scales identically with frequency products. Just my
>> $0.02.
>
> Steve (and others who replied similarly),
>
> Thanks for the feedback. Regarding slew rate, I was thinking through
> the following:
>
> If you have a 1 Vpp, 10 Hz sinewave, the slew rate through zero crossing
> is going to be ~30 uV/ns (assuming I did the math right), and trying to
> trigger anything with that slew rate and achieve ps jitter will be very,
> very difficult.
>
> Whereas if you have 10 Hz periodic ECL pulse with 800 mVpp amplitude
> and 200 ps rise-time, it will be much easier to trigger something with
> low jitter.
>
> Ken had asked about the fundamental frequency we're dividing, and
> that's typically coming from a Ti:S laser oscillator. We (or my laser
> customers) don't really care about absolute jitter wrt an absolute
> time reference; we/they care about the stability between the laser
> pulse and the divided signal. So we take a 80 MHz photodiode output
> from the laser oscillator cavity, feed that into a comparator to turn
> it into an ECL pulse train, and then feed those into a set of
> monolithic counters (On Semi MC100EP016AFA). By cascading enough
> counters we divide by 8,000,000 and put out an ECL edge every 100
> msec. That 10 Hz pulse is then used to trigger a second laser, for
> example in pump-probe spectroscopy, so the phase relationship is
> critical, and they want relative jitter down in the single-digit ps
> range. In practice my customers have been very happy with this
> solution, but they haven't sent me any 'scope shots or jitter
> measurements.
>
> But I did ask them what alternatives they had, and they've been using
> a combination of analog frequency synthesis and/or using programmable
> delay generators, with mixed results. The industry standard solution
> for delay generation is the SRS DG535, which everyone likes for its
> flexibility, but it has a jitter floor of 50 ps, and it only gets
> worse as period and delay go up.
>
> It was the analog frequency synthesis that piqued my interest, because
> locking two clocks that are 7 orders of magnitude apart *sounds*
> difficult to me, and my customers have nodded general agreement that
> it is. The caveats are that I lack the expertise to know if it
> actually is or not (and why), and that my customers are similarly
> out-of-field here, since they're typically laser folks and not EEs.
>
> That's the general point that I'm trying to make (and defend), so if
> that's
> not clear in my writing, I need to work on the phrasing a bit. Thanks
> again for all your input.
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