[Gb-ccb] Minutes from 07jul03 videocon

Fri Jul 11 15:28:34 EDT 2003

Thanks for your corrections- I clearly wrote those minutes up too
quickly.

In connection with the filters, perhaps this summary will be useful.
The basic choice being made is between a wider post-detection
bandpass, which allows higher time resolution at the ADC input and
thus less time lost to blanking, and a narrower post-detection
bandpass, which will alias less noise into your measurement.  Both
effects are at the few percent level.  We see no problem with the 2
MHz 8-pole filter; a 1/2 usec settling time out of 25 usec is quite
good, and with 10 MHz sampling at the ADC we have a comfortable margin
ofoversampling. In particular 20 dB at the Nyquist frequency of 5 MHz
is not unreasonable.

Although you might achieve better rejection of noise by pushing to a
narrower bandwidth-- which you would do if something other than the
post-detection filter limits the post-phase-switching slew time-- it
is reasonable to assume that this will not be the case.

A question (not meant to cause havoc): does the precise filter have
important impacts on other aspects of the system?  Thus can we swap
them out based on tests later?  Should we do lab tests with a few
different filters at first, and order the rest later (or are they
expensive/long lead items?)? I can imagine that the 3mm Rx might want
narrower video filters since the switching will be substantially
slower...  it might not matter or, it might not be feasible.

 Brian

Martin Shepherd writes:
 > Note that in addition to comments regarding the minutes of the
 > teleconference, there are some important points in this email,
 > including a decision on what type of low-pass filter to use.
 > 
 > On Wed, 9 Jul 2003, Brian Mason wrote:
 > > Background: We held the videocon in order to provide feedback on a
 > > number of specific issues that Caltech had raised now that they are
 > > getting on with the hardware design.  The main issues were: the
 > > overall architecture change (to a faster, lower-resolution ADC further
 > > up in the signal chain, followed by low pass filters and integrating
 > 
 > Actually the ADCs are preceded by low-pass filters, not followed by
 > them.
 > 
 > >...
 > > Clarifying our earlier definition of "fast" we said the step response
 > > should be 0.5 usec, settling to -84dB at 1 usec (-84 dB is a part in
 > > 4000 that is, 12 bits).
 > 
 > Actually, 12 bits is 72dB, not 84dB, but the 1 in 4000 is essentially
 > correct (ie. 2^12=4096).
 > 
 > >....
 > > backends which I didn't quite get down clearly (the signal is down by
 > > 15 dB at the nyquist frequency, which results in aliased signal being
 > > 30 dB down?)
 > 
 > Curious. How do you get from 15dB aliased signal strength to 30dB
 > down? I would have thought that it would simply be 15dB down. If I am
 > wrong, please tell me, since a doubling of the filter dB stop-band
 > attenuation would be great :-).
 > 
 > >-- and wonders whether the aliased stuff is signal or
 > > noise (worst case: some sort of systematic noise which doesn't average
 > > down).
 > 
 > Not systematic noise, simply higher-frequency receiver and detector
 > noise.  The key to understanding this is to realize that the time
 > window (tw) during which the ADC samples its input signal is shorter
 > than the sampling period (T). Without an anti-aliasing filter, the
 > duration of this sampling window sets the averaging time, whereas with
 > an anti-aliasing filter the time-constant (tf) of the filter sets the
 > averaging time.
 > 
 > A perfect anti-aliasing filter thus improves the SNR seen at the input
 > of the ADC by the factor:
 > 
 >   sqrt(tf/tw)
 > 
 > However a low-pass filter with poor stop-band attenuation is like a
 > filter with a smaller time-constant than that implied by the cutoff
 > frequency, and thus the SNR is degraded.
 > 
 > I have been researching filter choice for the last couple of days, and
 > have reached the following conclusions.
 > 
 > When choosing a filter for the CCB, it is important to keep in mind
 > that the primary goal of this filter is not anti-aliasing, but
 > averaging the input signal for a strictly bounded time period. In
 > other words the ideal filter for this application would have the
 > time-domain impulse response of a top-hat function, which would
 > dictate a sync function filter. Such a hypothetical filter (which
 > doesn't exist) would be a disaster in the frequency domain, since it
 > would ring very badly, well beyond the Nyquist frequency. As such one
 > needs a compromise filtering function. The best readily available
 > option is the Bessel filter, which is designed to exhibit virtually no
 > overshoot or ringing in the time-domain, and has a fast step-response
 > time, due to it having approximately constant delay as a function of
 > frequency. To counter the relatively poor frequency response of this
 > type of filter, I suggest that instead of using a 5Mhz anti-aliasing
 > filter, we use a 2MHz low-pass Bessel filter of 8 or 10 poles. At the
 > Nyquist frequency of 5MHz both 8 and 10 pole Bessel filters have
 > attenuations of about 20dB. At the sampling frequency of 10MHz, the
 > attenuation of a 10-pole Bessel filter is about 70dB.
 > 
 > Since there would be nothing detectable by the ADC beyond 10Mhz using
 > one of these filters, we know that the increase in the SNR due to its
 > imperfect frequency response is less than 40% compared to the ideal
 > SNR. In practice, given that the filter is already down by 20dB at the
 > Nyquist frequency, the noise degradation will be a lot less
 > significant than this.
 > 
 > Unlike most other types of filter, the theoretical time needed by a
 > 2MHz Bessel filter to settle to virtually any practical accuracy is
 > slightly over the reciprocal of its cutoff frequency. Thus a 2MHz
 > low-pass Bessel filter should settle within just a little over 0.5us.
 > 
 > So, the specification for the filter that I would like is a Bessel
 > filter with at least 8 poles, and a cutoff frequency of 2Mhz. Suitable
 > filters can be bought from TTE, at:
 > 
 >  http://www.tte.com/
 > 
 > The base of a 2MHz PCB-mounted 8-pole Bessel filter from TTE has
 > dimensions of 2.4" by 1". Thus 16 of these filters would occupy
 > upwards of 38 square inches of the PCB (ie. 6"x6"). The height of the
 > filter is 0.5".
 > 
 > > The full-scale voltage of the ADC being considered is 2.5 Volts.
 > 
 > This needs clarification. When operated differentially, each of the
 > two differential inputs of the ADC can swing over a 2.5V range between
 > the limits 1.25V and 3.75V (ie the common-mode voltage is 2.5V). The
 > optimum ADC noise performance is achieved for signals at the
 > common-mode voltage.  Ideally the signal would be offset to place the
 > signal level that is generated by the receiver when there is no sky
 > signal, at the common-mode voltage.  In principle I could base such an
 > offset on the output of a trimmer-pot attached to the ADC reference. I
 > am still pondering whether to do this, or simply to place the 0V of
 > the detectors at the common-mode voltage.
 > 
 > > We
 > > indicated to Caltech that from our point of view, the signal levels
 > > are open to specification by them. The ADC is clocked at 10 MHz
 > > (5xNyquist).
 > 
 > Hum, the Nyquist frequency is the ADC sampling rate divided by
 > 2. Maybe you meant to say 5 times the cutoff frequency of the low-pass
 > filter?
 > 
 > > Discussion of the control signals was somewhat less conclusive.  Will
 > > we use LVDS or TTL?
 > 
 > I thought that we had finally decided on opto-isolated TTL?
 > 
 > >...
 > > receiver.  Are both TTL and LVDS compatible with using the opto
 > > isolators?
 > 
 > I don't know of any opto-isolators that are designed to generate LVDS
 > outputs.
 > 
 > > Not clear.  How fast can the opto-isolators be driven?
 > > Martin will look into the last question.
 > 
 > As reported yesterday via email, suitable devices with 75ns
 > propagation delays are readily available (and cheap).
 > 
 > > ...
 > > are combined and sent.  A sketch of the control signals would be
 > > helpful and GB will produce this.
 > 
 > My previous documents already contain these diagrams.
 > 
 > > As to interconnection: the general feeling was that more connectors
 > > rather than fewer was good: this permits empirically optimizing the
 > > grounding and bundling scheme, and presents more options for
 > > troubleshooting should they be needed.
 > 
 > That wasn't the impression that I got. My recollection was that we
 > settled on using 4 cables containing 4 pairs each, with the caveat
 > that if subsequently found to be necessary, we could always retrofit
 > the CCB and the receiver to use 16 individual cables.
 > 
 > > Martin asked if we have some video filters lying around which we could
 > > have a quick look at.  We do, and we might be able to have a look at
 > > them; failing that we could probably give some pointers.
 > 
 > I don't think that this will be necessary now. I am comfortable that
 > the Bessel filters that I described above will do the job.
 > 
 > Thanks.
 > 
 > Martin Shepherd  (mcs at astro.caltech.edu)
 > 
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 > gb-ccb at listmgr.cv.nrao.edu
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