Detecting AC signal on top of a DC w

[Paul.Jorden at eev.com]

Posted to CCD-world:
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To Scott, et al,
 >
 > The ICCD is not an option for me.  Is there any way, using
 > an integrating detector like a CCD, to detect a modulated
 > signal over a DC (or slowly varying) background that
 > includes a continuum (e.g. incandescent) and discrete
 > line (e.g. fluorescent) sources?

There is one possible technique that astronomers have used (for polarisation 
or 'high speed' sampling).
You essentially divide the image area of the CCD into three zones, the 
centre of which is illuminated.  You project your signal onto the centre, 
and this can have a slow modulation (say 10Hz, square wave preferable).
After integrating for one half cycle you rapidly transfer to say the upper 
storage zone.
You then integrate for the other half cycle, followed by a rapid tranfer to 
the lower storage zone.
This process can be repeated for multiple cycles.  The upper zone could 
integrate  the signal + background, whilst the other would integrate only 
background.
At the end of the total sampling period, read out the whole area, and 
subtract the upper and lower zones to get the net signal.

I cannot tell whether the figures work out well enough for your application. 
 For example 128 rows could be shifted in ~5 uS each (600 uS total); this 
transfer time could be negligible compared to a zone integration time of say 
50 mS- corresponding to a 10 Hz total modulation cycle.  Perhaps all these 
rough figures could be improved upon.
Total number of integrations per zone is limted by dark current or ambient 
background.

 > The operating temperature of the CCD will be ~ -10 deg C
 >  so there is significant dark current (~ 50 electrons per pixel per
 > second),.......... Thus, I am dark current limited.
 >
I would have thought that dark current could be ~ 10 e-/pix/sec for an 
inverted mode device, with ~20 um pixels.  Less with smaller pixels.
Presumably you are concentrating the light as much as possible to maximise 
signal/pixel.

I'll be interested to hear any comments.

Regards,
Paul
_______________________________________________________________
Dr Paul Jorden,  CCD Applications, EEV
Waterhouse Lane, Chelmsford, Essex, CM1 2QU, UK
Tel:      44 (0) 1245 -453458 (direct), -493493 (switchboard)
Fax:      44 (0) 1245 453224
see also:-     http://www.ccd.eev.com/

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