Posted by
Peter Rupprecht on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Resonant-scanning-preamplifer-bandwidth-tp7584591p7584609.html
*****
To join, leave or search the confocal microscopy listserv, go to:
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopyPost images on
http://www.imgur.com and include the link in your posting.
*****
Hi,
Sampling at 80 MHz does resolve the beating pattern issue, but there is another advantage. If you sample at 80 MHz locked to the excitation pulse and do not significantly broaden the fluorescence decay (2-4 ns), you do not integrate over the noise that would otherwise be smeared into the signal when using e.g. a 10 or 20 MHz bandwidth. If you sample only the (2-4 ns of) signal, and not the noise of the remaining time, you can also increase the gain of the PMT to much higher regimes than for low-bandwidth. This makes a direct comparison of different configurations difficult, since the noise can "look" different ....
I tried this out for my system and realized that 80 MHz (Femto) and 60 MHz (Thorlabs) preamplifiers have not sufficient bandwidth (at least the preamps I used) to show this effect. For a 240 MHz bandwidth, however, (Femto), this effect was very prominent, but the low intrinsic gain of the 240 MHz preamp setting was not optimal. -- Avoiding the beating pattern works for any preamp bandwidth.
To the original question: "Matching" the preamplifier with the sampling rate ... hm. The bandwidth of a preamplifier is not a cutoff, but a smooth transition, and the "80 MHz" value indicates only a certain point for this frequency curve. And I have to say that for a preamp I'm using (Femto DHCP), the actual bandwidth is very likely lower than the indicated one.
Peter
Michael Giacomelli <
[hidden email]> schrieb am 16:10 Dienstag, 5.Januar 2016:
Hi Heping.
The idea is to sample synchronously with the excitation so that you
don't get beating between the sample and excitation clocks (imagine a
situation where one sample gets two pulses, then next zero pulses, the
next 2 pulses, ...) not necessarily to Nyquist sample the excitation
itself.
As for analog bandwidth, it should be set high enough to sample the
motion of the excitation spot across the sample. Since most systems
use a resonant scanner with a bidirectional line rate of no more than
16kHz and a laser with an 80 MHz rate, that's 5000 pulses per line,
but probably no more than 1024 pixels. Even accounting for the fact
that the scanner is moving faster in the center of the scan than the
edges, there is typically still at least 2 excitation pulses per
pixel, which means the analog bandwidth of the image will be around
half that of the excitation (~40Mhz) at the center and less on the
edges. Therefore you'll be ok with an amp with at least that much
bandwidth and an 80 MHz sampling rate, at least for many systems.
Keep in mind though that you don't want too much bandwidth either,
there is no advantage to amplifying frequencies that are not in your
signal, and you'll quickly start to pick up FM radio stations if you
go above 80MHz.
Mike
On Mon, Jan 4, 2016 at 11:53 PM, Heping Yuan <
[hidden email]> wrote:
> *****
> To join, leave or search the confocal microscopy listserv, go to:
>
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy> Post images on
http://www.imgur.com and include the link in your posting.
> *****
>
> Hello list users. I had a question about analog to digital conversion in resonant scanning two
> photon microscopes. I have seen older discussions involving some users of the open source
> ScanBox and ScanImage programs and they advocate for using sampling rates that match
> with the Ti-Sapphire laser repetition rate (80 MHz). However, they also suggest using
> preamplifiers with cutoffs that match this sampling rate (80 MHz). Should the preamplifier
> not be at most half of the sampling rate to avoid aliasing? I believe there may be more
> subtle details I am not aware of.
>
> Thank you for your time.
>
> Heping