Re: Update on laser distance meter project (master thesis) and question

>
> its actualy quite easy, much easier than you would expect, and as the
> circuit around the photodiode is so simple theres little room for problems
> to arise. the only complication is generating an extra frequency that is
> always 400khz diferent from your transmited signal, this would be easy with
> a second DSS programed 400khz apart.

Yes but say your carrier is 20Mhz generated with a DDS. your frequency
superponated on the reverse bias of 200V will be 20.4 Mhz. Now what
about the phase between the two because i work with different
frequenties (carriers in this aproach) the 20Mhz from the 20.4 must be
in phase with the 20Mhz send frequency. otherwise you always measure
an other phase delay because the second dds will never load up at the
exact same time as the first.




> i came up with it myself

very smart :)

>
> yes, but if you look at the slope of gain versus voltage it is very steep
> especialy in the region of maximum realistic gain, but this is exactly where
> you want to use it anyway, this means a smal variation in bias voltage ~
> 500mv wil cuase a very large change in gain, this is more than ample to
> provide for a very efective multiplier.


yes ofcourse, thats the part i still didnt got, i was still thinking
of mixing due the diode characteristic but you actually mix with the
voltage/gain characteristic.

>
> the beauty is that if your signal is to strong you can reduce the bias
> voltage and this reduces the gain of the photodiode and also the mixer gain,
> making very efective agc. for smal change in bias voltage wich means minimal
> change in apd capacitance.

yes but for a high dynamix range you sometimes have to reduce with
40-50Volts soo then the capacitance will change too much and the phase
delay will change significantly. Shouldn't it be better to reduce the
apmlitude of the AC signal on the reverse bias. Lets say this is 500mv
and if you lower it to 50mv this will give a 10 times lower signal at
the input of the amplifier but the capacitance will be the same (450mv
wont change much).

> >

> you can forget TIA becuase you get a single frequcncy out of your mixer
> stage. a simple voltage gain stage with no feedback resistor exept for dc
> biasing is then by far the best. in fact the signal realy is so strong with
> an apd runing at high multiplication you dont want too much amplification.
>

soo just amplify the voltage over the bias resistor. I am thinking now
about non linearity due the exponential APD characteristic but for
small ac signal amplitude it wont give a problem.

> 400khz bandwidth would be far to much, in fact the use of a narowband filter
> reduces the noise and the reduction of bandwidth of 30mhz down to 3 khz
> would give a further 100 fold reduction in noise.
>
but if you work with a narrow band filter the phase change will be
rapid due any frequency error (drift).Isnt it better to use a filter
with low quality(Q factor) soo ok more noise but less phase change, a
compromise between the two must exist.

> ah ic yes if you change the phase detector to a linear one then you might
> wel find a huge improvement anyway.

yes i suppose but this way is easy:)

> >
> I lost a bit of interest when i found they are comercialy available and
> havnt done anything to it for some time, however my initial goal was to try
> and resolve with much greater acuracy over more moderate distances for
> machining aplications and so i need to increase the carier frequcny from
> 30mhz to upto 1ghz wich cuases more of a chalenge, the idea of using the
> beat frequcny from 2 lasers is something i was considering, and wicked
> several stage frquency multipler to get the mixing signal.
>

its a shame it exist already.

I am going to write a whole chapter on this one (optoelectronic
mixing) for my book (thesis) because this is the way to go... to make
it work in practice in one month i dont gonne try, but next year on my
own i will try, i am really curious what this will give in resolution
when i step uy my carrier to 100Mhz orzo (with a better DDS)

its a challenging project , i am really happy i did it, its far more
difficult i ever could expect but thats good because i learned a lot.

thanks,

Yannick
.

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