Re: Photodiode wich is fast enough to detect +50Mhz analog (sinus) signal??

From: colin (no.spam.for.me_at_ntlworld.com)
Date: 07/31/04 

Date: Sat, 31 Jul 2004 14:37:32 GMT

"Yannick" <yannick_de_wit@pandora.be> wrote in message
news:cc50d220.0407310025.6cbc794d@posting.google.com...
> "colin" <no.spam.for.me@ntlworld.com> wrote in message
news:<MctNc.430$ld7.326@newsfe6-gui.ntli.net>...
> > "Yannick" <yannick_de_wit@pandora.be> wrote in message
> > news:cc50d220.0407210131.790375fb@posting.google.com...
>
>
> yes that's a point but only if u use 2 frequenties,then you will do
> like u said , first a low frequentie to detect in wich period our
> signal will be and then a higher frequentie to know where we are in
> this particulary period(soo we got high resolution) but the problem is
> that we only can do phase measurement within 0 and 180 degree, soo you
> cant really see in wich half of the period we are measuring, thats why
> u need at least three measurements...
>

when my signal gets converted to IF i can still use (positive) edge
detection and so use the whole length of the cycle. so 10mhz wld give me 15
meters, more than that and i would hate to think of how weak the signal
wouuld be, unles you have a reflective target, although tank turent mounted
range finders use single pulse to measure distances of many miles i beleive,
but then they use very high wattage pulsed laser devices. it wld be fun to
play around with but not so safe perhaps

i can easily measure the length of my lab, especialy so if i use the shiny
metal part of a cabinet in the corner.

> i maybe will use 100 different frequenties, for example i start with
> 50Mhz, then 51MHZ, then 52 and so on. then consider the distanced you
> measure a constant , and plot the phase difference between the sended
> signal and the received signal , you will see this will be a triangle
> wave (waving between 0 and 180 degrees) in function of the
> sendfrequentie. if you compare this wave (the wave with the received
> phase measurements) with the ideal wave, you can calculate wich is the
> right distance.(for every sample do : positive Root[(measured phase -
> calculated (ideal) phase)^2] then make the sum of it(yes indeed its
> like a fourier transformation but with triangle waves in stead of
> sinuses) the more samples you get the better your accuracy will be and
> noise will have less influence.
>

hmm this is very interesting way of doing it, in fact if you cycled through
a frequency range wouldnt you then get a phase change that varied with a
rate of change equal to rate of frequency change x distance, then you cld
convert this to a frequency wich is proportional to distance ... and use a
simple freq counter.

> yes indeed the higher the freqeuntie the better our resolution will
> be. but its hard to detect such high frequenties.

yes the signal to noise ratio gets worse thanks to the capacitance, unless
you tune it, but i think you would only need such high resolution over
shorter distances where returned signal shld be stronger anyway. using realy
high frequencies would only be of benefit if returned signal was so strong
you didnt need to wory about s/n ratio. my main area of aplication would be
in acurate positional sensing for use in robotics, machining etc. althogh
distances may be short the surface might be reflective and reflect most of
the light away.

as a further thought i may try using a frequency multiplier after the tuned
input stage so i cld select the fundamental or 3rd or even 5th harmionic
easily just by simply changing the LO frequency, wich wld give me 3 ranges,
and just have a 10mhz signal.

> indeed ofcourse that's also my plan. I am gonne program an atmel
> AT90S8535 microcontroller to send the signals to a DDS frequentie
> synthesiser wich gives the right frequenties in a minimum of time. the
> only problem with the analog device DDS synth is that the main clock
> is 120 Mhz soo when u want 60Mhz out you only get 2 samples , then
> after DA conversion thiss will make a square wave, and then u have to
> use antiimage filters to make a sinus again. but i am not sure how
> good this will be, what kind of DDS frequentie synthesiser will u be
> using?

at the moment im doing it the easy way, but then im not using so many
frequencies as you. im using two 30mhz voltage controled crystal oscilators,
i found that using a 10mhz crystal in 3rd overtone gives a good spread of
frequencies of just below 30mhz, with a bag of 10 crystals i can find two
that are able to be locked to produce a beat frequency locked to a 32khz
crystal in a PLL.

i wil probably change to a 455khz IF in order to use a 455khz filter, wich
will mean i will have to find two crystals wich are 455khz apart wich might
be more dificult. also il get much less delay multiplication, so i maybe
have to have a second IF but this cld be realy low like 1khz. wich wil mean
il need another LO of 454 khz, wich is more dificult, or maybe il just lower
the IF to 1khz anyway, i cant even seem to find 455khz filters anywhere
anyway wich is frustrating, but then image rejection becomes non existant,
but i gues this isnt to much of a problem, as theres no adjacent band,
although i have thought about an image descriminating mixer stage just in
case.

converting the square wavr to sine might mean you get variable phase shift
with filters, dont know about antiimage filters, but u cld either use
triangles and a very simple triangle to sine converter or use lower
frequencies and then use a mixer to generate the higher frequency. if u use
2 mixers and generate signals with 90' phase shift you can select either the
a+b or a-b frequency from the output with no need for filter. in fact i
might try do this anyway to generate the frequencies i need. ie 30mhz+90'
mixed with 32khz, plus 30mhz mixed with 32khz+90'. wld give me 30.032mhz i
forget the name of this type of mixer now, but i wouldnt be surprised if
theres an ic that does it all for you.

a micro wld also be usefull to drive a 7seg led readout or even a LCD
display.

> it's good someonse is also trying this challenge, gives me hope :),

yes, good to see someones trying a diferent way too. always good to bounce
ideas around, ppl used to think i was crazy trying to do it this way lol.

Colin =^.^=

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