20mhz-40mhz sine osilator please help

[matbymay]

hi everbody

i have ir led distance measurement project and i need 20 mhz sine wave oscillator to drive led circuit and for next step to modulate reveived signal with mixer so i need second osc. to modulate signal.

any idea for implement and if you would like to help me about project i can explain because sometimes i really stuck!!!!

firstly and urgently i need oscillator circuit or idea

thanks

 

[blueroomelectronics]

Not sure you can modulate an LED at 20MHz, they're not that fast.
What distance are you trying to measure? What accuracy?

 

[Hero999]

How accurate does it need to be?

Use an LC oscillator.

Yes, 20MHz is normally too fast for an LED to turn completely on and off but it you always keep the LED on a bit, it might work.

 

[matbymay]

thanksfor your anwsers


i m going to use ir led with driver circuit it will be as simple as i can.
range is between 1cm to 1 m and accuarcy will be 1mm or 3mm.

this project calls infrared rangefinder with phase measurment technics.

if you have any ideas i m sure you are all going to share with me. i really need help

thanks

 

[blueroomelectronics]

What design are you basing it on. Why not use a Sharp IR rangefinder?

 

[matbymay]

i cant use sharp ir range finder
the aim of the project is rangefinder with ir led i have to design all part and circuits(transmitter, receiver, modulator, pll for phase and low pass filter). so i need idea to implement all things. i m not good with analog electronic however it will be analog circuit design. i need theorical information an practical.

thank you

 

[blueroomelectronics]

This is a difficult project. Can you pick another?

 

[Hero999]

Here's how I'd do it.

I've never built it but it works in LTSpice.

You'll need to build it on a small PCB, don't even try breadboarding this because it won't work.

It might work because the LED never turns off, it just gets dimmer. LTSpice doesn't have an IR LED model (that I know of) so I used a red LED.

This circuit will oscillate at about 30MHz and isn't very stable. Build it in a screened enclosure (e.g. a metal box) to minimise RF emissions.

 

[matbymay]

yes it is hard project blueroom but i have already picked up. i need help.
i got your circuit hero999 thank you so much i ll try that circuit but
is there any ic for worrking range 30 or 20 mhz with sinewave output. and can i design this oscillator with 74hct4046a IC? there is vco inside the IC but how can i use?

 

[blueroomelectronics]

The 74HCT4046 can't get anywhere near 20MHz. What are you basing this design on?

 

[matbymay]

i want to use for oscillator but i m plannig to design with 74hct4046 for phase measurment circuit which has exor gate to gives pulses when detected phase diffrence. but firstly i have to design driver for IR led and receiver photodetector with opamp

thank you

 

[blueroomelectronics]

Do you have any links to a similar design? What are you basing your theory on?

 

[Hero999]

Why do you need a sine wave?

A pulsed waveform should be good enough.

There are many ways of designing a 20MHz to 40MHz oscillator, I just posted the simplest method.

Change the values of C1 and L1 to alter the frequency. The value of C2 will also need changing if you change the frequency significantly.

If you need an accurate oscillator, use a crystal and a harmonic filter. Assuming you're using a microcontroller, it should be fairly easy to tap the clock, thus eliminating the need for a separate crystal oscillator.

EDIT:
Please post a schematic of your idea.

 

[matbymay]

i just though maybe easy way i can do it with ic due to i need stable oscillator. i need sinewave because i ll modulate signal from output of receiver. i m not sure which way easy and minimum complicated. i can use xtal oscillator but problem is stabilite.
any suggestion for oscilaltor?
thank you

 

[blueroomelectronics]

As Hero999 stated, post a schematic or block diagram.

 

[matbymay]

hi again

i just quick drew sorry for messy design.
p1: object which reflects ir led beam
p2: ir transmitter 20 mhz with sine wave
p3: photodiode detector with op amp amplifier
p4:mixer which is modulated output of op amp and driver circuit
i forget osc. which is referance or carrier signal for mixer
p5 and p9: low pass filter to get low frequency fc-fo
p6: xor input for phase differences between led driver and phootodipde amplifier output

and finally computer interface with a/dc
final part not important
i dont use microprocessor only for interface part i ll use.
all design will be analog.
if you need further explanation i could give more detail.
thanks for help

 

[matbymay]

i think you can see picture?

 

[matbymay]

now it is ok

 

[blueroomelectronics]

Interesting, but the optical bits will probably not function properly at 20MHz to 40MHz

Post the part numbers of the IR LED & Detector.

 

[matbymay]

OPTEK|OP232W|LED, IR | Farnell United Kingdom -------led
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NXP | Semiconductors | RF and Video Functions | Up-Down Converter and Mixer | Audio/Video/RF
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