Will this 25 MHZ oscillator work?
Node 3 and node 5 should be connected. The junction is missing in the schematic.
Brian.
still i dont get 25 mhz
The problem is that any crystal based oscillator takes a long time to simulate due to the very high Q of the crystal.
Since you have also L3 and C1 in series, the simulator will show oscillations of around 106 MHz instead of 25 MHz.
This is because the holder capacitance of X1 is about 5pF. So the simulator at start up sees a resonant circuit
consisting of C1 + L3 + 5pF in series and immediately start to oscillate at around 106 MHz. If you remove the crystal
and replace it with a 5pF cap you will get the same simulation result.
To further prove this remove R1, L3 and C1 and connect the crystal directly to the base of Q1 and you should see oscillations
after a few mS of simulation time around 25MHz. It will take a while to before you will see oscillations. Watch the
transient oscillation time (bottom right of Multi Sim display) and you will see oscillations slowly building up after about 5mS
Thanks,
So should I assume that in reality my circuit would work and oscillate at 25 MHZ but the simulation cannot capture the true characteristics of the oscillator?
Why do you need L3 in the circuit? Unless you want to introduce extra phase lag to lower the crystal oscillation frequency it is not needed.
Normally you would choose a crystal with parallel resonant frequency slightly below 25MHz. C1 can then be made adjustable to trim
the frequency to exactly 25MHz
E-design,
So what would be your recommendatory for a revised design of the schematic? Should I just remove L1 and the 560 ohm resistor and the oscillator will work in real life?
---------- Post added at 14:48 ---------- Previous post was at 14:44 ----------
E-design,
So what would be your recommendatory for a revised design of the schematic? Should I just remove L1 and the 560 ohm resistor and the oscillator will work in real life?
Here is a simple circuit design often encountered in real life with some design guidelines.
Thanks, Edesign, greatly appreciated,by the way I got my original design from an RF design book which had an inductor included in the schematic.By the way on the schematic from the book, they did not specify the MHZ of the crystal but only capacitance and inductor values at the required MHZ, Why was that?
The main aim of the project is to attach an attena for transmitting electromagnetic waves i.e radio waves, I would probably have to attach a small capacitor on the emmiter to block DC, will this schematic be suitable for that? I though I should ask you this before discarding the original design. Many thanks.
It will be hard to comment without reading the section in the book you mention.
You can add a DC blocking capacitor. To get any meaningful range and impedance match you may need another RF driver stage before connecting to your antenna.
Do you mean here I need to amplify the signal before applying it to the attena? So what are the standard components should I apply for amplication without affecting the oscillations?
25 MHz won't be allowed for general purpose RF transmission in the ETSI region. You have to restrict the device to dedicated ISM frequencies, e.g. 27.12 MHz (26.957-27.283). 10 mW e.r.p. is permitted, respectively 0.7 V rms supplied to a 50 ohm load. You would also need filters to reduce the harmonic power.
I am aware of the frequency limitaions i.e to use 27 mhz, I used the 25 MHZ as a test since the value is not available on multisim but intend to use the former frequency.
If the output amplitude of the oscillator is sufficient, you can use a common collector buffer, otherwise a common emitter voltage amplifier.
Here is the schematic of the buffer amplifier, how do I calculate capacitor and resistor values?
Here some 27MHz RC circuits you can look at
THE RADIO BUILDER: RF Toys 27 MHz Circuits_Part One
There is one diagram from part-2 of the article which I marked the oscillator and amplifier section for you.
I have built this circuit and it does not work at all!
---------- Post added at 19:20 ---------- Previous post was at 18:58 ----------
E design,
I have built this but it does not work!
It should, provided you use sensible wire lengths. Did you include the IC ? Without it there is no bias source for the amplifier.
Brian.
Sorry, I built the e-design version but not the one with IC but does not work,
I remember, that you have proven extraordinary skill to make oscillator circuits fail that work well for others. In so far I'm not surprized. I'm sure that the circuit from post #8 can work well.