Urgent help needed in Crystal selection

[Abhishekabs]

Hi Friends,
I want to make crystal sine wave oscillator for 51MHz & 121Mhz.
I have BFR 92A transistor.

I am not sure which crystal I should use?
I read that Overtone crystals needs to be used. but did not found much info on manufacturer's website.
Can anybody suggest?
I found one crystal but I am not sure it is correct for my purpose?

I am expecting atleast 1 dBm output from this oscillator.
Then I will modulate this signal with sound for transmission.

 

[jiripolivka]

With a BFR 92A, you can try any suitable quartz crystal. In my designs I connected the (overtone) crystal as a ground to transistor base, then the output LC circuit to collector, tuned for a maximum output at the desired frequency. Across the emitter to ground resistor, I connected a capacitor trimmer by which I can optimize the chosen harmonic for the best output.
Typical RF output at 51 and 121 MHz with a BFR92A at +5...+12 V DC bias can be ~ 10 mW or more , depends upon collector current.

To modulate such oscillator, you would achieve AM with a narrowband FM. I would advise to use another stage for AM. For FM like in FM radio, using a crystal is not good as the high Q of the crystal allows only a narrowband FM. A LC circuit with a varicap diode across it offers a good wideband FM if you prefer.

---------- Post added at 19:47 ---------- Previous post was at 18:05 ----------

Only now I looked at your attachment- it shows NOT a crystal but a complete oscillator! You will need no BFR92A to make it work.

 

[Abhishekabs]

Thanks for the reply.
I am not doing any FM. Only AM.
51.2 MHz is SOS signal.

So the crystal which I found can directly give me the frequency I want?
SO I need to connect an amplifier in front of crystal to amplify the signal right?

To be honest I am confused.
Which system to be used.
I have not designed any crystal oscillator before.

Can you please advice how should I proceed?
component wise ?
Circuit wise?

 

[vfone]

**broken link removed**

 

[jiripolivka]

You can follow the nice schematics referred above. If you already have the oscillator, then you can use the buffer (second) stage also indicated in the schematics above, to get ~ 10 mW output.'
For the best stability, I would introduce the keying signal to the buffer stage; you can control transistor current by applying e.g. a TTL signal through a 10 k resistor to the base. You should adjust the on and off states for the best AM modulation index.
If you are going to buy a crystal, then again you can use the schematics above and make an oscillator; for 51 MHz, you can use a 3rd overtone crystal, 17 MHz.
If you control transistor max. current to prevent its damage, there is nothing to fear. Have only a receiver or spectrum analyzer ready to optimize your circuit.
Some circuits need tweaking; for example, after switching DC off, then they do not like to start again at the desired frequency. Be ready to adjust transistor bias as well as the capacitors.

 

[Dr.Drew]

**broken link removed**

Complex schematics. I often use UHS CMOS invertors TinyLogic series from Fairchild. With them an -175 dBc/Hz noise floor and -150dBc/Hz at 1 kHz offset at 100MHz is achievable. And schematic is very simple.

 

[vfone]

Even using a logic inverter, have to use an LC tank circuit (more complexity) to select the desired harmonic when design an overtone crystal LO, as Fairchild recommends in app note:

https://www.fairchildsemi.com/an/AN/AN-340.pdf

With 2 nsec rise/fall time of the Ultra-High-Speed inverter, when build a 50MHz LO, perhaps you get harmonics up to 10GHz.

 

[Dr.Drew]

Second stage of IC i use as buffer with impedance tranformer/LPF. Harmonics are -30-40dBc up to 10th harmonic Higher harmonics are lower than -70 dBc.

 

[davenn]

So the crystal which I found can directly give me the frequency I want?
SO I need to connect an amplifier in front of crystal to amplify the signal right?

yes thats correct. but you are going to have to figure out how you are going to modulate it.

To be honest I am confused.
Which system to be used.
I have not designed any crystal oscillator before.

and you are not designing one here, as that package already contains a fully operational oscillator
now depending on which one you have/will buy
the option N & I = can only trim the freq over small range close to the operating freq.
The option V = which has a control voltage pin (pin1) so that the oscillator can be tuned over a wide range
that onewould be your best bet asyou would be able to apply modulation to that control voltage line and FM the osc signal.
AM modulation would be a bit more difficult

cheers
Dave