# Question-crystal oscillator

#### samy555

Joined May 24, 2010
116
Hi
I have some questions about the following circuit:

1. Is it a common base oscillator?
2. If the center frequency of the tank is 27MHz, does this mean that the crystal operated at its fundamental frequency?
3. If I made the tank frequency = 81MHz, does this mean that the crystal will operate at its 3ed tone?
4. In fact I have a 27.175 MHz crystal, should I have to change the tank values to oscillate at 27.175 MHz?

Thank you very much

#### MikeML

Joined Oct 2, 2009
5,444
1. Is it a common base oscillator?
2. If the center frequency of the tank is 27MHz, does this mean that the crystal operated at its fundamental frequency?
3. If I made the tank frequency = 81MHz, does this mean that the crystal will operate at its 3ed tone?
4. In fact I have a 27.175 MHz crystal, should I have to change the tank values to oscillate at 27.175 MHz?
1. Yes, but only because at the frequency of oscillation, the crystal is like a series-resonant circuit which has a very low impedance, which effectively grounds the base at one frequency.
2.No. The tank is at the same frequency as the crystal because it is a 27MHz oscillator. The crystal is already operating on its mechanical 3rd overtone, because fundamental crystals typically are not made to go that high in frequency (the slab of quartz would be to thin).
3. No, see above.
4. No.

Here is a description of how it works: (From this article)

Fig 3 is a circuit of a grounded base oscillator using the crystal as a series resonator. This circuit is
usually used for overtone operation. It is simple in concept. The transistor Q1 is biased in the active region
as an amplifier. A resonant circuit L1C1 is placed in series with the collector. Note that Ce is a relatively
“small” capacitor (not the usual 470pF to 0.1 uf we might use in an RF amplifier stage). Ce is used to
bypass the emitter resistor. The internal collector to emitter capacitance Cc provides feedback to the
emitter, the ratio set by the values of Cc and Ce. At a frequency where the crystal is series resonant, the
base is effectively grounded at this frequency. This allows oscillation, the transistor acting as a common
base amplifier at this frequency. This oscillator is efficient and has a good power output (10-20 milliwatts)
but the stability and phase noise are not the greatest, since the base circuit resistance in series with the
crystal lowers the operating Q somewhat. It is also somewhat susceptible to stray oscillation modes, since
there is little selectivity in the crystal circuit other than the crystal itself. By tuning L1 and C1 over a wide
range, it is possible to produce oscillation on unwanted overtones and spurious modes. Also, stray
capacitance across the crystal and associated leads may allow for uncontrolled spurious oscillation, and off
frequency operation away from the true series resonant frequency. The crystal has to be specially ordered
with an offset in frequency to get the exact desired frequency. This has to be determined by experiment. A
series inductor or capacitor in the base circuit can be used to adjust or “pull” the frequency somewhat.
Replacing the crystal with a dead short allows the circuit to function as a straight common base oscillator.
This circuit is OK for general-purpose use but is not recommended where critical frequency control or
phase noise is an issue.