Question-crystal oscillator

[samy555]

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

 

[amayilsamy]

1. Transistor working in self biasing oscillator,
2. Its is working in 27 MHz Frequency, Because transistor base control the collector output.

Tank circuits can operate at much lower frequency,we can achieve in the VLF range

 

[jiripolivka]

This type of transistor oscillator can be named "common base" as the base is grounded at crystal resonance.
If you want to obtain 3rd harmonic, tune collector resonant circuit to 81 MHz, and use a capacitor trimmer in emitter (in the schematic the value is 82 pF for 27 MHz output. For 81 MHz optimum, the capacitance may be only 20 pF. Using a trimmer allows to peak the desired harmonic output.
I have used this harmonic crystal oscillators for many frequencies as needed. It is a good idea to adjust also the base resistors, to try several transistor types and also several crystals. Not all crystals are happy for overtone use.
When you start testing, use a mA meter to see when the RF oscillation peaks in collector current. If you can, use a spectrum analyzer to see the best harmonic output and to minimize unwanted harmonics.

- - - Updated - - -

To tune the circuit for 27,125 MHz, only the collector circuit should be fine-tuned for a peak output (use a trimmer or a tunable coil).

 

[E-design]

Crystals in the 27 MHz band are usually CB radio type crystals. Most of the time they are designed to operate at their 3rd harmonic. In other words, to get 27.125 MHz the crystal's fundamental frequency will be roughly 1/3 of 27.125 MHz.

 

[D.A.(Tony)Stewart]

There are 2 resonant modes in crystals. The manufacturer tunes the frequency for one mode only as it impossible to tune for both modes.

1st f is the series mode frequency then slightly higher f (e.g. 0.1%) is the anti-resonant high impedance mode. In Series mode ESR is critical for gain margin, and in parallel mode, load capacitance is critical for frequency accuracy.

For more Crystal Theory https://www.ecsxtal.com/store/pdf/oscir_des.pdf

If you already undertand, try the quiz.

https://www.learnabout-electronics.org/Oscillators/osc26.php

When buying crystals they are usually specified as S or P type where Parallel type also needs a load capacitance, so they have in part number instead S or pF load to denote series or parallel tuned.

Good luck on the tricky quiz.

If you understand , let me know.

- other quality factors besides Q are usually stated by Series Resistance where lower is better, but in Series Mode, consistency is best, when multiple vendors and SMT sizes are considered, thus 150 +/-50 Ohms is best in smallest SMT package from one vendor vs than 25~250 from different vendors with different SMT case sizes.

- other test metrics include, >5x Gain margin due to Negative Resistance/Xtal R ratio and temperature stability and temperature range of cut.

The "S" tempco. curve of "AT cut" crystals can be made symmetric or asymmetric around 25'C so custom ordered for a ppm stability limit over a required T range, such as 25, 500, 100, or 200 ppm over a various ranges of T, e.g. --40~+85 or 0~70 etc.

if the OP is still with me... The tuning core of L is used to optimize gain in the given schematic to ensure adequate gain margin at Xtal series mode frequency. This is fairly wide bandwidth compare to Xtal frequency and C value does not change for offset f of < 10% , just value of L and use tuning.

 

[samy555]

1. Transistor working in self biasing oscillator,
2. Its is working in 27 MHz Frequency, Because transistor base control the collector output.

Tank circuits can operate at much lower frequency,we can achieve in the VLF range

Thank you

This type of transistor oscillator can be named "common base" as the base is grounded at crystal resonance.
If you want to obtain 3rd harmonic, tune collector resonant circuit to 81 MHz, and use a capacitor trimmer in emitter (in the schematic the value is 82 pF for 27 MHz output. For 81 MHz optimum, the capacitance may be only 20 pF. Using a trimmer allows to peak the desired harmonic output.
I have used this harmonic crystal oscillators for many frequencies as needed. It is a good idea to adjust also the base resistors, to try several transistor types and also several crystals. Not all crystals are happy for overtone use.
When you start testing, use a mA meter to see when the RF oscillation peaks in collector current. If you can, use a spectrum analyzer to see the best harmonic output and to minimize unwanted harmonics.

- - - Updated - - -

To tune the circuit for 27,125 MHz, only the collector circuit should be fine-tuned for a peak output (use a trimmer or a tunable coil).

Thank you very much jiripolivka, it was a very good answer I benefited from.

Crystals in the 27 MHz band are usually CB radio type crystals. Most of the time they are designed to operate at their 3rd harmonic. In other words, to get 27.125 MHz the crystal's fundamental frequency will be roughly 1/3 of 27.125 MHz.

This speech is very important, and I want you to clarify a point:
I got my crystal from a used chinese waky-toky board printed on its outer surface 27.125. Does this mean it's fundamental = 9.41MHz?

There are 2 resonant modes in crystals. The manufacturer tunes the frequency for one mode only as it impossible to tune for both modes.

1st f is the series mode frequency then slightly higher f (e.g. 0.1%) is the anti-resonant high impedance mode. In Series mode ESR is critical for gain margin, and in parallel mode, load capacitance is critical for frequency accuracy.

For more Crystal Theory https://www.ecsxtal.com/store/pdf/oscir_des.pdf

If you already undertand, try the quiz.

https://www.learnabout-electronics.org/Oscillators/osc26.php

When buying crystals they are usually specified as S or P type where Parallel type also needs a load capacitance, so they have in part number instead S or pF load to denote series or parallel tuned.

Good luck on the tricky quiz.

If you understand , let me know.

Thank you
Certainly I'll tell

 

[D.A.(Tony)Stewart]

Here is a 27.125 Walkie Talkie crystal schematic here.

If it is a series type such as above then it will be a "3rd Overtone" if it is a "fundamental f" then it is parallel. Each gives the best characteristics required.

So if you put it in a Colpitts parallel mode CMOS Osc. Design and find it running at 9MHz that would confirm what I said.

Overtone mode requires more current but less voltage obtained by series resonance.

 

[samy555]

To SunnySkyguy
Hi
Crystal particular, I could not understand that much about it, especially regarding to does it work in the series mode or parallel.
Even the PDF file that you gave me I felt difficulty in understanding from the first page, when I saw figure (a)
But may be I understand an important point here, that: which is written on the body of the crystal will not necessarily be the fundamental frequency

Fo the Quiz, my answers were correct only in points 3, 5, 6, 7
The rest were my answers wrong! (my level = 40%)
Thank you for your help