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I want to increase the frequency of the crystal oscillator oscillating at 25MHz frequency. For this, I added an LC tank circuit to the collector in parallel with each other. I experimented to see if the frequency would change.

But, even though I tried many times for capacitor and coil values, the frequency never changed. The amplitude changed, but the frequency remained 25MHz. Why did the frequency not change? Where am I going wrong? Here is the circuit I designed.

enter image description here

I replaced the coil in the collector with a coil with 3 or 5 turns of winding. But, still the frequency did not change. I played with the capacitor values, and the frequency did not change. When I removed the capacitor from the tank circuit, it oscillated again at 25MHz without any change in the signal. The presence or absence of the capacitor did not change the result. Below is an article about this method.

http://www.koreascience.kr/article/JAKO201509057414006.pdf

In the article in the link, the crystal oscillator increased its frequency to 2 or 3 times. How did he achieve this? I built a similar circuit. But, I could not increase the frequency. In the circuit in the article, using an inductance at the emitter, your circuit. I added a coil to the emitter, but still the result did not change.

Maybe someone can't open the article. I am attaching a picture from the article.

enter image description here

Although I soldered the circuit on the PCB, I could not increase the frequency.


@Tony Stewart EE75 @glen_geek

I have done trial and error many times. I changed the eelman values of many circuits. I have been dealing with this for 1 month. No result.I read whatever I found on the internet to see if something was missing. I even have a lot of information about the history of frequency.The fact that the real experiments do not match with the ones in the books makes me very tired. It is very easy to say "look, this experiment gave this result" in simulation. It makes me sad that I can't set up the circuit and get results from the experiments.

Here is the actual circuit I built, the circuit diagram and the oscilloscope image. enter image description here

/****************************************/

edit:17/02/2022 I built the circuit on a perforated plate and soldered it. In the tank circuit, I used 1uH coil and 10pF capacitor. In this case, the oscillation frequency of the LC tank circuit is 50Mhz. But 50 Mhz oscillation did not occur. The output is still 25 Mhz. Therefore, soldering the circuit did not work either. enter image description here

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    \$\begingroup\$ Can you tell us what frequency you wish to achieve? \$\endgroup\$
    – glen_geek
    Feb 12, 2022 at 13:31
  • \$\begingroup\$ >I cannot increase the output frequency of the crystal oscillator circuit< should be: "I would multiply the output frequency ... by n=1,2,3 ... NB: some n do, some don't. Some transistors can do, some can't do it well ... \$\endgroup\$
    – Antonio51
    Feb 12, 2022 at 13:35
  • \$\begingroup\$ @glen_geek ,I want to increase the frequency from 25 Mhz to 50 or 100 Mhz. \$\endgroup\$
    – OzGtZ t
    Feb 12, 2022 at 14:50
  • \$\begingroup\$ X2 frequency (50 MHz) should be achievable. Be aware that any measuring device attached to the collector tank will seriously shift its resonant frequency - that tank must be resonant at 50 MHz. If you intend to drive another device with this 50 MHz collector voltage, some kind of impedance matching is required. 100 MHz is more difficult, since collector current at that frequency is much smaller than current at 50 MHz. \$\endgroup\$
    – glen_geek
    Feb 12, 2022 at 15:18
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    \$\begingroup\$ Operation on a breadboard even at 25 MHz is liable to be marginal. Operation at 100 Mhz is very unlikely. Capacitances of 1 to 10 pF are likely between strips. Inductances will also be present. \$\endgroup\$
    – Russell McMahon
    Feb 13, 2022 at 11:50

5 Answers 5

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Why did the frequency not change. Where am I going wrong?

The tuned circuit added in the collector is resonant at 27.7 MHz. This is due to: -

$$f = \dfrac{1}{2\pi\sqrt{LC}}$$

And, the basic crystal oscillator running frequency is 25 MHz. The whole idea is that your tuned circuit in the collector is meant to pick up on harmonics of the basic crystal frequency but, 27.7 MHz is not a harmonic of 25 MHz.

If you look at the circuit in the linked article, they are tuning to 4x the crystal frequency: -

enter image description here

In other words they are tuning into the 4th harmonic of the basic crystal frequency and, as I said earlier 27.7 MHz is not a harmonic of 25 MHz.

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  • \$\begingroup\$ I wrote in the comment above. .I changed the inductor value and capacitor value many times to observe if there is a change in frequency. But the frequency did not change. \$\endgroup\$
    – OzGtZ t
    Feb 12, 2022 at 12:49
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    \$\begingroup\$ You didn't try the right combination I suspect. \$\endgroup\$
    – Andy aka
    Feb 12, 2022 at 12:56
  • \$\begingroup\$ @OzGtZt have you got an answer to this question that you can formally accept or do you still have some things unexplained? \$\endgroup\$
    – Andy aka
    Feb 19, 2022 at 14:35
  • \$\begingroup\$ Although I soldered the circuit to the plate, I could not obtain multiples of 25mhz frequency. As a result, I could not succeed. \$\endgroup\$
    – OzGtZ t
    Feb 19, 2022 at 14:39
  • \$\begingroup\$ @OzGtZt if we are done here you should choose an answer and formally accept it; this is what is expected from you on this site. \$\endgroup\$
    – Andy aka
    Mar 23, 2022 at 9:20
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There are 2 critical parameters in harmonically tuned resonators.

  • Q or Zo and n*fo.

  • The impact on choice selection is the tolerance of the passive components and harmonic content of gain and attenuation in the fundamental oscillator at the desired harmonic.

  • If Q is too high from the R/X(f)=Qp for the parallel resonant circuit then the value of LC is critical hit or miss. with 1% to 20% typical range then you have to add or subtract capacitance with a varicap or trimp cap to tune it. This sensitivity can be reduced by shunting the collector current sink with a parallel R. But this also affects the attenuation of distortion from more harmonics. So 100 is a reasonable limit for LC tuning Q unless extreme selection or tuning is used with temperature compensation.

  • It is also critical to have short leads when you get down to < 0.1uH as wire is ~10 nH/cm and put an RF decoupling Cap across the supply close to the oscillator.

  • Here I simulate it with a voltage source and a series R, where you can see the effects with a mouse-wheel tuning R over its value.

For a simple tuning take your 3.3 uH and /4 [uH] for 2f or /9 [uH] for 3f or /16 [uH] for 4f then use very tight construction and short leads. But each time you increase n x f the Zo reduces and you must shunt the collector with a suitable R for Q<=100 or the inverse of your tolerance error for tuning.

  • the choice of transistor also has an effect with GBW or current gain hFE-BW product, but your choice of SS9018 is an excellent one. Worst case loop gain >1 is your goal.

enter image description here

Conclusion

Tolerances and Q (tuned by your R2) are critical tradeoffs, as well as layout improvements.

  • Possibly all you had to do was change your load from 10k to 1K or less to pass the signal thru the harmonic filter

I assume you know all the equations for Q, f, and X(f). Refs: https://www.cs.ccu.edu.tw/~cwlin/courses/electronics/notes/CH13.pdf https://www.wikiwand.com/en/RLC_circuit

Other info

  • Saturation and cutoff may prevent square waves by limiting gain and false oscillation on 3rd harmonic, but that is prevented by the LC filter.
  • Most Xtals purchased >> 20 MHz are actually harmonic frequencies or an overtone crystal, guaranteed by filtering.
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  • \$\begingroup\$ "Possibly all you had to do was change your load from 10k to 1K or less to pass the signal thru the harmonic filter"I tried this method. I did not connect any resistor to the output. The result did not change. Then I tried the resistance values of 1k, 100ohm, 333ohm, respectively. Again, the result did not change. There was a signal oscillating at 25 Mhz on the oscilloscope screen. \$\endgroup\$
    – OzGtZ t
    Feb 12, 2022 at 16:53
  • \$\begingroup\$ What LC values did you use? 50MHz would be 1.1uH then 470 uH //10pF for 75 MHz etc.. Simulate it then realize that . and if fails then your assumption on value is wrong. Measure it, \$\endgroup\$ Feb 12, 2022 at 17:11
  • \$\begingroup\$ even bending the coil wire can tune it when < 1uH, so it must be more rugged and the cap must be an NP0 or P120 for temp compensation \$\endgroup\$ Feb 12, 2022 at 17:13
  • \$\begingroup\$ As in the article, when I connect a coil (3.3uH) to the emitter, a frequency of about 37mhz is generated. Or when I short-circuit the emitter with the jumper, a frequency of 45 Mhz is formed. I realized that these frequencies have nothing to do with the crystal oscillator. Because when I turn off the crystal, it continues to oscillate at 45Mhz or 37Mhz. In addition, removing and adding the capacitor in the tank circuit in the collector does not create a change in frequency. Let me also mention that the circuit is built on a breadboard. \$\endgroup\$
    – OzGtZ t
    Feb 13, 2022 at 9:38
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schematic

It is unclear what you're trying to achieve from the various schematics you posted. Are you trying to pull the crystal frequency or use a harmonic of the crystal frequency. You can only pull a crystal a small amount. Normally, you choose a harmonic of the crystal frequency.

In the top schematic, the center frequency of the RLC circuit is \$ {1 \over {2 \pi \sqrt{LC}}} = 27.7 MHz\$. With the extra capacitance and wiring inductances, the resonant frequency is probably closer to 16MHz.
The lower schematic is in the 100s of MHz.

Some things to watch out for. In the red box, there are two capacitances to watch out for. One is the breadboard and wiring capacitance. On a breadboard, the capacitance between rows can be significant. You also have lead inductance which is significant when you use inductances under 1uH.

At the frequencies you're operating at, get rid of the breadboard and either prototype this on a perf board or dead-bug style (no board is used). Trim the lead lengths to minimize lead inductance.

Add an emitter follower buffer on the output to minimize the scope probe capacitance on the tank's center frequency.

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  • \$\begingroup\$ R0 : Output Resistor. The Output Resistance will be equal to the inductance of the coil. R0=2pifL=2pi.25.10^6.3,3.10^(-6)=518 ohm. The output capacitance is approximately equal to the sum of the scope probe and breadboard capacitance.Let the capacitance of the board be 30pF So, Ceq=13pF+30pF=43pF. Let the high frequency cutoff point be "fh". fh=1/(2pi*518*43.10^(-12)=7,1 Mhz .signal weakens after 7.1Mhz.In order to neutralize the probe and board capacitance at the output, why don't we connect a (for example 1pF) small capacitor in series with these two capacitors. \$\endgroup\$
    – OzGtZ t
    Feb 14, 2022 at 16:50
  • \$\begingroup\$ Thus, we increase the high cutoff frequency. We get rid of making an emitter follower circuit.Doesn't that make sense? \$\endgroup\$
    – OzGtZ t
    Feb 14, 2022 at 16:50
  • \$\begingroup\$ I guess what I said doesn't make much sense. If we connect a 1pf capacitor in series, the amplitude of the signal will become very small. \$\endgroup\$
    – OzGtZ t
    Feb 14, 2022 at 16:53
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Here is an example of what you search for, but be aware that VHF multipliers are "difficult" to "work".
These are found in the old amateur radio "schematics".
See the lower part of this schematic (x3 used). And this.
I use an "old" transistor in this schematic ...
And A "breadboard" is not an option.
Simulation may be "long", very "long" ... and not "representative" of success.
For some reasons, multiplication factors are generally odd numbers (1,3,5 ...).

Here is an amplifier (first stage) at 25 MHz, the second stage is a 4x multiplier at 100MHz.

enter image description here

A more complete spectrum view analysis shows the waveform at output and a view of the spectrum content. :-) --> filtering needed.

enter image description here

And AC analysis

enter image description here

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    \$\begingroup\$ The 2N3866 transistor may be "old" but it's still very good for it's intended tasks :-). \$\endgroup\$
    – Russell McMahon
    Feb 14, 2022 at 12:18
  • \$\begingroup\$ I made a circuit similar to this, but it did not work properly. Because the signal was weakened due to the miller effect. Also, what is the name of this program you are using. LTSpice is insufficient to simulate RF signals. \$\endgroup\$
    – OzGtZ t
    Feb 14, 2022 at 16:55
  • \$\begingroup\$ I used this transistor 2N3866 for a little transmitter on 144 MHz (adapting input/output impedances is necessary, but also a difficult task). Program is now free microcap12 spectrum-soft.com/download/download.shtm \$\endgroup\$
    – Antonio51
    Feb 14, 2022 at 17:01
  • \$\begingroup\$ See this example electronicsforu.com/electronics-projects/… \$\endgroup\$
    – Antonio51
    Feb 14, 2022 at 17:07
  • \$\begingroup\$ See also these examples using multipliers on 144 MHz band and one based on 96.7 MHz oscillator -> 773 MHz. - (13 cm beacon). See the varactors used. \$\endgroup\$
    – Antonio51
    Feb 14, 2022 at 17:20
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Q1 is very much in the class A mode due to R4. Class A is renowned for low distortion which means low harmonics compared to the fundamental. Place an experimental resistor from base of Q1 to batt neg to starve Q1 pushing it into more nonlinearity meaning more harmonics which should be easier to find. If Q1 starts class AB and runs class C it would be better.

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