Problems with crystal oscillator circuits

shaorc

Problems with crystal oscillator circuits [Copy link]

 

The resistor between the input and output pins of the crystal oscillator circuit in the figure is as large as 1 megohm. What is its function? If it is used for feedback, does the feedback resistor need to be so large?

chunyang

It plays a role in reducing the gain of the internal amplifier, making it easier to start oscillation and more stable. The author thinks that the resistance of this resistor is large, which should be affected by the negative feedback resistance of the op amp. Without the resistor, it is equivalent to connecting an infinite resistance in parallel. Is 1M ohm large or small compared to infinity?

maychang

The purpose of using this resistor is to provide a DC bias to the input of the amplifier. Most quartz crystal oscillator chips already have this resistor inside, so there is no need to add it externally. This resistor is only needed when the chip does not have this resistor inside, such as when using 74HC04 (inverter) as an amplifier.

annysky2012

chunyang posted on 2021-5-20 15:48 It reduces the gain of the internal amplifier, making it easier to start oscillation and more stable. The author thinks that the resistance of this resistor is large, which should be affected by the negative feedback resistor of the op amp, ...

Sometimes you don't have to.

shaorc

maychang published on 2021-5-20 16:32 The purpose of using this resistor is mainly to give a DC bias to the input of the amplifier. Most quartz crystal oscillator chips already have this resistor inside, so there is no need to add it externally...

Which part of the book can I read in detail about this knowledge? Including the DC bias for the input of the amplifier, does this amplifier refer to the amplifier inside the crystal oscillator (the Y2 device in the first post)?

maychang

shaorc posted on 2021-5-20 18:51 Which part of the book can be read in detail for this part of knowledge? Including the DC bias for the input end of the amplifier, this amplifier refers to the crystal oscillator (first...

This knowledge is scattered throughout the analog circuit textbooks.

Quartz crystals and quartz crystal oscillator circuits are found in all analog circuit textbooks. Earlier textbooks mostly describe quartz crystal oscillators made of discrete components. The quartz crystal oscillator made of integrated operational amplifiers is shown in the figure below, which is from page 453 of "Electronic Technology Fundamentals. Analog Part. (Kang Huaguang. 5th Edition)". The AC path is mentioned, which means that the DC path is not drawn. It is common sense to use operational amplifiers that there must be a DC path at the input end of the operational amplifier.

LBJ-lx

Not every place where a crystal is used needs to add this resistor. This is partly due to the crystal drive circuit, and partly due to the different types of crystals or processing techniques. This resistor is not always 1M, some are larger and some are smaller, and it also depends on the working conditions. Basically, its role is to act as an impedance match, so that the crystal drive circuit or the crystal itself is in a better working condition.

maychang

LBJ-lx posted on 2021-5-21 10:28 Not every place where a crystal is used needs to be added. Part of it is due to the crystal drive circuit, and part of it is due to the different types of crystals or the origin of the processing technology...

"Part of it is due to the crystal drive circuit, and part of it is due to the different types of crystals or the processing technology."

It has nothing to do with quartz crystals or the processing of quartz crystals. All quartz crystals are the same.

maychang

LBJ-lx posted on 2021-5-21 10:28 Not every place where a crystal is used needs to be added. Part of it is due to the crystal drive circuit, and part of it is due to the different types of crystals or the origin of the processing technology...

"Basically, its function is impedance matching"

It has nothing to do with "impedance matching".

chunyang

annysky2012 posted on 2021-5-20 18:31 Sometimes you don’t need it

Most crystal oscillator circuits do not need it. Please refer to the device manual for details. If the manual states that it is used, it must be used. If it is not specified, it is not used.

se7ens

It is common that this resistor has a limit on the oscillation amplitude of the crystal oscillator.

In addition, it is a bit difficult to understand what the third floor said about providing a DC bias for the op amp

In addition, can the 2nd and 3rd floor explain the negative resistance test method and significance of the crystal oscillator?

shaorc

maychang published on 2021-5-20 20:39 This knowledge is scattered throughout the analog circuit textbooks. Quartz crystals and quartz crystal oscillator circuits are in every analog circuit textbook. Earlier...

When laying out the PCB, the capacitors C1 and C2 in the first patch should be close to the crystal oscillator Y1, and also close to the pins of the connected chip?

maychang

shaorc posted on 2021-5-24 15:12 In the PCB layout, the capacitors C1 and C2 in the first post should be close to the crystal oscillator Y1, and also close to the pins of the connected chip?

"When laying out the PCB, the capacitors C1 and C2 in the first patch should be close to the crystal oscillator Y1, and also close to the pins of the connected chip? "

Yes. These components should be placed as close to each other as possible and as close to the chip pins as possible.

But the frequency of quartz crystals is not very high, so the requirement of "as close as possible" is not very strict.

maychang

se7ens posted on 2021-5-21 16:55 It is common that this resistor limits the oscillation amplitude of the crystal. It is a bit difficult to understand what the third floor said about providing a DC bias to the op amp. The second and third floors can...

"Also, it's a bit hard to understand what the third floor said about providing a DC bias to the op amp."

On the third floor, I said, "This resistor is only needed when a chip does not have this resistor inside, such as when a 74HC04 (inverter) is used as an amplifier." I did not mention op amps, but general amplifiers that do not have this resistor inside.

The internal circuit of 74HC04 is shown in the figure above. It can be seen that the input is the gate of the MOS tube. If there is no resistor in the first post, the input end of the inverting amplifier is "floating" (there is a capacitor connected to the ground, but the capacitor does not provide a DC path), and its potential is uncertain. Such a state is not allowed. If there is a resistor in the first post, then the resistor will provide a DC path for the two MOS tube gates, and the circuit can generate oscillation.

maychang

se7ens posted on 2021-5-21 16:55 It is common that this resistor limits the oscillation amplitude of the crystal. It is a bit difficult to understand what the third floor said about providing a DC bias to the op amp. The second and third floors can...

The internal circuit of TTL inverter 7404 is shown in the figure above. If there is no resistor in the first figure, the emitter of T1 in the figure above is also "floating", and the input end of the TTL circuit is "floating" at a high level. Obviously, this state cannot work normally. In order to make the TTL circuit 7404 produce oscillation in conjunction with the quartz crystal, a DC bias must be provided to the input end, that is, the resistor in the first figure is added. Moreover, for TTL circuits, the resistor cannot be too large, and it is impossible to be as large as tens or even hundreds of kilo-ohms, which is different from the amplifier of MOS process. However, TTL process chips are rarely used now, so there is no need to consider them in new circuits.

shaorc

maychang posted on 2021-5-24 15:47 "In addition, it is a bit difficult to understand what the third floor said about providing a DC bias for the op amp." On the third floor, I said "Only the chip without this resistor inside, ...

Thanks for your guidance

The following questions are

[1] In my first post, I only mentioned the separate circuit of the crystal oscillator part and did not give any peripheral applications. However, you mentioned the connection between the crystal oscillator and the inverter on the 3rd and 14th posts. Was this for the purpose of giving examples?

【2】What if the OSC-IN and OSC-OUT signals in the first post are connected to the pins of a microcontroller or other control chip?

maychang

shaorc posted on 2021-5-24 16:19 Thank you for your guidance. The following questions are as follows [1] In the first post, I only mentioned the separate circuit of the crystal oscillator part and did not give any peripheral applications. However, you mentioned the following on the 3rd floor...

OSC_IN and OSC_OUT are marked in the first picture. OSC is the abbreviation of oscillator, which shows that the circuit is connected to the oscillation circuit.

Besides, quartz crystal cannot produce oscillation without an amplification circuit.

maychang

shaorc posted on 2021-5-24 16:19 Thank you for your guidance. The following questions are as follows [1] In the first post, I only mentioned the separate circuit of the crystal oscillator part and did not give any peripheral applications. However, you mentioned the following on the 3rd floor...

OSC_IN and OSC_OUT are connected to the microcontroller. These two pins on the microcontroller are the output and input of the inverting amplifier circuit, and a bias circuit has been added internally. For details, see the internal block diagram of the microcontroller.

se7ens

maychang posted on 2021-5-24 15:54 The internal circuit of TTL inverter 7404 is shown in the figure above. If there is no resistor in the first post, the emitter of T1 in the figure above is also "floating", and TT ...

Thank you for your reply

The question you asked is about the resistance added between the two ends of the crystal output, which is different from the resistance of the single-ended input of the inverter you answered.

Moreover, the input impedance of general amplifiers is very large, so there is no need to connect additional large resistance resistors.

So the resistor mentioned by the OP should be used to adjust the gain or limit the oscillation amplitude.

maychang

se7ens posted on 2021-5-25 09:41 Thank you for your reply. The question you asked is about the resistance added between the two ends of the crystal output, which is different from the resistance of the single-ended input of the inverter you answered and...

The resistor in the first circuit is not necessary for the quartz crystal oscillator circuit. Some amplifier circuits already have their own bias, so there is no need to add this resistor. Only amplifier circuits without bias need this resistor.