What is the point of a picofarad capacitor?

[Jononomous]

So if I have a 22uF capacitor that naturally does not allow DC current to flow, what is the point of schematics having lets say a 6pF capacitor before a crystal oscillator? If a component allows for only a 6pF load, is that to limit the voltage? or the current? the voltage should stay the same but just build up quickly... so how do I knock down a 1amp 6v load to 6pF so I don't blow the component? I guess the point of the 6pF capacitors is what's confusing me.

Thanks for any help!

- Jon

 

[crutschow]

The point of the 6pF cap is to provide the required AC load for the crystal so it will oscillate properly at its correct frequency. Google crystal oscillators for more info.

You don't have to adjust the voltage or current to accommodate the capacitor. You can't blow the component by ordinary circuit voltages.

 

[Jononomous]

What about blowing the oscillator though? If it has a load capacitance or whatever of 6pF - 30pF...how do I (get it down to)/(control) those ranges?

 

[blueroomelectronics]

What's it for and post a schematic.

 

[Jononomous]

This isn't exactly what I'm building, but regardless here is the schematic and component list for this specific schematic.
I want to change the frequency and toy around with outputting specific frequencies ect ect.

Parts:

Resistors: all 0.25 watt, 5% tolerance Maplin code
R1 180 kilohms (k) M180K
R2 15 kilohms (k) M15K
R3 6.8 kilohms (k) M6K8
R4 1 kilohm (k) M1K0
R5 2 kilohms (k) M2K0
R6 150 ohms () M150R

Capacitors
C1, C2, C3 1 nanofarad (nF) or 1000 picofarads (pF) WX68Y
C4 100 nanofarads or 0.1 microfarad (F) YR75S
C5 12 picofarads (pF) WX45Y
TC1 60 picofarads (pF) trimmer WL72P

Semiconductors
TR1, TR2 2N3904 npn QR28F
IC1 7473 or 74LS73N dual JK flipflop YF30H
D1 5.1V 500 mW Zener QH07H

Additional items
100 kHz crystal
B7G valveholder (found at most rallies)
14-pin DIL IC socket BL18U
Phono socket YW06G
PP3 battery connector HF28F
PP3 battery
On/off switch FH00A
Veroboard JP47B
Metal case as required

Trying to get a handle on frequency control, and from many schematics I can see the flow of things but don't quite understand them or crystal oscillators.

**broken link removed**

 

[Brevor]

There should be a resistor or coil on the collector of TR-1.

 

[blueroomelectronics]

What frequency are you after and what will it drive?

 

[Jononomous]

Thanks Brevor, but (if it's not too much to ask) could you explain why R5 and R6 aren't enough to dull the current?

Thanks blueroom,

Any frequency between 1Mhz - 200Mhz, I ordered a collection of different crystals from digi-key and I would just like to be able to
play around with the numbers n such... just driving a simple transmitter

 

[RCinFLA]

There should be a resistor or coil on the collector of TR-1.

see R5, it is the collector load resistor.

The 6-60 pF trimmer is to adjust oscillator center frequency. If you want exactly 100 kHz you need to have a crystal cut for series resonant frequency with a 32 pF series loading resistor. Many are cut for 32 pF load for parallel operation.

 

[Jononomous]

Ok RCin, that makes sense... but lets say I bought the following crystal:
Digi-Key - CTX083-ND (Manufacturer - MP101)

30pF capacitance load... the concept as I understand it would be to place a 30pF capacitor in series with the crystal, and then adjust the harmonics after the crystal with a NPN transistor?
This particular crystal does not have a load variance/range so I assume using a trimmer on such a crystal would burn it out if I went anywhere above 30pF... am I starting to get the gist of all this?

 

[MikeMl]

The drive level in these oscillators is so low that they cannot hurt the crystal regardless if the capacitors are too large or too small. If the capacitors are wrong, either of two things will happen, the oscillator wont start oscillating when power is applied, or will be off the frequency marked on the crystal by +- a few tens of Hz per Mhz of Frequency.

In other words, if you use the trimmer to "pull" the crystal frequency, you will be luck to move say a 5MHz crystal by more than +-100Hz. If you try to make the trimmer bigger in an attempt at pulling the frequency further, you will eventually get to a trimmer value that will prevent the oscillator from starting reliably.

Readings:
**broken link removed**

TWO

Search terms: Pierce Crystal Oscillator, ColPitts Crystal Oscillator.

 

[Jononomous]

Awesome... I think I understand it now, Thanks Mike for the explanation and readings!

 

[RCinFLA]

Ok RCin, that makes sense... but lets say I bought the following crystal:
Digi-Key - CTX083-ND (Manufacturer - MP101)

30pF capacitance load... the concept as I understand it would be to place a 30pF capacitor in series with the crystal, and then adjust the harmonics after the crystal with a NPN transistor?
This particular crystal does not have a load variance/range so I assume using a trimmer on such a crystal would burn it out if I went anywhere above 30pF... am I starting to get the gist of all this?

All the ones with capacitance number are cut for anti-resonance (parallel mode). The ones with 'series' are cut to operate in series resonance mode.

The motional capacitance, Cm, is very small (fraction of picofarad). The motional inductance (Lm) is very large. This along with the series resistance (Rs) make up the series resonance circuit of the crystal. Slightly above this series resonance combination frequency the crystal looks inductive, the case capacitance along with the external load parallel capacitance form the anti-resonance (parallel resonance) in a parallel mode oscillator.

 

[MrAl]

Hello there,

I am surprised to see a variable cap in series with the crystal. For the parallel cut crystal (the MP101 device) the way to adjust the frequency would be to adjust one of the parallel connected capacitors, not a series capacitor. Normally one cap would go from one lead of the crystal to ground, and the other cap would go from the other lead of the crystal to ground, and the total load capacitance would be the series combination of these two. Thus, for 30pf load capacitance you would need two 60pf caps. One of those caps could be the adjustable one.
It also appears that the 2nf cap is way too large, and there is no other cap on the other side of the crystal in that schematic.

Note that the only problem with not being able to tune the crystal frequency is that the oscillator frequency may be off by 100ppm or maybe a bit more. If that is not a problem for your app then it doesnt matter if you tune it or not.

Note also that series crystals dont have load capacitance, only parallel ones do. That's just to allow a different form of oscillator considered more stable, and with the addition of those caps the oscillator frequency comes closer to the crystal's stamped value.