Low-noise oscillator

I think there was a thread about this some time ago.......just can't find it. Anyway, it is probably too old for any of us to remember what we were arguing, er, ah, discussing.

Anyway......here is a slightly different version. The idea was inspired (or ripped off.....) from yet another ham radio book. The change is that it uses a common-base stage (Elso.....are you reading this......?) at Q2; instead of a 22 ohm resistor to pick off the crystal current. The input Z of this stage is around 5 ohms, so there is somewhat less Q degradation.

Jocko
 

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Spec'ing crystals

I know that sooner or later, some wiseguy will fuss about a capacitor in series with the crystal. So for those of you who are new at this.....and for those that ought to know better, but don't.......

This circuit operates in the series-resonant mode. When ordering a crystal, you can either specify series-resonant frequency, or a parallel-resonant one.

Both cases refer to the frequency at which the crystal exhibits a series resonant frequency, usually in the range of tens of ohms. This is not the anti-resonant frequency, which is the equivalent of a parallel tuned circuit, and therefore in the range of many kilohms.

For a crystal spec'ed for series resonance, they measure the resonant frequency with only a resistive load, usually 50 ohms. The only capacitance is the stray holder capacitance, etc., usually around 6 pF.

When you spec for "parallel resonant", they measure the series resonant point using the specified load capacitance in parallel with the crystal. Typical values are 20 and 32 pF. This frequency is a few kilohertz higher when measured this way.

Most crystals you see in parts catalogs are spec'ed for parallel resonance, as the most common clock circuit around uses that type. It might be easier to order a crystal that way, because it will work in any circuit. In the case of a Butler oscillator, you can simply put a capacitor of the proper value in series with the crystal. The circuit will work just as well.

Jocko
 
Then square it up once inside.

Reciever info recieved and thankyou.
If using your oscillator circuit inside a one box cd machine (CD-94), would you square the sinewave before feeding the DSP/DAC osc input pin, and if so how ?. (some kind of data slicer/schmitt/74HCT etc or something or discrete ? - keeping in mind your previous snippets/advice of some gate devices causing zero/little backfeeding - can you remember ?).
A two box CD player is on the future wish list - I have various and plenty standard mechanisms and servo/dsp/system junked machines to play with and experiment with.
On this subject, I reckon different Laser pickups cause different sounds.

Eric.
 
At what point (if any) would one need to adjust values for higher freq. crystals (~24Mhz) and is this freq. still in a good space for this design? I'm trying to put together a circuit with the CS8420 on the front and the output freq. is a fixed 256Fs. I'd like to be able to 'play' with the higher Fs ranges.

Also, Jocko mentions gain on a 7404, this is something I don't understand? It as simple as adding a R2/R1 input/feedback resistor ratio for an inverting opamp? I thought output level was independent of input level in TTL?

Thank you.
 
Other frequencies

Scale C1 and C2. For 24 MHz, cut them in half. C3 & C4 are adjusted so that the crystal will resonate into the load it was cut for. You will need to know that.

As for the inverter........yep, you can configure it just like an op-amp. The output level of that circuit is too low to make a square wave without some gain somewhere.

Make sure that whatever you do has a stable PSU, or all that work to get low phase noise will be gobbled up by jitter in that circuit.

Jocko
 
Almost all modern circuits place the crystal in the series resonant mode. The confusion arises when you start to specify the frequency.

A crystal that is said to be series resonant has its series resonant mode measured into a resistive load, say 50 ohms.

When you see a crystal with its resonant frequency specified at a parallel load of X pF (usually in the 20 - 32 pF range), the series resonant mode is measured with a load of that specifed capacitance, instead of the resisitive load.

Jocko