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Old 1st May 2004, 02:23 PM   #1
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Default OCXO clock ?

Hi,

what about a OCXO clock ?

Bernhard
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Old 1st May 2004, 07:55 PM   #2
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Found a TCXO ( seems to be one, there is some parts attached to the crystal with silicone ) it had a 12,8MHz crystal but frequency is divided by a flipflop, also printed on the hermetically sealed case is 6,4MHz.

I swapped the crystal for the 11,28...MHz and it works, unfortunately after a short between the psu leads, the smd 74S74 flipflop died.

Can I use the divided frequency as clock ?

Or should I put a buffer in place of the flipflop ?

Will buy a new smd on monday...
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Old 1st May 2004, 10:18 PM   #3
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Removed the dead smd chip and wired the thing with a 74HC14 gate.

Without the gate the swing is 0,5V to 1,3V.
Will SAA7220 accept that ?

With 74HC14 gate:
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Old 2nd May 2004, 01:47 PM   #4
JohnW is offline JohnW  Hong Kong
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Hi Bernhard,

Greater absolute accuracy offered by OCXOS is not normally important for audio applications and greater long-term accuracy is no guarantee of good Short Term phase noise (jitter).

In fact, I normally find the converse is true (unless you have a mega expensive OCXO). Due to the greater complexity of the oven oscillator circuit (using variable capacitance diodes), the short-term phase noise can be much worse then standard XO circuits.

The SAA7220 will clock on the HC14 O/P, but I dont understand why the rise & fall times on the O/P are so poor? What probe are you using (a x1 cheap thingy)?

Looks like your using a Tek 7000 series scope and I guess from the green CRT Phosphor a storage type maybe 7834, or one of the smaller three bay unit?

You can pick-up second hand Tek 900MHz active probes for these for around $100 if your using a 7834 / 7934 with say 7A19 or 7A29 you can easily use them to 1GHz then at least you will get decent waveform edges.

Anyway, be kind to your Tek 7000, there one of the best examples of analogue engineering in the world! Bettered only by the Tek 11302A (500MHz self calibrating MCP - image intensified CRT!), dont offend them by using them with a cheap piece of string as a probe

John
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