XO controlled 480Hz Oscillator

"The Journey is the reward"

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This is the actual crystal I ordered from digikey:

If I get the thing dialed in, the 30ppm is tolerable for my application. I will try 25pF fixed capacitor for the PI network part of the circuit. If I am paranoid should I still figure a value to limit the current so that @ 12V the max power is ~2mw? Am I running into a greater chance that I will not be driving it enough to oscillate? Or is the minimum power so far below the max power limit that it does not matter? I will use a 2Meg resistor for the foldback. Maybe a 25pF trimmer in parallel will let me dial the freq right in. The specified crystal frequency is what I need. I am not trying to shift it off the package frequency. I just want to put some variability in so that I can compensate for stray capacitance after I have mounted it in the tape deck.

Thanks Guys, Chris Maness

Yes, particularly if you care much about drift. This particular xtal is rated at 1mW maximum.

Yes. You could empirically increase it until it doesn't start then go down by a fairly large safety margin (maybe 3:1). Or work it out from the transconductance of the oscillator amplifier (I think NXP specifies typical values on their HC datasheet).

DC feedback = bias.

--
In a Pierce oscillator, since the load capacitance is presented in
series with the crystal, if the crystal has a specified load capacitance
of 17pF the load capacitors should be about 35pF each.

, e r g

=A0"The Journey is the reward"

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Here is what I have so far. 2 Meg to bias. Two 20pF for the Pi network. These have an equivalent of 10pF. I would imagine that any stray capacitance would add to this in parallel. Hopefully the stray capacitance is less than 7pF. That would satisfy the 17pF that the crystal wants to see to be on freq. I will add a 20pF trimmer parallel to the crystal for fine tuning. I would imagine I would need to take any frequency measurements off of the next divider stage so as not to introduce more stray capacitance and throw the freq. measurement. As far as the current limiting resistor, I will use a high value like a 20k pot with a 10k resistor in series. I will decrease the value until the circuit starts oscillating. In the final circuit I will use half that value.

Here is a photo of the circuit:

Thanks, Chris Maness

I finished the this part of the project. The design with the 3.9Mhz crystal with a CD4060 works perfectly. My tape deck is running again. However, there is a small 480Hz hum in the record/play audio now. This is very slight hum, not noticeable when there is dialog. I did not notice this when the unit was running from a tuning fork time reference. However, the tuning fork itself was audible outside the chassis, and maybe masking my ability to hear the hum through the headphones.

There is a voltage divider resistor (1k) in series with the circuit. This was in place because the tuning fork ran off of 12V and the supply was 24V. My circuit is using a voltage regulator to drop the voltage down to 12V. There is now about a 8V drop across the resistor. If I remove the resistor, the hum gets much louder. If I shunt the power with a 1000uF cap after the resistor the hum gets louder. If I shunt before the resistor the hum drops into the Nyquist noise when I monitoring through the preamp. It is still there, but buried. Is there a better way to get rid of this hum/ripple? The cap is rather big and not as effective as I would like.

Thanks, Chris Maness

ou can

It is already built and installed. I just need to solve the little hum problem that it created. However, overall it is working good.

Thanks, Chris

Got a schematic of the power supply and the regulator?

Ed

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cap

Yes I will post them shortly. The unit runs off of two separate battery packs. 1 24V pack with a negative ground, and one 12V pack with a positive ground. The schematics I have are separate because I was modeling them in LTSpice.

Chris

Problem could be that when using a tuning fork oscillator, the signal was a sine wave. Now dealing with digital divider you have a square wave and what you are hearing are harmonics. Look up 'super filters' these are capacitor/pass transistor arrangement to effectively increase capacitance through gain of the transistor.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Use only Genuine Interocitor Parts" Tom Servo  ;-P

=20

You could use a parallel T (sometimes called twin T) filter with a high Q to get the sine wave back.

On Feb 18, 7:41=A0pm, "JosephKK" wrote: [....]

If you use a couple of flip-flops to make a 45 and 90 degree delayed version, making the filter can be a lot easier. If you combine the 3 signals with

14K,10K,14K resistors, the 3rd harmonic drops to zero. 0 * 3 =3D 0 45 * 3 =3D 135 AKA 180-45 90 * 3 =3D 270 AKA -90

If you draw the 3 vectors, you will see why the 3rd harmonic is zero.

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The output of this tuning fork was digital (square).

Chris

Are you grounding the new circuit to same (presumably digital) ground point as the original?

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Use only Genuine Interocitor Parts" Tom Servo  ;-P