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Old 20th June 2012, 01:48 PM   #1
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Default Help this imbecile with some math

Painful as it is to admit, I have the math skills of a "no child left behind" 4th grader.

Can someone please help me figure out (ok, figure out for me) what component values I need to get 750kHz from this HEF4060B as a crystal oscillator (Fig 7 in the pdf)?

Thanks

http://www.nxp.com/documents/data_sheet/HEF4060B.pdf
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Old 20th June 2012, 02:17 PM   #2
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Try Rt=27 ohms, Ct= 22 nF, R2=270 ohms and C2= 220 pF. These values give f= 732 kHz
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Old 20th June 2012, 02:38 PM   #3
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Quote:
Originally Posted by willysan View Post
Try Rt=27 ohms, Ct= 22 nF, R2=270 ohms and C2= 220 pF. These values give f= 732 kHz
Ah thanks, but I'm using the circuit in Figure 7 that has an external crystal.

I think I see one thing I'm doing wrong though; the max input frequency of RS is ~8MHz for my supply voltage (5.6V) and I'm using a 12MHz crystal.
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Old 20th June 2012, 02:45 PM   #4
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In Fig 7 you will only get reliable oscillation at the frequency of the crystal. The trimmer cap will give small variation but not that much.

You might be better off going for a crstal multiplication of the frequency that you require and then use a divider to get it down to where you want it.

i.e 3MHz / 4 = 750 KHz
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Old 20th June 2012, 03:06 PM   #5
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The original circuit had a 6MHz crystal with an output of 375kHz taken from pin-7. In my ignorance and complete lack of understanding of the circuit I was hoping a 12MHz crystal would give double that.

Maybe I'm overlooking something and one of the other outputs is already at my desired frequency?
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Old 20th June 2012, 03:40 PM   #6
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Quote:
Originally Posted by theAnonymous1 View Post
The original circuit had a 6MHz crystal with an output of 375kHz taken from pin-7. In my ignorance and complete lack of understanding of the circuit I was hoping a 12MHz crystal would give double that.

Maybe I'm overlooking something and one of the other outputs is already at my desired frequency?
You are on the right track, this is a divider and the outputs Q3..Q13 each give a different output frequency. Basically each one is half the frequency of the previous one. The output Q3 gives 1/16 the crystal frequency (Q0 would be 1/2, Q1 1/4, Q2 1/8, Q3 1/16, etc.). Note that only 10 of the 14 "Q"s are available as outputs. You can see in Figure 2 that Q3 is connected to the output of Flip-Flop 4 (FF4), therefore the input frequency (6 MHz) has been divided by 2 four times, to give you 1/16 the input frequency. Unfortunately, the output of FF3 is not available (no Q2 output pin) so you can't get 750 kHz with 6 MHz in. For that you need to jump to a 12 MHz input, and use Q3 output.

Use the recommended components in Figure 7. Rbias is listed as 100k to 1M, and it is usually not critical, so try values in that range and pick one that works well. It is usually not necessary to have a trimmer capacitor unless you want to fine-tune the output frequency, for which you will need a very accurate, recently calibrated frequency counter. So you can just use a fixed capacitor instead of a trimmer. You may be able to find a variant of this IC that will do 12 MHz at 5 VDC.

Last edited by macboy; 20th June 2012 at 03:43 PM.
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Old 20th June 2012, 03:59 PM   #7
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Thanks Macboy, wish the datasheet was as clear as your reply.

I would just up the supply voltage but the device this feeds can only tolerate a 6V input. Possible to use a resistor divider on the output?
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Old 21st June 2012, 04:34 PM   #8
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A voltage divider would probably work depending on the drive capability of the output and on the input of the next device. Adding the resistance will limit current drive into the next device's input which limits slew rate, but since this is not very high frequency, you can probably get away with it.

Also look at different ICs, especially 74 series, since they are almost exclusively designed for 5 V operation. The 74xx393 (e.g. 74HC393) will give you 750 kHz from 6 MHz or 12 MHz input, and will do it on 5 V.
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