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#1
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Using CMOS inverters to build oscillators works best when using "unbuffered"
inverters. These can be identified by the letter "U" in the part number, as in "74HCU04". If you use a buffered part or a part that has Schmitt trigger inputs, then the input-output transfer function becomes very steep (too much gain) and the oscillation becomes hard to control. When this happens, the circuit often oscillates, but at some frequency much higher than the crystal frequency. I've made oscillators out of XOR gates, but if it doesn't come up on the right frequency it can be hard to debug without a scope. To determine if it's oscillating at all, I suggest measuring the DC current consumption with the feedback removed (crystal out of circuit), and then with the feedback closed (crystal in circuit). When the circuit is oscillating, the current will be markedly higher. Once you know it's oscillating, then you have to determine where it's oscillating. This might be challenging if all you have is a receiver. Joe W3JDR "PaoloC" wrote in message ... [Slightly off-topic request] Hi. As of the other post of mine, another part of my weekend was spent trying to get a XOR gate to oscillate with a XTAL. I have a working 74HC14 oscillator: "One gate of a 74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL. 10pF and 33pF (from the junkbox) capacitors to ground on each side of the XTAL." Since my project calls for a frequency doubler, which I want to implement with a XOR gate, I wanted to use one XOR gate of a 74HC86 as oscillator. Remaining gates would work as buffer, delay line, digital mixer. (I have Googled newsgroups and found an interesting discussion about XOR frequency doublers, so I am aware of its limitations.) So I moved the oscillator circuitry (R; C; R, XTAL; C) from the 74HC14 to the 74HC86, pulling the other input of the XOR gate to "1", so that I would produce an inverter. Nothing happens. I used a 10.0 MHz XTAL. I understand that the 74HC14 has Schmitt trigger inputs and that a 74HC86 might have higher propagation delays. Still, is it possible to use a XOR gate as an inverter and oscillator? Do I overlook something? In the end I recovered the 74HC14 oscillator, but this adds one component to the final circuit I have in mind that wastes energy, space and is underutilized. Looking forward to your always helpful replies! Paolo IK1ZYW |
#2
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On Mon, 19 Apr 2004 18:53:16 GMT, "W3JDR" wrote:
Using CMOS inverters to build oscillators works best when using "unbuffered" inverters. These can be identified by the letter "U" in the part number, as in "74HCU04". If you use a buffered part or a part that has Schmitt trigger inputs, then the input-output transfer function becomes very steep (too much gain) and the oscillation becomes hard to control. When this happens, the circuit often oscillates, but at some frequency much higher than the crystal frequency. I've made oscillators out of XOR gates, but if it doesn't come up on the right frequency it can be hard to debug without a scope. To determine if it's oscillating at all, I suggest measuring the DC current consumption with the feedback removed (crystal out of circuit), and then with the feedback closed (crystal in circuit). When the circuit is oscillating, the current will be markedly higher. Once you know it's oscillating, then you have to determine where it's oscillating. This might be challenging if all you have is a receiver. Certainly would be. The other thing about using Schmidt type inputs is the likelihood of causing longer-term damage to the crystal. They 'prefer' not to be driven to hard and the sharpness of the buffered gates ain't good for 'em. |
#3
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On Mon, 19 Apr 2004 18:53:16 GMT, "W3JDR" wrote:
Using CMOS inverters to build oscillators works best when using "unbuffered" inverters. These can be identified by the letter "U" in the part number, as in "74HCU04". If you use a buffered part or a part that has Schmitt trigger inputs, then the input-output transfer function becomes very steep (too much gain) and the oscillation becomes hard to control. When this happens, the circuit often oscillates, but at some frequency much higher than the crystal frequency. I've made oscillators out of XOR gates, but if it doesn't come up on the right frequency it can be hard to debug without a scope. To determine if it's oscillating at all, I suggest measuring the DC current consumption with the feedback removed (crystal out of circuit), and then with the feedback closed (crystal in circuit). When the circuit is oscillating, the current will be markedly higher. Once you know it's oscillating, then you have to determine where it's oscillating. This might be challenging if all you have is a receiver. Certainly would be. The other thing about using Schmidt type inputs is the likelihood of causing longer-term damage to the crystal. They 'prefer' not to be driven to hard and the sharpness of the buffered gates ain't good for 'em. |
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