XTAL oscillator with a XOR gate
PaoloC
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
JLB
Go back to fundamentals. An oscillation occurs when there is positive
feedback with a circuit gain of 1. That is, the signal is not inverted at
the desired oscillation frequency. Another way to say this is that the
phase of the signal at the input and output are the same. If the gain is
less than one, the oscillation will die out---inother words it will not
oscillate at all. If the gain is greater than one the circuit will 'lock
up' at either 0 volts or the power supply voltage.
Most oscilaltors have some type of phase shift device between the input and
output to ensure that the phase of the input and oputpuit signal are the
same.
Note that in some circuits the feedback and phase shift may not be obvious.
A vacuum tube oscillator, for example. Here the feedback is sometimes
provided by the capacitance inside the tube.
Everything must be just right for the oscillation to occur and be stable.
With an oscillator built from inverting logic gates, you need two gates so
there is no overall phase inversion through the gates. If you are using XOR
gates, I would recommend tying one of the input lines on each gate high,
That way it will act as an logic inverter.
Jim
N8EE
"PaoloC" <oll...@hotpop.com> wrote in message
news:c5vt05$66npd$1...@ID-199778.news.uni-berlin.de...
JLB
Most oscillators that use a single active device (transistor or tube) are
linear analog circuits. You are trying to use a nonlinear (that is,
digital) device in an analog application.
The circuit using two inverting gates chasing each other's tail is called a
ring oscillator. It can be made to work without a crystal but will be
relatively unstable. Most such circuits I have seen using a crystal are in
non ciritical applications. I suspect that it will be not too stable
(temperature wise) and may not be suitable for use in a radio circuit.
Also, you will need to use a good filter becasue the signal will be full of
harmonics.
Jim
N8EE
"PaoloC" <oll...@hotpop.com> wrote in message
news:c5vt05$66npd$1...@ID-199778.news.uni-berlin.de...
Tim Wescott
Frankly I'm surprised that you're getting it to work at all with the
74HC14. I'd recommend that you use a 74HCU04 (an _unbuffered_
inverter), or a transistor oscillator with an output tuned to get the
signal strong enough to tickle a CMOS gate.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Chuck Olson
tying one input high. Then you can get it to act like a linear inverting
amplifier by putting a 1 Meg resistor from output to the other input. Now,
if you have two of these linear inverting amplifiers in cascade, a feedback
path from the second output through a crystal to the first input should
produce oscillation. You may even find that just one of the linear
inverting stages will be sufficient, since there will be a phase shift if
you put a 30pf capacitor in series with the crystal. You can control the
feedback with the capacitors to ground that you mentioned.
To run a doubler, you need a non-symmetrical square wave or pulse waveform,
since a symmetrical square wave has no second harmonic content (only odd
harmonics). With vacuum tube amplifiers, this was called Class C operation,
since conduction occurred for considerable less than 1/2 the cycle and
doubling was easy. Doubling requires about 90 degree conduction (1/4 cycle).
You should get yourself an oscilloscope, if you are a sighted person.
Chuck
W3JDR
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" <oll...@hotpop.com> wrote in message
news:c5vt05$66npd$1...@ID-199778.news.uni-berlin.de...
Steve Nosko
"JLB" <IHat...@nospam.net> wrote in message
news:YRRgc.26000$pk5....@fe03.usenetserver.com...
>
> ...With an oscillator built from inverting logic gates, you need two
gates so
> there is no overall phase inversion through the gates.
There are two types of oscillators with gates. Series and parallel.
The former requires no inversion the latter does.
> If you are using XOR
> gates, I would recommend tying one of the input lines on each gate high
He said he was.
I think the biggest issue here is thr Schmidt input. It can't be biased
*in the active region*.
--
Steve N, K,9;d, c. i My email has no u's.
Paul Burridge
>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.
PaoloC
thanks to all those who replied and discussed this topic, providing me
with a lot of good suggestions.
In the end I might use a BJT/FET oscillator instead, especially if it
works with the ex-CB XTAL of my other post. Troubleshooting with a
receiver and a DVM is fun, but time consuming and often demotivating.
I dream of an oscilloscope as much as I dream of an HF dipole up on the
roof. Hopefully both will come with time, as well as children, a better
wage, ... ;-)
73,
Paolo IK1ZYW
Steve Nosko
"Steve Nosko" <suteuve.n...@moutouroula.com> wrote in message
news:c63prk$qjn$1...@newshost.mot.com...
> like Figure 7:
>
> http://www.northcountryradio.com/PDFs/column007.pdf
>
>
> --
> Steve N, K,9;d, c. i My email has no u's.
>
>
> "Steve Nosko" <suteuve.n...@moutouroula.com> wrote in message
> news:c61bkj$11$1...@newshost.mot.com...
Steve Nosko
http://www.northcountryradio.com/PDFs/column007.pdf
--
Steve N, K,9;d, c. i My email has no u's.
"Steve Nosko" <suteuve.n...@moutouroula.com> wrote in message
news:c61bkj$11$1...@newshost.mot.com...
>
Steve Kavanagh
74HC04. Yes I know the 'HCU-4 should give less trouble, but I didn't
have any at the time. For an example, see
http://www.arsqrp.com/ars/images/backissue_images/2002_images/0602_images/ve3sma_images/Vfo1.gif
It should be possible to insert a crystal somewhere in the feedback
path to get a single-inverter crystal oscillator, though I have not
tried it. The key is that the tuned circuit is used to provide an
extra 180 degree phase shift.
Steve VE3SMA
PaoloC <oll...@hotpop.com> wrote in message news:<c62jek$73cbv$1...@ID-199778.news.uni-berlin.de>...
Steve Kavanagh
morning, in my link if you remove all the tuning components and
buffers, short the 330pF cap, and replace the inductor with a crystal
it becomes a standard crystal oscillator circuit used with the 'HCU04.
Seems to me it should work with the 'HC86 but there is always the
possibility of spurious oscillations with such buffered gates.
Steve VE3SMA
PaoloC <oll...@hotpop.com> wrote in message news:<c62jek$73cbv$1...@ID-199778.news.uni-berlin.de>...
Hans Summers
> 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."
I have successfully made a circuit using a single gate of the 74HC86 similar
to you describe. I used 1K from the gate output to the 10MHz crystal. 12p
capacitor on either side of the crystal to ground. It works very nicely! See
the circuit diagram here:
http://www.hanssummers.com/radio/qrss/qrv/mod2/index.htm .
73 de Hans G0UPL
http://www.HansSummers.com