We appreciate your cooperation with the FAQ improvement questionnaire. Were these FAQs helpful? We would like to hear your opinions and requests regarding these FAQs. Opinions from customers will be used to improve the FAQs. We will not respond to inquiries and requests received by this form. Well, op amps are not necessarily bad as comparators, but they sure are slow, and the LM is among the slowest. Not only is its slew-rate slow, but if you put in an overdrive of just 5 mV more than VOS, the output will respond at only 0.
An LF35 1 or one-fourth of an LF will respond somewhat faster. So if you want to use an op amp as a comparator, you'd better need merely a slow comparator. But, even so, some people do use op amps as slow, precision comparators. Even though op amps are generally not characterized as comparators, you can engineer such a circuit successfully.
For example, the LM minus its compensation capacitors is a surprisingly competent, fairly quick comparator. But, please don't overdrive and damage the inputs. Conversely, I am occasionally asked, "Can I put some damping capacitors on an LM and use it as a unity-gain follower? But I have used the slower LP and LP successfully this way, as a slow inverter or slow follower. Any circuit that adds current gain can oscillate--even a buffer.
Let's agree that a buffer is some kind of linear amplifier that has a lot of current gain. Some have a voltage gain around 0. Others have gains as high as 10 or 20 because their outputs must swing 50 or V p-p-or more. Even emitter followers, which you'd expect to be very docile because their voltage gain is less than 1, have a tendency to "scream" or oscillate at high frequencies. So whether you buy a buffer or "roll your own," watch out for this problem.
Also, a buffer can have a high-frequency roll-off whose slope increases suddenly at 40 or 60 MHz and thus can contribute phase shift to your loop, back down at 6 or 10 MHz. You can beat this problem, but you have to plan.
A buffer can also add a little distortion, which the op amp cannot easily cancel out at moderate or high frequencies. Since buffers don't usually have a spec on this distortion, beware.
Also, if you're running the output's quiescent bias current as Class AB, you must be sure that the DC operating current is stable and not excessive. You must set it high enough so that you don't get distortion but not so high that power consumption becomes excessive. One of my standard procedures for stabilizing a unity-gain follower stage is to put feedback capacitance around just part of the loop FIG. This circuit tolerates capacitive loads, because the buffer decouples the load while the feedback capacitor around the op amp provides local stability.
Most unity-gain buffers, whether monolithic, hybrid, or discrete, are unstable with inductive sources, so keep the input leads short.
A series resistor may help stability, as it does for the LM, but it will slow down the device's response. Driving these loads can require a lot of current, which leads to overheating.
Plan your heat sinks carefully to keep the device from exceeding its rated maximum tempera-ture. Most buffers don't have any thermal shutdown feature, but the new LMs and LMs show that a good op amp, at least, can have such features designed in.
When using buffers to drive remote loads, be sure that the transmission lines or cables have suitable termination resistances on both ends to prevent reflections and ringing. If you can afford the voltage drop, it's best to put about 50 ohm between the buffer and its cable. When your buffer provides a lot of extra voltage gain, you must make sure that the gain rolls off in a well-engineered way at high frequencies, or the loop will be unstable. If the buffer-amplifier has a positive gain, as in Figure 9Sb, you can use capacitive feedback around the main amplifier.
But when the buffer-amplifier has a gain of FIG. In some cases, you can achieve stability by putting a series RC damper from the noninverting input to ground to increase the noise gain, but this trick doesn't always work. Damping this loop is tricky, because there is so much gain stacked up in cascade. But the feedback capacitor to the negative input makes this safe and easy.
Depending on the gain of the buffer, you can use these three schemes to stabilize a buffered amplifier. A circuit that inadvertently latches up presents a problem exactly opposite that of an oscillating circuit. Or, you could correctly say that a latched-up circuit is an oscillator with zero frequency. Although latched-up circuits demand troubleshooting, the good thing about them is that they sit right there and let you walk up to them and touch them.
And you can measure every thing with a voltmeter to find out how they are latched. This state of affairs doesn't mean that troubleshooting them is easy, because sometimes you can't tell how the latched-up circuit got into its present state. And in an integrated circuit, there can be paths of carriers through the substrate that you can't "put your finger on.
Two approaches for attacking destructive latches are:. Turn off the power quickly, so the latched-up circuit cannot destroy anything. Try turning on power for short pulses and watching the circuit as it approaches the destructive latch condition. Use an adjustable current-limited supply with zero or small output capacitance, such as the example in Section 2 , so when the circuit starts to latch, the fault condition can easily pull the current-limited power supply's voltage down.
Another way to inadvertently generate a latched-up condition is to turn on the outputs of your multiple-output power supply in the "wrong" sequence. Some amplifiers and circuits get quite unhappy when one supply usually the positive one turns on first.
Automatic power-supply sequencers can help you avoid this problem. An anti-reversal rectifier across each supply can help, too, and is always a good idea for preventing damage from inadvertently crossed-up power-supply leads or supply short circuits. It was always painful for me to tell them, "Don't ship it--junk it. And, next time put antireversal diodes on each supply.
Also add an antireversal rectifier across each power supply's output to protect the supplies FIG. Some people think that leaving parts out is a good way to improve a circuit's reliability, but I've found that putting in the right parts in the right places works a lot better. Refer also to a running commentary and debate on this topic in Section If you have any doubt that your anti-oscillation fixes are working, try heating or cooling the suspected semiconductor device.
In rare cases, passive components may be sufficiently temperature-sensitive to be at blame, so think about them, too. Even if a circuit doesn't get better when heated, it can get worse when cooled, so also take a peek at the circuit while applying some freeze mist. My point is that merely stopping an oscillation is not enough. You must apply a tough stimulus to the circuit and see whether your circuit is close to oscillation, or safely removed from any tendency to oscillate.
This stricture applies not only to regulators but also to all other devices that need oscillation-curing procedures. For example, if a 47 ohm resistor in the base of a transistor cures an oscillation, but 24 ohm doesn't, and 33 ohm doesn't, and 39 ohm still doesn't, then 47 ohm is a lot more marginal than it seems. Maybe a 75 ohm resistor would be a better idea-just so long as or or ohm resistors are still safe. An oscillator as recited in claim 1 wherein said capacitance means included a voltage variable capacitor.
A crystal controlled overtone oscillator as recited in claim 3 wherein said amplifier includes a transistor having base, emitter and collector electrodes, said input terminal being connected to said base electrode, said common tenninal being connected to said emitter electrode, and said output terminal being connected to said collector electrode.
A crystal controlled overtone oscillator as recited in claim 4 wherein said first capacitor is a voltage variable capacitor. A crystal controlled overtone oscillator as recited in claim 5 further including means for applying a modulating voltage to said voltage variable capacitor. A crystal controlled overtone oscillator as recited in claim 6 further including a resonant circuit tuned to a predetermined harmonic of said predetermined overtone operating frequency connected to said output electrode.
A crystal controlled overtone oscillator as recited in claim 3 wherein said amplifier has a predetermined transconductance proportional to the square of the predetermined overtone operating frequency and the capacitance of said second and third capacitors.
A crystal controlled oscillator as recited in claim 3 wherein said first and second resonant circuits each have first and second terminals, said first terminal of said first resonant circuit being connected to the first terminal of said second resonant circuit to form a first junction, said second terminal of said first resonant circuit being connected to the second terminal of said second resonant circuit to form a second junction, said resistor being connected between said first and second junctions.
A crystal controlled oscillator comprising: a piezoelectric resonator having a predetermined operating frequency and other spurious operating frequencies; inductance means and capacitance means connected in series with said piezoelectric resonator to form a first resonant circuit, said inductance means and said capacitance means being series resonant at said predetermined operating frequency; second inductance means and second capacitance means connected in series to form a second resonant circuit, said second inductance means and said second capacitance means being series resonant at said predetermined operating frequency; resistance means; means connecting said resistance means in shunt with said first and second resonant circuits; and amplifier means connected to one of said first and second resonant circuits for energizing said resonant circuits to oscillate at said predetermined operating frequency.
A crystal controlled overtone oscillator as recited in claim 3 wherein said amplifier includes a transistor having base, emitter and collector electrodes, said input terminal being connected to said base electrode, said common terminal being connected to said emitter electrode, and said output terminal being connected to said collector electrode.
A crystal controlled oscillator as recited in claim 3 wherein said first and second resonant circuits each have first and second terminals, said first terminal of said first resonant circuit being connected to the first terminal of said second resonant circuit to form a first junction, said second terminal of said first resonant circuit being connected to the second terminal of said second resonant circuit to form a second junction, said resistor being connected between said first and sEcond junctions.
US USA en USA en. Frequency-controlled oscillator comprising a piezo-electric element and having an extended frequency variation range. Crystal controlled overtone oscillator having a rejection circuit for preventing oscillation at undesired overtones. Prior Art There are many applications wherein it is desirable to prevent an oscillator from oscillating at undesired frequencies. One such application is in a frequency modulated crystal controlled overtone oscillator wherein the modulation of the oscillator can cause the oscillator to switch to a spurious mode of operation.
Several techniques for reducing spurious oscillations are known. One such system employs tuned circuits to prevent oscillation at the undesired spurious modes, whereas another such technique utilizes a resistor in parallel with the crystal to introduce losses in the oscillator circuit to prevent oscillation at the undesired modes. Whereas these techniques provide means for reducing spurious oscillations, the first technique is costly, requires precise adjustment and does not effectively remove spurious responses that are closely spaced in frequency to the desired frequency of operation.
The second technique causes undesirable loading of the oscillator, and only reduces spurious responses that have an amplitude characteristic well below that of the desired oscillation frequency. SUMMARY It is an object of the present invention to provide an improved frequency modulated crystal oscillator providing modulation linearity and freedom from spurious oscillations.
It is another object of this invention to provide a frequency modulated crystal controlled oscillator that provides increased frequency deviation without spurious oscillations.
In accordance with a preferred embodiment of the invention, a resistance-inductance network is added to a Colpitts type oscillator. The values of the inductance and resistance are chosen to provide power dissipation at undesired spurious frequencies, and to allow only minimal losses to occur at the desired operating frequency. The gain of the oscillator circuit is selected to prevent oscillation at the undesired spurious frequencies at which the power loss is present.
A second resonant circuit comprises a second inductor 16 and a pair of feedback capacitors 18 and The two resonant circuits are coupled together by means of the coupling capacitor 22, and are further coupled to the base of a transistor 24 by means of a coupling capacitor
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