LC Oscillator Questions

The waveform in a high Q tank that's lightly coupled to the amplifier should be very nearly sinusoidal. If in addition, the amplifier remains linear and represents a constant impedance over the whole cycle of the waveform, then the waveforms should everywhere be sinusoidal. If the amplifier+tank has barely enough loop gain to sustain oscillation, then clipping will be minimal, but it's also possible to detect the level and control the gain of the amplifier. You could, for example, use a light bulb like HP did in their original audio oscillator. Beware, though, that best oscillator performance in other regards may not be achieved the same way you achieve lowest harmonic distortion. Be careful that you optimize the right things for your application.

In the work I do, I need to measure distortion, and the generators I use don't have low enough distortion in their outputs to be directly useful. The distortion levels in the "raw" outputs are generally about -40 to -50dBc. I use filters to make things better, and can get to -140dBc distortion levels fairly easily. If it's low harmonic distortion you want, I'd suggest that it may be better to just put a filter on the output of the oscillator that has only moderately low harmonic output, and not worry so much about that aspect of oscillator performance. Filters work well when the oscillator frequency range is about 1.5:1 or less. Much more than that and you'd need to switch in different filters depending on the oscillator frequency.

Cheers, Tom

Hello Anthony,

1. Please set the following option to sitch off data reduction/compression in the result file..

.options plotwinsize=0

1. You have to set a small maximum timestep in the .TRAN line too. Maybe a value of 0.01*Period of oscillation if you hunt for very low distortion.

Can you send me your file (.asc-file and model-file?) to check it?

Best regards, Helmut

"Anthony Fremont" schrieb im Newsbeitrag news: snipped-for-privacy@news.supernews.com...

The secret to a beautiful waveform is -- you usually don't need it straight from the oscillator.

There are a lot of things that you want out of an LC oscillator. Low phase noise, frequency stability, consistently strong oscillation, pure tone, etc. Of these, the only two that you can't clean up later in the following amplifier chain is low phase noise and frequency stability. Concentrate on those, & don't sweat the nice waveform.

Frequency stability and phase noise performance are often achieved by intentionally designing the amplifier so the active element operates in class C, without ever going into voltage saturation. This keeps it's drain (or collector) impedance high, yet delivers a large voltage swing to the gate (or base) to keep phase noise low. It also gives you a more or less consistent standing voltage in the tank, which helps the design of the following buffer stages.

If you absolutely positively must tap the World's Most Beautiful Sine Wave off of the oscillator section, consider a parallel-tuned tank that's loosely coupled to the active element. Then loosely couple your output tap to that -- it's your best chance.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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"Applied Control Theory for Embedded Systems" came out in April.
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After reading the other replies, it seems aparent that the shape of the signal from the first stage is not that critical, it is stability and phase noise that are most important. So, I should put things back where there is clipping to be sure that the oscillator oscillates and then clean up the signal in subsequent stages.

Thanks. :-)

Helmut Sennewald wrote:

In alt.binaries.schematics.electronic I have posted the schematic, the asc-file and an oscilloscope screen shot from an actual circuit. Here is the asc-file contents:

Version 4 SHEET 1 880 708 WIRE -704 -96 -784 -96 WIRE -400 -96 -704 -96 WIRE -224 -96 -400 -96 WIRE -704 -16 -704 -96 WIRE -400 -16 -400 -96 WIRE -224 32 -224 -96 WIRE -544 80 -592 80 WIRE -400 80 -400 64 WIRE -400 80 -464 80 WIRE -288 80 -400 80 WIRE -592 128 -592 80 WIRE -400 144 -400 80 WIRE -784 160 -784 -96 WIRE -400 240 -400 208 WIRE -224 240 -224 128 WIRE -224 240 -400 240 WIRE -80 240 -224 240 WIRE 48 240 -16 240 WIRE -784 272 -784 240 WIRE -704 272 -704 48 WIRE -592 272 -592 208 WIRE -400 272 -400 240 WIRE -224 288 -224 240 WIRE -592 384 -592 336 WIRE -400 384 -400 336 WIRE -400 384 -592 384 WIRE -224 384 -224 368 WIRE -224 384 -400 384 WIRE -400 448 -400 384 FLAG -784 272 0 FLAG -400 448 0 FLAG -704 272 0 FLAG 48 320 0 SYMBOL voltage -784 144 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 5.8 SYMBOL res -416 -32 R0 SYMATTR InstName R3 SYMATTR Value 100k SYMBOL npn -288 32 R0 SYMATTR InstName Q3 SYMATTR Value 2N3904 SYMBOL cap -416 144 R0 SYMATTR InstName C1 SYMATTR Value .01µ SYMBOL res -240 272 R0 SYMATTR InstName R7 SYMATTR Value 1k SYMBOL cap -416 272 R0 SYMATTR InstName C2 SYMATTR Value 500p SYMBOL ind -608 112 R0 SYMATTR InstName L1 SYMATTR Value 20µ SYMATTR SpiceLine Rser=.1 SYMBOL cap -608 272 R0 SYMATTR InstName C3 SYMATTR Value 200p SYMBOL cap -16 224 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C4 SYMATTR Value 270p SYMBOL res -448 64 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value .001 SYMBOL cap -720 -16 R0 SYMATTR InstName C5 SYMATTR Value 10µ SYMBOL res 32 224 R0 SYMATTR InstName R2 SYMATTR Value 10000k TEXT -792 360 Left 0 !.tran 50uS

Thank you for your time.

"Anthony Fremont" schrieb im Newsbeitrag news: snipped-for-privacy@news.supernews.com...

Hello Anthony,

The large capacitance of C1 (10nF) has caused an interrupted oscillation. Please change its value to 1000p and the oscillator will work as expected. I have also added MEASURE-commands to measure the frequency. View -> SPICE Error Log

Another method is using the FFT in the waveform viewer.

Best regards, Helmut

Save as "osc1.asc".

Version 4 SHEET 1 880 708 WIRE -688 -96 -784 -96 WIRE -576 -96 -688 -96 WIRE -304 -96 -576 -96 WIRE -784 -64 -784 -96 WIRE -688 -64 -688 -96 WIRE -576 -16 -576 -96 WIRE -304 32 -304 -96 WIRE -784 48 -784 16 WIRE -688 48 -688 0 WIRE -576 80 -576 64 WIRE -480 80 -576 80 WIRE -432 80 -480 80 WIRE -368 80 -432 80 WIRE -576 128 -576 80 WIRE -432 144 -432 80 WIRE -576 240 -576 208 WIRE -432 240 -432 208 WIRE -304 240 -304 128 WIRE -304 240 -432 240 WIRE -240 240 -304 240 WIRE -160 240 -240 240 WIRE -64 240 -96 240 WIRE -32 240 -64 240 WIRE -576 272 -576 240 WIRE -432 272 -432 240 WIRE -32 272 -32 240 WIRE -304 288 -304 240 WIRE -32 368 -32 352 WIRE -576 384 -576 336 WIRE -432 384 -432 336 WIRE -432 384 -576 384 WIRE -304 384 -304 368 WIRE -304 384 -432 384 WIRE -432 416 -432 384 FLAG -784 48 0 FLAG -432 416 0 FLAG -688 48 0 FLAG -32 368 0 FLAG -64 240 out FLAG -240 240 e FLAG -480 80 b FLAG -576 240 lc SYMBOL voltage -784 -80 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 5.8 SYMBOL res -592 -32 R0 SYMATTR InstName R3 SYMATTR Value 100k SYMBOL npn -368 32 R0 SYMATTR InstName Q3 SYMATTR Value 2N3904 SYMBOL cap -448 144 R0 SYMATTR InstName C1 SYMATTR Value 1000p SYMBOL res -320 272 R0 SYMATTR InstName R7 SYMATTR Value 1k SYMBOL cap -448 272 R0 SYMATTR InstName C2 SYMATTR Value 500p SYMBOL ind -592 112 R0 WINDOW 39 36 108 Left 0 SYMATTR InstName L1 SYMATTR Value 20µ SYMATTR SpiceLine Rser=.1 SYMBOL cap -592 272 R0 SYMATTR InstName C3 SYMATTR Value 200p SYMBOL cap -96 224 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C4 SYMATTR Value 270p SYMBOL cap -704 -64 R0 SYMATTR InstName C5 SYMATTR Value 10µ SYMBOL res -48 256 R0 SYMATTR InstName R2 SYMATTR Value 100k TEXT -824 -152 Left 0 !.tran 0 200uS 0 4n TEXT -824 -184 Left 0 !.options plotwinsize=0 TEXT -816 472 Left 0 !.measure tran t1 FIND time WHEN V(out)=0 TD=90u RISE=1 TEXT -816 504 Left 0 !.measure tran t2 FIND time WHEN V(out)=0 TD=90u RISE=101 TEXT -816 536 Left 0 !.measure tran f0 PARAM 100/(t2-t1) TEXT -816 576 Left 0 ;View -> SPICE Error Log \\nfor the measured frequency TEXT -520 -184 Left 0 ;C1 changed to 1000p!

Okay..

Okay, that certainly explains why all the sample circuits I find don't expend any great effort at creaing a nice sine wave, and none at explaining why. What you say certainly makes sense, especially if there are no really negative consequences of having the oscillator make a "less than perfectly shaped" wave.

Ok, thanks for the information. :-) I did allot of googling but found nothing that explained it like this. I was thinking of building a little single conversion superhet WWV receiver for 10MHz, if I continue with that I'll just concentrate on cleaning it up in another stage.

Some material I read suggested keeping Xl of L1 at ~300Ohms, the series Xc (C3) at ~200Ohms and Xc of C1/C2 at 45Ohms. Do you have any thoughts on that? Right now I have way too much inductance for 3.5MHz by that theory, and judging from other circuits I've seen.

I have never seen clipping. These things are supposed to limit in cutoff, not saturation. As the signal build up, the conduction angle gets smaller and smaller until the device runs out of gain. That is another way of saying that the DC value of the gate voltage gets more negative the bigger the amplitude. This works out automatically with a JFET. You need about 10K -

100K DC resistance from gate to ground. Using a bipolar transistor is not a good idea.

Tam

That sounds more or less right. With a Clapp oscillator the main tank is isolated by the series cap, so more of the energy is kept in the coil and C3, and less of it shows up in C1, C2, and the transistor.

If you're driving a balanced mixer you want to have an LO signal that doesn't have much even-harmonic (2nd, 4th, etc.) energy in it, but for a casual receiver that's the least of your worries. Since you're operating at a fixed frequency it may be a good idea to just feed the oscillator output into a single-tuned resonant circuit to clean it up, then send it on to the mixer.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

I was wondering about the load that a bipolar would present. I will see if I can find a JFET in my junk pile, thank you. :-)

In some LC oscillators, the amplitude of the oscillation is controlled by a feedback loop. For example, a rectifier can be used to create a DC voltage proportional to the oscillation amplitude on the LC tank, and then an op-amp can be used to compare the rectifier output signal to a reference voltage. The output from the op-amp can be filtered and then used to control the current in the oscillator core. It is difficult to do all of this in a way that keeps the phase noise low, but given the right simulation tools (e.g. SpectreRF which is rather expensive), good results can be obtained. In particular, a well-defined oscillation amplitude can help to keep the KVCO well controlled, which is useful in PLLs.

Chris

This can more difficult to acheive well than it first seems, adjusting the bias conditions to adjust level is often not stable, as it draws more current as the oscilations build up, a fast feedback loop wich monitors the current can be used to adjust the bias to keep the average current at a set point, wich can be just a single npn transistor, an op amp can then be used to set the set point, although just ensuring a set current is usually sufficient to maintain a very clean stable waveform, and yet not have any startup problems, for all but very wide range oscillators.

Colin =^.^=

Thank you very much. :-) I have now switched to using an MPF102 JFET instead of the bipolar and much less capacitance for C1 (now 470pF). I only get a 2V peak to peak signal out now, but it's quite nice looking.

Ok, I've now put in an MPF102 and changed R3 to a pull-down. I lowered C1 to 470pF and I get a nifty 2V p-p sine wave on the output. It really tamed the tank circuit voltage down as well. Which brings up a question, with the tank now completely DC blocked from Vcc and Vss, where does it get it's energy. I assume that it must come thru the gate. How does that happen? :-? My circuit is much like Figure 1 here, without the diode though:

(cyan)

R1

crystal

around

is

transistor

but

peaks

use

The prettiest waveforms come from balanced oscillators. Distortion then turns up as 3rd 5th 7th etc harmonics which are far less ugly than the 2nd

3rd 4th 5th etc generated by the single ended types. Balanced ALC is also easier and more effective. My own experience says that 'prettier' is better. Those oscillators offering gross distorted outputs also seem to suffer badly in other areas and gross distortion always causes problems further down the line. Procuring good quality is a classic black art, one aspect is to allow the LC just an occasional vague glimpse of the maintaining amplifier. Another is to cause limiting by use of an amp having a gentle gain change (eg Fet v bipolar) and the other is ALC. (Or all three together). Failing that, there is always the cop-out of an output filter :) john

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It comes from the source, through the coupling capacitors -- Cfb-a and Cfb-b in your link.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

I have now changed it to an MPF102 that I've had laying around for many years. It works great, thanks. :-)

Glad it worked out. By the way the feedback path is through the capacitive network between source and gate. That would be more obvious in the configuration that uses a tapped inductor, but works the same way. Leaving out the diode was the right thing to do; it just adds to the noise. I don't know what kind of stability and linearity you need, but if that is important, do not use the common type of ceramic capacitors that are meant for bypassing. They are lossy, and their value varies with applied voltage. Use mica, NPO ceramic, or Mylar and similar for larger values.

Tam

Without going to the purity levels that Tom requires, I've always found that bipolars can be used to produce a fairly reasonable "visibly sinusoidal" (see note) waveform. Follow the oscillator with an amplifier stage which drives a limiter/clipper, and use that to control a gain element in the oscillator. It's like the incandescent non-linearity arrangement except the oscillator stage waveform remains fairly clean.

(Note: Harmonic distortion not readily discernible on a CRO)