Poor Mikey. The only way you can redeem yourself is with a HAND analysis of the MC1648 that shows you actually have a single brain cell in your body. NO simulations ;-)
...Jim Thompson
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| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Increasing the supply voltage and the emitter resistance can help with this. Also making sure that you don't have any mid-length time constants around for squegging to find is good idea.
They don't tend to have the problem you descibed above but they are not without their own troubles. With JFETs, you need to be careful about the bias to make sure that you don't have it driving the gate into conduction. A JFET can have a very high gm near Idss that still increases as the gate goes towards forward bias. "Gate leak" bias tends to yeld a noisy oscillator.
If you control the average emitter current, you will likely do even better. For large signals, you get a reduction in gm as amplitude increases. Basically, any gain that goes to making harmonics, is making as much fundamental.
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I've seen that happen. It can destroy the transistor if nothing stops the run away condition.
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You need to solve the positive feedback issue mostly before you try to close the loop. You can make your RF amplitude detector circuit as a combination of a fairly quick inaccurate one and a slower more accurate one. You can stand a 10% variation in the "P" term of a "PI" controller. In the long term it is the "I" term that sets the amplitude.
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What is the swing on the base? I had assumed it would be small since you are doing an AGC. Decreasing average base voltage with increasing signal is a sign of nonlinear operation.
L1,C1 and C2 form the frequency selection circuit. The C2/C1 ratio is largish. Y1 is running in series mode and sees low impedance on both of its ends..
Thanks, its a grounded emitter for the sake of minimum parasitics, a reasonably high voltage is needed to get it to oscillate over a wide range and keep Ccb low. Im not sure about the squeging I dont think ive seen any since its behaved.
In effect the fairly quick inacurate one is monitoring the collector current. reducing the RF gain with the 200R resistor in the feedback was the key to success in taming the positive feedback, but ive not seen this used in the countless oscillator circuits ive looked at.
Im not sure what the swing on the base is, exept that it is high when uncontrolled. measuring such signals is problomatic. It should be very small when it is controlled as it goes through another gain stage before it is level controlled to .5vp-p
The non linearity doesnt seem to be a problem now, its said this improves the phase noise anyway. The problem before was uncontrolled limiting in subsequent stages cuasing awfull harmonics wich exceeded the fundamental in the final stage, now the output looks at worst like very rounded squarewave wich is fine to drive my laser.
Yes, assuming the current rises when it oscillates that will do it. The emitter resistor I had assumed effectively does this same thing.
If you need wide tuning you may not be able to do so but normally I would suggest that changing the capacitive divider in the feedback path be used to lower the gain.
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I was expecting a calculated estimate. You could do a spice model of the circuit. If everything else is accurate, the base swing is likely to be also. In the startup condition, you want to make sure you can't get into breaking down the EB junction.
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Personally I have my doubts about the non-linearity making phase noise better. The non-linear action makes the stage operate as a mixer it takes noise from near harmonics and mixes it down to near the carrier. This seems like a bad idea to me.
You didn't respond to the question. Show me a by-hand mathematical analysis. Otherwise I relegate you to blow-hard status.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
I was trying to keep parasitic inductance as low as practicably possible, as it considerably lowers the range of tuning, conveniently these transistors have 2 emitter leads.
Yes thats true, the tuning is done by 2 varactor as a capacitor divider, I was unsure as to the necessary ratio of this divider as many circuits had quite different ratios,
1:1 seemed to be inbetween. I could try a higher capacitance one accros the emitter. I had a circuit with series diodes in the collector side and parallel in the emitter but this wasnt very good at 1ghz.
It should work well enough for now to test the limit of the laser/detector anyway.
IME the accuracy of the spice simulation would be entirley dependant on my estimate of the parasitics wich isnt very good, Ive usualy added parasitics to the sim till it works as the real thing, sometimes far more than would seem realistic. I think the main problem is the model of the varactor diode, the series resistance varies drasticaly over the tuning range.
Seems to make it considerably better judging by the figures in a few reports, at least for close in phase noise. The maths suposedly shows the tank is least susceptable to a noise impulse during the peak voltage. I cant say ive measured it for myself though.
I remember I had a similar discussion about this some time ago, I ended up being more confused than before. Its also interesting that the collector current monitoring transistor suposedly negates some of the RF transistors 1/f noise.
From a DC point of view, the ground plane of the oscillator circuit can be above the emitter resistor. This can let you use the "opposed emitter" as you normally would by taking them straight to the plain. The bypass capacitor on the emitter resistor will include the capacitance from the oscillator's ground plain to the overall PCB ground plane.
You could just assume that the EB junction has 1/2 the swing of the collector and see if that would break down the transistor. The loading of the input drive will ensure that the signal is less than that.
In terms of phase shift caused by an impulse, the effect varies as the sine of the angle from the peak. As you go further into class-C, the angle at which there are noise impulses decreases. On the other hand, the amplitude (or number of) the noise impulses is increased. This makes it sound like it may matter what the noise source's spectrum looks like.
Yes, moving the ground plane leeds to many possibilities just like all the different colpits configurations. Ive used a 100R collector load instead of the inductor/33R wich probably has the same effect, and the base bias is now derived from this wich otherwise is reasonably conventional bias circuit, although it is further controlled through a pnp.
I chose common emitter for the colpits becuase it means the capacitances from each end of the inductor to ground are effectivly in series wich means the effective parasitic parallel capacitance is half that compared to one end of the inductor being grounded, wich has less efect on the tuning range, also the blurb about noise advantage of dual emiter being conected directly to grnd was in the back of my mind.
When it was uncontrolled I did put my RF sniffer probe on the base and found about 1.7v p-p wich surprisingly was higher than at the collector, this probably means it was exceeding the 1.5veb on the spec sheet, the impedance probably limited any damage.
When it is controlled to low level the waveform looks quite sinusoidal wich means the base drive must be small ( angle at which there are noise impulses decreases. On the other hand, the
The noise spectrum would also be dynamic, probably the biggest part of noise is related to sqrt of collector current, therefore a higher peak current would mean lower shot noise contribution for the same average current.
Actually, you also chose common-base and common-collector. The oscillator doesn't 'know' where its circuit is grounded.
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OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
Yes but the parasitic capacitances to ground, biasing components and also the output connection all have diferent influences as they are referenced to wichever transistor conection is grounded.
But but but..... when you go further into class C you include noise components from higher and higher frequencies. The noise increases as the sqrt() of the bandwidth so there is another sqrt() term to multiply the sqrt(I) by removing the advantage.
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