damping resistor

[6666]

I have a strange situation at the moment, I have built a simple bog standard PI cct inc preamp, to test some coils and it does not need a damping resistor on the coil, yet it gives a nice damped wave form, so not sure whats going on

[moodz]

An interesting idea, right on I thought so between pulses to insert a sine signal I intend to try it,I'll let you know what I did! It came to my mind yes from frequency I get a converter dc voltage and that subtract from rx oscillator frequencies constant oscillator frequency of course before that and to convert that frequency into voltage means I have two reference oscillators one mix with rx ,and the other reference I would use for subtraction...
Thanks Eric!

There is more than one patent on this idea ... like this one http://www.freepatentsonline.com/8629677.pdf .. or maybe not :-)

[Orbit]

[Куоте = моодз; 280174] Постоји више од једног патента на ову идеју ... овако један хттп://ввв.фреепатентсонлине.цом/8629677.пдф .. или можда не: -) [/ куоте] Thanks Moodz! For a link..

[Ferric Toes]

I have a strange situation at the moment, I have built a simple bog standard PI cct inc preamp, to test some coils and it does not need a damping resistor on the coil, yet it gives a nice damped wave form, so not sure whats going on

I have a potted coil like that and there are two possibilities. The first is that there is short circuit across the gap in the woven copper fabric tape that is helically wound around the coil bundle. With the gap, I measure across it 1.5 ohms, so if the gap were shorted the we have a single turn conductor with a 1.5 ohm resistance. I tried this on an unpotted coil and to my surprise it gave an almost perfect damped response.

Previously, I have experimented with a damping resistor across the shield gap as the peak voltage across the resistor is then much lower. The problem is that most resistors have metal end caps which give a signal when near the coil. There are some high quality carbon resistors made by Arcol that do not have metal caps, and they would work.

The other possibility in my potted coil, which incidentally uses high end guitar coax cable, is that I did not pare back the semiconducting layer sufficiently, and that it is in contact with the live core. There is no way of finding out now. The coil is quite stable though, but shows no ringing even if externally excited.

In a bog standard PI circuit the preamp input resistors, together with their associated protection diodes will always damp the oscillations until the diodes cease conducting. This is in parallel with the other damping arrangements..

Eric.

[Carl-NC]

In a bog standard PI circuit the preamp input resistors, together with their associated protection diodes will always damp the oscillations until the diodes cease conducting. This is in parallel with the other damping arrangements.

And if you use an inverting-type preamp, the input resistor continues to damp even after the diodes turns off.

[6666]

And if you use an inverting-type preamp, the input resistor continues to damp even after the diodes turns off.

Hi Carl-Eric, yes I was using a bog standard inverting pre amp, in the past I've generally used the recommended coil and damping resistor for the project I was building, as it turns out mostly inverting preamps, looking at preamp out usually looked good enough, it wasnt untill recently when I was going for exact damping resistor for my 3dss coil (trying to match Godigit's results) that this continuing damping phenomenon has shown up, after removing the damping resistor completely, but useing a non inverting preamp gives much more precise results. Why do we use inverting preamps ?