Effect of Coil Shielding on rf Interference.

[Anonymous]

A question on another forum, raised the point of the effectiveness of conductive shielding on the search coil of a PI detector and whether it cancelled noise. Here is my reply, and as this is a PI Classroom, let's have it posted here as a lesson too <img src="/metal/html/glasses.gif" border=0 width=15 height=15 alt="">
It is perhaps incorrect to describe shielding as cancelling noise. It is better to refer to shielding as attenuating noise. We also need to consider what frequency noise we hope to deal with by this method. With current designs, the front end of a PI has to be broad band. Not necessarily down to d.c., but it could be from a few hundred Hz, up to 100kHz or more, depending on how fast a decay curve we want to look at. This means that any coil shielding must be transparent within that frequency band. The aluminium shielding referred to in the first post, obviously was attenuating frequencies at 50kHz as it was increasing the useable delay to 20uS or more. In this case it was impeding the higher frequency components of the TX pulse from getting out. If the shielding starts attenuating at 100kHz, then for higher frequencies, there will be ever greater levels of attenuation, depending on the skin depth characteristic of the particular material used, and how thick it is. Aluminium or copper tape could be used, but it would have to be very thin, more like the aluminium film deposited on Mylar, for decorative material.
Even when the shielding is giving high levels of attenuation, it depends how near the source of the interference is, as to whether there is any effect. i.e if the shielding reduces the interference amplitude by a factor of 100, but the source is brought nearer, so that there is 100 fold increase in signal, you are no better off. R.f. interference has two effects on a PI detector. One is a beat note effect, such as experienced in here in England with the high power 200kHz transmitter at Rugby. A PI detector running at a pulse frequency which is a close sub multiple of 200kHz will experience beat interference. e.g. 200Hz, 2kHz, 20kHz. A low frequency warble of the audio will be the result. That is why a TX frequency control is very useful, as a small adjustment can increase the frequency of the beat note to the point where the integrator smooths out any residual effect. Obviously any reduction in the interference amplitude as seen by the receiver input, helps enormously. You can certainly see the reduction in the 200kHz signal when a coil is shielded, as compared with an unshielded one.
The other type of r.f. interference results from much higher frequencies, even to 100

 

[Anonymous]

Eric,
If this is a classroom lession,ILL ask a dum question.
1)At what circuit in PI would you mount the TX frequency control,what would it look like.use a coman IC in a simple drawing.Thank you.
Frank.

 

[Anonymous]

Hi Frank,
This is always done in the primary clock generator. Your CS6PI has a frequency control that also shifts the sample delay. If you experience the effects of rf interference, then just shift this control a bit. Often a couple of hairsbreadths will do. That way there is no noticeable change in the detector's performance. In the above unit, it is just a pot in the timing circuit of a TLC555 IC.
Eric.

 

[Anonymous]

Eric,
Thank's.I am at public libary,trying to think where the clock IC would be adjusted.only 2 places are at pins that go to fets,
Pot at 555 timer,its blocked.to run timer at max and be able to use battery check.R5 and R6 are maxed out,to lower time delay.
On the unite above that you were refering to,will you draw a simple drawing of the control,for people who dont ask questions.
Frank.

 

[Anonymous]

Frank,
On your CS 6, the frequency control on the faceplate of the detector is the control that adjusts the pulse timing frequency.
Eric's designs are a little different in how the timer is ultimately used, but the basic design is such that a 555 timer sets the pulse repetition rate, otherwise known as the pulse frequency.
I have copied and attached a pic of the 555 timer circuit used in Carl Moreland's Hammerhead PI which is somewhat similar to Eric's design. Carl's design is good basic design that shows how it can be done. In the pic, R 2 sets the pulse frequency, while R 3 adjusts the pulse width. The output of this timer normally pulses on and off a transistor that then biases the main FET gate.
Reg

 

[Anonymous]

Reg,
Thank's.I was hopeing that he would put a earler drawing of the goldquest 555 and pots.So I could compair the changes I made in CS.Was wondering if 555 timer was still used for AC to +5.
After looking at Hammerhead 555.It look like I adjusted my 555 the hard way. But I learned a lot by lowering it a K and ohm at a time.Thank's again.
Hope that your drawing helped some one else.
Frank.