- sounds like good job security there.
- can you explain more about your circuit? Is it some type of high pass filter? What is the 3rd overtone?
1/ Except the pager industry in GB disappeared many years ago with the advent of 2G mobile phones! I worked on them as well....
2/ No. It's a series resonant LC circuit which appears capacitive if tuned one way - which increases the system resonant frequency, and vice versa - (inductive - reduces it), at this nominal frequency.
At resonance it appears as a short circuit.
You can model a crystal as a high inductance ( eg 5mH ) in series with a very tiny capacitance. This can be measured with a special instrument, together with it's equivalent series resistance R - a few ohms..
This has a resonant frequency. Adding series inductance decreases it, capacitance increases it.
F = 1 / (2pisqrt(L*C) )
The ratio Z(L)/R gives the 'Q' or quality factor of the crystal. Since Z(L) is large and R small this leads to values of 10-100,000 , which is why crystals are so stable and give such clean oscillators.
3rd overtone. Most crystals are run here - ie at about 3 times their natural resonant frequency. Just like a guitar string a piece of quartz will resonate at it's 1st, 2nd , 3rd etc harmonic . Odd harmonics are favoured as they lead to symmetrical distortion of the blank.
Allan
PS Modern mobile phones seem to break at the slightest tap. A pre-production test for all the pagers we made was that they should be dropped from 6 feet onto solid concrete 12 times ( twice on each nominal 'face' ) and still work perfectly. I even made a special 'precision drop' gadget for doing this!
Once I had made up a small batch for a salesman to give to a client for trials. He brought one back a few weeks later and said it didn't work. Of course this is of concern to a designer, so I took it apart and found that the antenna ( a piece of silver/gold plated brass strip ) was bent. I bent it straight again and re-tuned it . It worked fine - up to spec.
I asked the salesman if he had any idea how this happened. He confessed. His kids had played tennis with it against a brick wall! Not sure how many G that is.....
Another spec was the 'spark test' . There is a an ISO standard spark generator which is meant to approximate the effect of a human charged with static to a given voltage ( I thnk it's 200pF in series with 200 ohms ) .
In very dry climates such as northern Europe in the winter, where the humidity is almost zero, this is a severe problem. Our pagers had complicated labyrinthine paths in the mouldings to try and make it hard for sparks to find the antenna - the RF front end was vulnerable.
We generally squeezed through at 4kV, but much more was luck. One client wanted 10kV - no chance.
But.
Then I thought - the centre of the antenna is at earth RF potential - it drove a differential input stage. So I earthed the middle with 200 ohms. It didn't make any difference to the RF performance, but now you couldn't break it with truly frightening 30kV sparks! Remember the energy is 1/2 CV^2, so a 30kV spark has 56 times the energy of a 4kV one. And 10 times that of a car ignition spark......
Not bad for a 1/2p resistor!
Allan
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