دوستانی که از اینترنت DSL استفاده می کنند حتما با واژه و دستگاه اسپلیتر آشنا هستند. این وسیله برای جداسازی Voice از Data در خطوط تلفن استفاده می شود که همانطور که میبنید یک فـیلتر ساده است و ساخت اون شاید ۵ دقیقه بیشتر زمان نبره .مدار زیر شماتیک همان دستگاه اسپلیتر است که باید با هر گوشی تلفن سری بشه یا در ورودی تلفن منزل نصب بشه. شاد باشید .
Commercial ADSL filter example: ELEXI ADSL filter
The following photos, circuit diagram and text try to describe how ADSL filter sold by ELEXI (product number 440072 "ADSL puhelinsovitin USA/USA GP4C") works. This ADSL fitler is picked as example because I happened to own one and the construcion of this filter is quite simple (=easy to understand).
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Photo 1: ELEXI ADSL filter outside viewe
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Photo 2: ELEXI ADSL filter opened
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Photo 2: Close-up of the circuit board iside the filter. The telephone line connecting wires go to top right and the wires going to telephone connector leave at bottom left.
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Figure 1: Circuit diagram of the filter circuit board electronics.
The phone line to central office (that carries ADLS and PSTN signals) goes to the left side of this filter and the phone goes to right side. This filter consists of an LC low pass filter made of the series connected coil (total impedance of 10.260 mH per wire) and the 22 nF capacitor across the wires. Those form the actual filter. There are resistors after this LC filter to make the filter properties have proper impedance matching for voice frequencies (should be around 600 ohms). This filter seemed to have one 1A fuse on the output to protect the circuit agains some catastrophic damage (what this would protect agains..).
The total series resistance this kind of filter cause to one telephone wire was around 45 ohms, meaning that having this filter adds around 100 ohms to the telephone wire resistance. The series resistors and internal coil resistance reduces the impedance mismatches that just putting a capacitor across telephone, wires would cause. If we take a look at the circuit construction with only resistances and capacitances in it, it is pretty close to a simple model of 0.5 km of 0.5 mm telephone cable wire. The coil impedance at voice frquencies would be at 20-200 ohms, causing some mismatch and attenuation at high telephone frequencies. The capacitor has impedance of aroudn 250-2500 ohms at voice frequencies. Return loss at voice frequencies (against 600 ohms) would be acceptable with this kind of circuit.
The coils in this circuit are built to ferrite cores, boppin cores for highest inductance values and the 40 uH coils are wound to small ferrite toroids. My quess is that the coil part is constructed from three separate coils to make the filter to work well at high freuquencies. The 10 mH coil has lots of turns, so at high freuquencies it's perfomrance might not be best (potential coil resonances etc.), but at those high freuquencies those smaller value coils have already attenuted the signal enough. Getting three different coils in series gives best performance on all freuquencies.
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