Circuit provides ADSL frequency reference

Bert Erickson, Fayetteville, NY -- EDN, 5/25/2000

The discrete-multitone (DMT) frequencies that asymmetrical-digital-subscriber lines (ADSL) use are integral multiples of a common frequency, and the symbol period is the inverse of this frequency. Integration over the symbol period allows the sine and cosine orthogonal waveform products to vanish for all multiples of the common frequency except for those having the same frequency. As the ADSL standards (TI.413) specify, the 256 channels are separated by 69/16 kHz. You can generate the midchannel frequencies with a PLL, but the reference frequency differs from that of crystals for computers and clocks. However, by using the circuit in Figure 1, you can generate the frequency by using a 3.58-MHz crystal to control the horizontal scanning rate in television sets. A typical 3.58-MHz crystal has a tolerance of ±50 ppm and a load capacitance of 18 pF. This tolerance provides a frequency of 3.579366 to 3.579724 MHz. If you multiply this common DMT frequency by 830, the result is 830x69/16 kHz, or 3.579375 MHz, which is 9 Hz above the crystal's lower tolerance limit. Assuming that you can select the C_S and C_T capacitors at either side of the crystal to tune the frequency near the lower tolerance limit, you can also select them for the desired frequency.

In other words, reduce the oscillator frequency with bistable flip-flops and combine the outputs in a NAND gate to divide by 830. For the 3.58-MHz crystal, design values for C_S and C_T were 23.6 and 75.7 pF, respectively. We chose 22 pF for C_S and 68 pF for C_T. A trimmer capacitor in parallel with C_T reduces the frequency. When C_T increased from 22 to 90 pF, the frequency decreased by 448 Hz and handily bridged the 3.579545- nd 3.579375-MHz frequencies. Tests showed that the lower frequency was more than 100 Hz below 3.579357 MHz, but the exact number depends on the calibration of the counter. Because 830 is a 10-bit binary number, the circuit divides by 415 first to permit combining with an eight-input NAND gate. A strobe applied to a flip-flop then creates a square wave for the reference-frequency output. (DI #2531)