Fixed EQ Selection

Fixed Value Equalizer Selection

Quality RF Services now offers QLX fixed equalizers to replace the Jerrold JLX (EQ-*) equalizer. The QEP-ADP adapter lets these low-cost equalizers replace the SEP and QEP family of variable equalizers. The advantages are better system noise figure due to lower insertion loss and NO adjustments to cause system response errors.

The QLX equalizers are offered in 2 dB incremental values from 2 to 30 dB in 330, 350, 400 and 450 MHz series. The 550 MHz series for line extenders range from 2 to 28 dB. Equalizer values are selected to provide a flat input to the trunk module. Use the following formula to select the values after measuring the levels of the highest and lowest frequency video carriers. The difference in the carrier levels is dB SLOPE. LF is the frequency of the low-end carrier and HF is the frequency of the high-end channel.

Since the bottom of the fraction is always constant for a given cable system, a constant (K) factor may be computed for later division into tilt values measured in the field. For a 450 MHz system the value is:

System (K) = 1 - {SQRT (55.25/445.25) = 0.6477

If a signal level measurement for channel 2 is +21 dBmV and channel 61 (445.25 MHz) is +10 dBmV, the forward slope of the carriers is the difference of 11 dB. Using the K factor from the equation above, perform the division below:

11 / 0.6477 = 16.98 dB equalizer required

Choosing the closest value equalizer in 2 dB steps would suggest a 16 dB EQ value. But since 16.98 is almost exactly between 16 and 18, it would be wise to take advantage of the lower noise figure of the amplifier input stage at lower frequencies by choosing the 18 dB EQ, attenuating the lower frequencies slightly more, which is in line with the natural noise figure of the module. The intent is to provide nearly flat input signals to the amplifier module. In Jerrold SJ systems the levels can be checked with the trunk module removed and measuring the signals after the EQ. The equation is generic and works with any brand of amplifier using fixed equalizers.

Since this method is measuring only the RF levels at the input of the amplifier, it is not concerned with the length of the preceding cable or passive losses. Changing the output slope of the preceding amplifier will alter the value of the following equalizer. Click here for EQ selection chart on page 2.