Comtech EF Data EQ90 Series - Manual (Page 43)

EQ90 Amplitude Equalizer Operation
Rev. 2 3-19
IF FREQUENCY (MHz)
DE
L
A
Y
(N
S)
Figure Chapter 3-18. Ideal Composite Delay Result
Figure Chapter 3-19 shows the typical group delay response of a 17.5 MHz bandwidth
carrier such as half transponder video. In this case, the RF carrier is shifted lower in
frequency (relative to transponder center frequency) by about 8 to 9 MHz. The resultant
transponder delay contains a mixture of linear and parabolic delay coefficients. The
required equalizer delay must be the inverse of the transponder delay as shown in Figure
Chapter 3-20 resulting in an ideal composite delay result as shown in Figure Chapter 3-
21.
Once the equalizer has been pre-aligned to a required inverse delay characteristic, it is
advisable to perform a station loopback delay test through the assigned transponder with
the equipment which will be used to transmit the actual carrier. Adjustments may be
required depending upon the resultant measured delay characteristic.
Figure Chapter 3-22 through Figure Chapter 3-24 illustrates how the composite delay
would appear if the transponder or equalizer frequency were shifted slightly with respect
to one another. The resultant composite delay appears as a linear slope. The direction of
this slope is dependent upon the direction of frequency offset. In this case, a negative
slope signals that a positive frequency shift is required of the equalizer characteristic to
achieve a flat composite result. Frequency offsets of as little as 0.5 MHz can result in
substantial delay slopes. In this case, a very slight increase in every equalizer section
frequency is required to preserve the delay characteristics while altering the composite
slope.