Section 3.23

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3.24 Offset Steps

Marginally different techniques were used for the COFDM and 8-VSB systems to measure the receiver tuning offset step size.

1. The COFDM receiver was instructed to change frequency up one offset step.
2. The Rig LO frequency was then adjusted in 1 kHz steps to find the centre of the frequency relock range.

The COFDM receiver was found to have a 200 kHz offset step with a total of 8 steps available. 3 of these were above the centre frequency while 4 were below the centre frequency.

It is noted that the offset step size exceeds the AFC lock range by around 20 times for the COFDM equipment. This means that the offset function would be no use for correcting a mis-tuned receiver unless the required frequency fell very close to one of the offset step increments.

1. The 8-VSB receiver the RF input cable was directly connected to the spectrum analyser and the LO frequency determined by its leakage out the antenna RF port.
2. As the 8-VSB offset was varied the steps in the LO frequency were noted.

The 8-VSB receiver was determined to have a first IF of 921.0 MHz. The LO was found at 1.1125 GHz for VHF channel 8 (191.5 MHz). A total of 16 250 kHz frequency steps are available with the 8-VSB receiver 7 above the centre and 8 below giving a tuning range of -2 to +1.75 MHz about the channel centre.

The 8-VSB receiver offset steps are smaller than the AFC range allowing a broader set of frequencies to be received.

Section 3.25

Index Page

Lab Reports Index