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Old 20-Jan-2021, 10:11 PM   #3
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Join Date: Feb 2013
Location: Western NY State
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Additional details

Taken from Justo Buffalo-Toronto plan
by Justo Rodal Santiago, Spain
The AVANTx can accept signals in its input ranging from -40 dBmV to +20 dBmV, that is to say, 60 dB. The block diagram of the AVANTx, which is depicted as follows, is helpful to understand why the balancing cannot be achieved.

The AVANTx has 4 inputs. Each one of the inputs has a first block of analog filtering, VHF high&low and UHF, after which a high margin amplifier prepares the whole set of received signals to the digital filtering block. After these 4 amplifiers, and before the digital filters, there are 4 Analog to Digital Converters (ADCs), one per amplifier, that give off the digital samples of each input to the digital filtering block.

This digital filtering block encompasses 32 filters, which are programmed by the user depending on the channel map in the location. The AVANTx can operate without channel conversion (i.e. same output as input channel) or performing channel conversion, moving the output channel to a different frequency. The digital filters bandwidth depends on the presence/absence of adjacent channels to the wanted channel. If there aren’t channels, this bandwidth is wider.

The problem of this schema is that the AVANTx won’t be able to cope with input imbalances higher than 30 dB in the same input1, because input 1 receives signals from Buffalo and from Toronto. Due to the high level of the signals received from Buffalo, the amplifier attenuates input 1 instead of boosting, not to overwhelm the ADC. That makes that the low level signals received from Toronto are attenuated, which makes the balancing of the whole set more difficult.

Ideally, to solve the problem, we should be receiving the signals coming from NW from two different antennas, separating Buffalo and Toronto. However, this is not possible, as the beam width of reception is very narrow.

The solution to use traps to attenuate individual signals in channels 15, 16, 31 and 32 seems therefore the only option reasonable not to override the input ADC of the AVANTx. However, the frequency response of the traps (picture below) added to the fact that channels 15 and 16 are adjacent to channels 14 and 17 coming with low levels from Toronto affect the level of these latter, as the trap is not strictly speaking narrow band.

Therefore, the use of traps attenuates effectively the higher level channels, yet impairs the adjacent ones. In the scenario we have, the trap in channels 31 and 32 would be useful, impairing channel 30, whilst the use of traps in channel 15 and 16 impairs channel 14 and 17 from Toronto. There is a channel 33 in use, yet received by the second antenna pointing to SW. Therefore it would not be affected by the use of the trap 31-32, provided that this second antenna uses a different input of the AVANTx to process the channels.

(For the following, see the attachment)

Input 1 in AVANTx would process the channels from Buffalo with the highest received levels. If needed, an attenuator could be placed in order not to override AVANTx input, as the level could reach the maximum of +20 dBmV. The channels from Toronto are disregarded in this input.

Input 2 in AVANTx would process the channels from Toronto. It is necessary to attenuate the levels of the channels 15,16, 31 and 32 by using the traps. If possible, the frequency response of the traps could be modified in order to achieve a minimum impair in the adjacent channels. However, channel 30 is extremely difficult to process, as the imbalance regarding nearby channels is too high.

Input 3 in AVANTX would process the channels from Buffalo received by Antenna 2

Attached is the proposed config plan for the 3rd input. Instead of using a 'splitter', I'll use a 16db 'tap' to offset the loss on the output for the weaker Canadian stations.

Attached Images
File Type: jpg Antenna input config reduced.jpg (69.3 KB, 353 views)
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Last edited by videobruce; 20-Jan-2021 at 10:16 PM. Reason: typo corrections
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