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Thread: T5 Multiview (new, thin box) RF output slope issue.

  1. #1

    T5 Multiview (new, thin box) RF output slope issue.

    Hi Guys,

    I've been setting up a pair of Multiview units, aiming to provide 10 analogue channels, together with all 6 digital MUXs.

    I'm seeing slope issues on both (new, thin) T5 units - signal measurements are as follows:

    UHF Ch (MUX): 42 45 51 53 57 60 (Beacon Hill Tx)
    dBV (aerial): 68 70 69 71 69 73 (input signal)
    dBV (V51-205): 74 76 73 76 72 79 (-30dB test port on Vision amp without Multiview)
    dBV (amp + T5): 70 72 64 62 57 59 (-30dB test port with Multiview & amp gain reduced)

    1st line of measured data shows reasonably flat aerial input signals (5dB range).
    2nd line of data shows amplifier has good linearity (7dB range).
    3rd line of data shows Multiview giving downward slope with increasing frequency, which is identical for both units and doesn't change if one unit is powered off (15dB range, with slope of -1.25dB per 8MHz channel).
    N.B. only one RF though loop has been used here (to avoid phase matching problems), with the link swapped onto the second box to check it worked similarly.

    To further compound the problem, there also seems to be a negative slope on the analogue signal outputs:

    UHF Ch: 21 24 28 31 34 38 40 49 55 65
    dBV (amp + T5): 80 76 79 79 78 75 74 72 71 76

    Over the UHF channels that's a range of 9dB, and is the "wrong way" to help with cable losses (which are higher at higher frequency). The slope isn't as steep as the MUX loopthru (-0.3dB per UHF channel), but the net result in the system is that the low frequency channels are fine, and the high frequency channels aren't (after old cable & splitter losses ~15dB worse than low frequency channels). This applies to both the analogue signals being generated and the digital MUXs trying to be maintained.

    The T5 tuner section isn't the problem here - signals measured at the antenna loop out port are just as good for level as the RF in. The problem seems to arise between the RF loop input and RF output connectors. A slope going the opposite way would actually help here, but a drop of 15dB on the digital MUXs passed though the box isn't ideal.

    If I avoid using the RF loop thru connection on either box and route some aerial signal directly to one of the other UHF inputs on the amp, I can recover digial MUX levels that are sufficiently flat. This doesn't, however, match the product spec and still leaves me with a slope issue to address for the analogue signals.

    I'd appreciate your comments.

    Jim.

  2. #2

    Re: T5 Multiview (new, thin box) RF output slope issue.

    Hi Jim

    I've just had the factory check a new production unit for the Analogue output & the Loop Feedthrough gain.

    ANALOGUE RF OUTPUTS
    Measured 90~94dBuV across channels 21, 24, 28, 31, 34, 38, 40 ,49, 55, 65 which is in line with the specification.

    RF LOOP FEEDTHROUGH GAIN
    This measured +10dB at the bottom of UHF to +3dB at the top of UHF which again is within specification.
    The loop through gain does tail off towards the top of UHF, this can be seen on the plot on the back of the datasheet for the T-35.

    Looking at your analogue channel allocation I noticed that you used 40 (31+9) & 49 (40+9).
    These may not be ideal when adhering to the +/- 1, +3, +9 rule of thumb for allocating analogue channels.

    The most interesting thing though is the output level using the amp. as this is actually below the T-35 output level.
    You may well find that there is a matching problem or the amplifier is being over driven from the T-35 and this is causing the problem.

    In such a complex installation though we would always advise to use an external combiner to combine the antenna into the system along with the T-35 and not to use the loop on the unit as this will provide better isolation.

    So you would have the amplifier feeding all the TV's including the T-35.
    The antenna connected into the amplifier using a combiner along with the output from the T-35.
    By using a 3 way combiner you can then add in another T-35 to the installation.

    What you would have to do with this arrangement though is attenuate the output from the T-35 by ~20dB to level this up with the other signals from the antenna.

    Another option would be to use channelised amplifiers which would allow you to add more channels onto the network too.

  3. #3

    Re: T5 Multiview (new, thin box) RF output slope issue.

    Quote Originally Posted by CMS_ADMIN View Post
    Hi Jim

    I've just had the factory check a new production unit for the Analogue output & the Loop Feedthrough gain.

    ANALOGUE RF OUTPUTS
    Measured 90~94dBuV across channels 21, 24, 28, 31, 34, 38, 40 ,49, 55, 65 which is in line with the specification.

    RF LOOP FEEDTHROUGH GAIN
    This measured +10dB at the bottom of UHF to +3dB at the top of UHF which again is within specification.
    The loop through gain does tail off towards the top of UHF, this can be seen on the plot on the back of the datasheet for the T-35.

    Looking at your analogue channel allocation I noticed that you used 40 (31+9) & 49 (40+9).
    These may not be ideal when adhering to the +/- 1, +3, +9 rule of thumb for allocating analogue channels.

    The most interesting thing though is the output level using the amp. as this is actually below the T-35 output level.
    You may well find that there is a matching problem or the amplifier is being over driven from the T-35 and this is causing the problem.

    In such a complex installation though we would always advise to use an external combiner to combine the antenna into the system along with the T-35 and not to use the loop on the unit as this will provide better isolation.

    So you would have the amplifier feeding all the TV's including the T-35.
    The antenna connected into the amplifier using a combiner along with the output from the T-35.
    By using a 3 way combiner you can then add in another T-35 to the installation.

    What you would have to do with this arrangement though is attenuate the output from the T-35 by ~20dB to level this up with the other signals from the antenna.

    Another option would be to use channelised amplifiers which would allow you to add more channels onto the network too.
    Hi Guys,

    Thanks for the detailed reply.

    I've now changed the setup to use a 2-way tap on the aerial feed to give a healthy input signal directly to one amplifier input, and the two tapped feeds for the T5 units (with no RF loopthru connected on either box).

    Ampified outputs for the digital MUXs now follow the input levels nicely, much better than using the loopthru.

    I don't follow how your loopthru gain measurements of +3dB to +10dB follow the spec. (min 3dB, max 7.5dB gain)? Not an issue anymore though.

    The revised configuration now has the two outputs from the T5 units connected to a 2-way combiner and then connected to a second amplifier input via a 6dB attenuator (amp has separate gain controls on each input).
    Output levels now measure as follows:

    UHF Channel: 21 24 28 31 34 38 40 49 55 65
    -30dB test : 83 83 84 84 80 80 77 75 77 82

    Output levels to the system are 30dB up on these numbers since I'm measuring on the test port - I don't think I have a matching problem here.

    The output level has been set to provide analogue pictures just before the onset of cross-modulation effects becoming visible (i.e. gain of amp at max, but still linear). Max launch level of 114dBV is consistent with specified 121dBV with appropriate de-rating for all the carriers.

    The output levels show a similar trend - max power at low frequencies, there seems to be a "dip" in the UHF channel 50 to 55 range, and then the power recovers but not quite as high as the low frequency levels.
    The output powers still show a range of 9dB (same as yesterday). Do your 90~94dBV measurements show a similar sort of trend?
    It looks a bit weird that the dip coincides approximately with the digial MUX inputs.

    The output UHF channels aren't my original choice (which did attempt to avoid 5 & 9 separations), but various other signals were causing interference, so the channel plan was tweaked to try and find quieter spots. I agree that on paper it doesn't look perfect - it may benefit from some further playing.

    Jim.

  4. #4

    Re: T5 Multiview (new, thin box) RF output slope issue.

    Quote Originally Posted by jim_fraser View Post
    I don't follow how your loopthru gain measurements of +3dB to +10dB follow the spec. (min 3dB, max 7.5dB gain)? Not an issue anymore though.
    That was my mistake, I noted the wrong measurement for gain, should have been 7.

    Quote Originally Posted by jim_fraser View Post
    The output powers still show a range of 9dB (same as yesterday). Do your 90~94dBV measurements show a similar sort of trend?
    It looks a bit weird that the dip coincides approximately with the digial MUX inputs.
    Seems strange, the input/output is completely isolated so the input should not have any effect on the output.
    On the measurement we did we seen a 4dB variation on the measured channels.

    I take it with the current configuration the installation is performing to an acceptable level?

  5. #5

    Re: T5 Multiview (new, thin box) RF output slope issue.

    Quote Originally Posted by CMS_ADMIN View Post
    That was my mistake, I noted the wrong measurement for gain, should have been 7.



    Seems strange, the input/output is completely isolated so the input should not have any effect on the output.
    On the measurement we did we seen a 4dB variation on the measured channels.

    I take it with the current configuration the installation is performing to an acceptable level?

    Hi Guys,

    Yes, current configuration is working as well as the existing coax cabling will allow (most TVs have good signal, but a few are on the end of dodgy cables with reduced picture quality. These will need the duff cables replacing - looks like water ingress).
    Headend configuration is really neat with the 2-way tap and dual input amp; setup is very straightforward, and results (on good cable) are excellent.

    Jim.

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