Someone take care about power regulation ?

  • To word it in another way: reading the ITU-presentation of Es'hailSat concerning the user requirements Melco was assigned the task to build transponders complying with these requirements.

    Thus, if 100 channels of 2.5 kHz are occupied with 100 users, and each user generates 20 dB SNR (inside 2.5 kHz) with 5 - 10W RF and -say- 20 -23 dB dish gain (50 - 75cm) , it's enough (?)

  • Ok i think Leila will indicate overpowered HAMs what's wrong.

    Makes me even more hoping that the NBTP is soon switched on and we are allowed to transmit over it. Then we will see. If the sat does receive in a similar strenght like the NBTP noisefloor was received down here then a few W on a middle sized dish should be enough. Amsat UK wrote something about 5-10W on 75cms dish.

    BTW: accidentally i found your real callsign some days ago, goog'lin for 13cm SSPAs ;)

    vy73 DB8TF

  • Output Back-Off. In my own words: the full capabilities of the TWTA will not be used but (in this case) output is 6 dB down. This is to improve linearity and improve NPR (Noise Power Ratio), so that in the passband there will be less IM (intermodulation) due to other (different) modes and users (very briefly said).

    So, O/P of the HPA (TWTA) is 100W (50 dBm) - 6 dB (OBO) = 44 dBm - 1.5 dB losses = 42.5 dBm + 17 dBi (EC horn) = 59.5 dBm (= 29.5 dBW).

    Divide this into 50 channels (however ... 50 x 2.5 kHz = 125 kHz) <-- ?

    But say ... 50 x 5 kHz channels you've 29.5 - 10log(50) = 12.5 dBW per channel.

    The rest follows from the sheet.

  • Allright, its similar like AM mod silent carrier. If you want a clean mod you need to swing up and down in pwr, but if you already have the power close to or at the max transmitter power you can't go further up and never get a good mod.

  • I believe like that, but now imagine you've 100 signals (in 100 'channels') and each signal/channel wants its own protection/quality/comfort.

    That means considerable efforts on the linearity of the transponder/amplifier/contraption to avoid 'in band' IM (so that not another user suffers from your IM). It's not only with satellites but also with mobile telephony, so the 'handy' you are using.

  • Well ... this power regulation has everything to do with the capabilities and QoS (Quality of Service) of the transponder per user requirements. So .. I reckon it's not (too) off topic ; -)

    And yes, from what I've seen concerning transponder noise and (illicit) signals, I fully agree, Melco did a good job!

  • Well the point is that we should not approach the problem from the high side, make far too much EIRP and then devise a solution to attenuate the signal even more than the usual transmitter control can do, but rather start from lower power and see if we can make it and if not maybe add some more power capability.

    When done this way I frankly do not see the need for much more than 10dB of power variation and it should be possible to do it using a power output control and/or some variation of the audio level (this of course should not be used to cover a too wide range as it would cause poor carrier suppression and maybe noise).

    When you have a 50W output transceiver and want to drive a 13dBm input level converter, OF COURSE you do not connect the two directly together and then reduce the level using the power control knob. That would be insane.

  • Nice, i search for Payload Engineers ;)

    73 de Kai

    • Official Post

    I'm optimistic MELCO has done a very good job on this and the TP will work very well.

    Yes... rest assured..

    Achim DH2VA and me have been in Japan at MELCO for the original specifications and critical design reviews... we have some folders full of paper-- but NDA NDA..

    For the time being we can only stick to the documents published by Es'hailSat itself and wait for the great opening ceremony..

    We are still waiting for customs clearing of our groundstation equipment in Qatar.. lot of paperwork too..

  • Ah, yes, non-trivial RF test engineering, one of my favourites :)

    Regarding NPR (Noise Power Ratio) linearity measurement:

    This was the toughest possible test back in the days of carrier transmission in telephone networks (ITU-T G.228, "Measurement of Circuit Noise in Cable Systems using a Uniform-Spectrum Random Noise Loading".

    For HF radio, I usually use a modified Wandel Goltermann RS-5 or a modified Hewlett Packard 3708A with WG crystal or homebrew, purpose build notch filters.

    HF setup for testing SDR-IQ:

    Example of a the notched spectrum (linear) above (10 kHz/div, 10dB/div):

    Full HF span:

    Insufficient measurement points in the SPA plot to correctly indicate depth of the narrow notch - once again, an analogue spectrum analyzer with a realy CRT would be the winner...

    Obviously it is possible to upconvert this signal to VHF/UHF, even SHF, provided your upconverter is vastly more linear than the DUT ;)

    For SHF I usually build a wideband noise generator/amplifier/filter combo to achieve the necessary spectrum for loading/linearity testing. The rest of the measurement is straight forward RF stuff with couplers, power splitters, power meters and spectrum analyzer.

    As an example, here is a linearity NPR test setup for a QRO 13cm SSPA:

    NPR is just the ultimate linearity measurement method ;)

    But for most purposes and intent, the traditional two-tone test or just plain -1 dB compression measurement is sufficient for setting bias etc. Some are happy with just guessing the bias and drive, but personally, I prefer facts ;)

    /Michael, OH2AUE


    "If you have data, you have something, if you do not, you have nothing." (Bengt Hultqvist, SK 24.02.2019)

    Edited once, last by oh2aue ().

  • Hi !

    Power control built into Up-converter.

    I have a step attenuator in the RF chain in my up-converter.

    The mixer is followed by som BPF:s and amplification an then fed into a HMC624, a 6 bit attenuator with 0.5 dB steps so max attenuation is 31.5 dB. Using 5 bits so I have 1dB steps. Up and down stepping is controlled by up/dwn counters (2 x 74HC193) feeding the HMC624 parallell input.

    The upconverter (and down converter) will be mounted at dish focus behind the feed, so I have remote control of the attenuator by pulses to the up/down counter inputs.

    Attenuator/counter setting is read by a DAC and feeding a DC-voltage back to the shack, 30V = 30dB attenuation.

    Thus I can control my output power from 9 dBm up to 37 dBm without the need to change anything like at the IF rig like MIC gain/RF power. It will always be running the same settings. IF (TX) rig is IC7600 running at 28MHz + xverter to 144 MHz fed via 25 m cable to the up-converter at the dish.

    At the power amplifier output I have a coupler feeding a log detector AD8313 with some signal conditioning so I get 5 V for 40 dBm, 4V = > 30dBm and so on, this will show the actual RF power feed to the antenna port.

    73 - Hannes - SM6PGP