[hpsdr] feedback signal level for pure signal

[Bill Diaz]

Gerhard,
    See below:
On 5/19/2015 08:32, Gerhard Burian wrote:
>
> Hello,
>
> I received my new ANAN100D last week and made first tests with pure 
> signal, the main reason why I changed from my FLEX5000A to the ANAN100D.
>
> Pure signal without external Linear Amplifier works ok, using the 
>  internal crosstalk.
>
> Using an external amplifier and feedback coupler shows following problems:
>
> My setup:
>
> PA with 2 kW (63 dBm).
>
> Feedback coupler: -43 dB (as one with 63 dB didn’t work, always in red 
> zone at S-ATT 0 dB).
>
> Feedback signal on EXT1 and “ETX1 on TX” checked, S-ATT set to 31 dB, 
> autocalibrate on.
>
         The newer PA/Filter boards (REV24) require the sampler/coupler 
to be attached to the rear panel "BYPASS" connector.  Since you recently 
received your ANAN-100D, it is likely you have a REV24 board.

         The EXT1, EXT2, and XVRT RX ports were disabled for both RX and 
TX on the REV24 boards until Doug added the "Disable Bypass" option to 
PowerSDR to enable these ports during receive only.  This option was 
enhanced in PowerSDR 3.2.27.   Note that connecting your sampler to EXT1 
would not likely work as intended with the port disabled during transmit 
on REV24 boards.

     SNIP
>
> Documents on Wiki say I must not have more than +13 dBm feedback 
> signal on the RX preamp (with S-ATT at a high level).
>
> If I calculate 63 dBm minus 43 dB minus 5 dB, I get +15 dBm on the RX 
> preamp.
>
> Otherwise Wiki is requesting -17 dBm after the step attenuator.
>
> Is this correct? Why does version 27 react this way?
>
        Since ADC overload occurs at -11dbm, a signal level of +13dbm 
would certainly be excessive.  A "green" feedback level requires a 
signal level of about -17dbm.  Therefore, applying +13dbm to the sample 
port would require ATT on TX to attenuate a signal applied to your 
Bypass port by 30db (+13dbm + -17dbm).  Since the ATT on TX maximum 
attenuation is 31 db, this would not allow much leeway.

        With your +63dbm signal, the AC2IQ coupler (-43db coupling loss) 
and an additional 20-30db of attenuation would lower the sample applied 
to the Bypass port to about -10 to 0dbm.  The Linearity form "Auto 
Attenuate" option would then automatically adjust ATT on TX to obtain 
the desired green feedback level (-17dbm) for a sample provided to your 
bypass port.

HTH
Bill KC9XG
>
>
> 73’s
>
> Gerhard OE3GBB
>
>

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