[hpsdr] feedback signal level for pure signal

[Scott Traurig]

Gerhard,

Just slightly contrary to what Bill is suggesting, even with the Rev. 24
PA/filter board I'd suggest erring on the high side with respect to the
signal level at the bypass connector in order to ensure that any internal
crosstalk on the PA/filter board, improved as it may be in Rev. 24, is
effectively suppressed. To that end I'd recommend selecting an attenuator
on the output of your -43dB coupler such that you achieve somewhere between
+7 and +10dBm at the bypass connector. This would require 10 to 13dB of
additional attenuation on the output of your coupler.

The -11dBm ADC overload value and -17dBm Puresignal "green" levels are both
referenced to the OUTPUT of the step attenuator, not at the bypass
connector. The higher you run the step attenuator the better within reason
with regard to crosstalk. Obviously you want to maintain a few dB of
headroom where the step attenuator is concerned. Configuring for +10dBm at
the bypass connector requires the step attenuator to be set to 27dB leaving
4dB of headroom for minor adjustments between bands. If you are
uncomfortable with this and want a little more then shoot for +7dB at the
bypass connector giving a nominal step attenuator setting of 24dB and
headroom of 7dB.

73!

Scott/w-u-2-o


On Tue, May 19, 2015 at 12:25 PM, Bill Diaz <william.diaz at comcast.net>
wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
>
>  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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