[hpsdr] Hermes RX questions

[Joe Martin K5SO]

Hi Byron, 

Yes, when using OpenHPSDR receivers with high power transmitters you do need to use a T/R switch to remove the receiver from its antenna when the local transmitter(s) are operating, or at least you should provide some type of antennuator switching when the local transmitters are transmitting to ensure that the input level to the Hermes receiver is not excessive.  Such T/R switching relays are provided on the Alex Tx filter board and on the ANAN-series PA boards, as examples of OpenHPSDR hardware.  Alternatively, you may certainly use your own external T/R relay but whatever arrangement you use, the T/R relay should be activated by the PTT lines of the local transmitters in order to achieve the necessary protection.  

The ADC overload warning on Hermes (and Mercury and Angelia) comes on when a signal is greater than about -10 dBm at the input of the receiver.  The warning indicates that the input signal has exceeded the amplitude conversion range of the ADC.  Levels greater than that maximum level result in distortion in the output of the ADC, of course.  

Personally, I don’t apply rf levels to my OpenHPSDR receivers greater than the “alert” warning level even though the receiver inputs can likely take ten times that amount or perhaps more before actually destroying devices.  

For reference, typical test equipment inputs  (e.g., on high end spectrum analyzers, etc) have max rf input limits around +7 dBm; that fact might give you an idea of what is generally considered to be a dangerous rf input level.  

73,  Joe K5SO

On Mar 3, 2013, at 6:05 PM, Byron Engler wrote:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> Hi all,
> I am looking at using my Hermes in a contest station. There will be 2 1kw transmitters that can be transmitting at any moment. I plan on using Hermes to monitor which bands are opened and not for the actual QSO's and plan to use some form of random long wire antenna. Should I be concerned about 'burning out' the ADC? 
> Any precautions I should take to protect the Hermes board. 
> What is the maximum safe magnitude of Rf at the RX connector?
> 
> 73
> Byron, ZL3DXT
> 

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