Hello everyone,
I’d like to clarify is it safe to connect directly Tx with Rx for the LimeSDR board?
As I can see in the Data Sheet the maximal allowed input level for all Rx channels is 0 dBm and at the same time, the maximal output level is 10 dBm. According to the above, it is not safe to connect Tx and Rx in general. But, in my opinion, it is possible to do it carefully in one of the following ways:
- limit output level via LimeSDR controls up to 0 dBm;
- use an additional 10 dBm attenuator between Tx and Rx.
Could you confirm my understanding, please?
Similar question: Safe input power level .
That is for the LMS8001 which is not the LMS7002M chip used in the LimeSDR-USB and LimeSDR-mini. I’d point you at this thread (Which is about the maximum input power for the LMI7002M chip. My current thinking is that it would be good to stay at least 10dB to 20 dB below the absolute maximum above which permanent damage will eventually occur, due to initial power on spikes that may exceed that limit, but that is just my opinion). There is a link on that thread that mention the additional hardware attenuation added by people using the LimeSDR-USB and LimeSDR-Mini as a VNA where they add 10dB, 15dB, 20dB, 21dB (15dB+6dB) attenuators to avoid damage.
The problem with relying on software gains to limit output power, is that as a board initially powers up the gains could be in any random state before being initialised to the required setting by the software. Most of the time all will be well, because statistically not all gains will be at maximum every time the board is powered up, but eventually the chip will fail because all the gains were randomly at their maximum.
Hello mzs,
That is for the LMS8001 which is not the LMS7002M chip used in the LimeSDR-USB and LimeSDR-mini.
Thank you for pointing this out! You are totally right, I’m interested in LMS7002M chip.
The problem with relying on software gains to limit output power, is that as a board initially powers up the gains could be in any random state before being initialised to the required setting by the software. Most of the time all will be well, because statistically not all gains will be at maximum every time the board is powered up, but eventually the chip will fail because all the gains were randomly at their maximum.
Thank you for the explanation. I understood that it is not safe to rely on software gain.
I’d point you at this thread (Which is about the maximum input power for the LMI7002M chip. My current thinking is that it would be good to stay at least 10dB to 20 dB below the absolute maximum above which permanent damage will eventually occur, due to initial power on spikes that may exceed that limit, but that is just my opinion). There is a link on that thread that mention the additional hardware attenuation added by people using the LimeSDR-USB and LimeSDR-Mini as a VNA where they add 10dB, 15dB, 20dB, 21dB (15dB+6dB) attenuators to avoid damage.
Thank you for the provided links! I’ve looked through them. I understood that the minimal attenuation for safe connection is 10 dBm, but ensure confidence it is better to use about 20 dBm.
Thank you very much!
Normal receiving values (for every receiver, not only Lime) are between -40 dBm and -100 dBm. (And even lower).
So it is not only that you can damage the receiver but what will it be worth if your receiver is overloaded with a signal from -10 dBm.
For normal receiving operations -40 dBm is already a high signal. If you have a receiver with an s meter on it you should see already around S9+50 dB signal on your meter and that is almost full deflection.
