Re: [Discuss-gnuradio] USRP B210 TX gain

[Marcus Müller]

I'd also like to chip in that effective TX power is very dependent on frequency for devices that span several orders of magnitude in frequency -- so you can't just say "the max TX power is x dBm", you need to measure for every frequency, with every bandwidth you use, for the specific signals you transmit; often, application-specific aspects define what TX power is, and it's not always trivial to say what TX power means -- for example, the B210 being good, but not perfect, it of course has emissions
a) inside f_target +-/f_sample/2 (intentional)
b) outside a) but close to it
c) further away
how much bandwidth do you observe when saying "TX power is x dBm"? a) only? That might make sense from a specific receiver's perspective, but not do a pure RX power observation right. a+b+c) ? Impossible to measure. a+b) makes sense if you're really interested in how much power leaves the device (e.g. for legal limits), but really depends a lot on factors like how you tune, what sampling rates you're using etc.

This all contributes to the fact that when determining the signal strength, you must first careful define what you want to describe, and then measure it.

Best regards,
Marcus

On 02.08.2015 21:09, Marcus D. Leech wrote:

On 08/02/2015 03:01 PM, Samith Abeywickrama wrote:

Hi,

I am confused with 89dB tx gain of USRP B210 and what is the meaning of 89dB gain? How this value relates to maximum output of 50mW in B210? Because this value is too much high, as a example: If the RF output power of AD9361 agile transceiver is -29dBm and after we apply 89dB gain, transmit power should be 50dbm, But 50dBm is too much higher than 50mW.

--

Best Regards!

Samith

The maximum output power of the AD9361 is about +17dBm.

Gain control is implemented as attenuators in the gain chain, so that 89dB of gain-control range should be applied to the maximum output power at any
  given frequency, so that a setting of 0dB gain means that, notionally, your output power is  +17dBm - 89dB = -72dBm, and at 89dB "gain", your output
  power would be +17dBm.

In RF circuits, it's very typical to use variable attenuators to effect gain control, so that the individual gain stages operate at fixed bias and matching
  levels, and the inter-stage attenuators collectively effect some range of gain control for the entire chain.

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