# USRP B210 TX gain

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.

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

Best Regards!
Samith

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
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.

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