# USRP B210 TX gain

**URL:** <https://www.ruby-forum.com/t/usrp-b210-tx-gain/242850>\
**Category:** GNU Radio\
**Created:** [August 2, 2015, 9:02pm UTC](https://www.ruby-forum.com/t/usrp-b210-tx-gain/242850 "2015-08-02T21:02:31Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![Sadaf\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/858c86/32.png) [@Sadaf\_N](https://www.ruby-forum.com/u/Sadaf_N)\
**Post date:** [August 2, 2015, 9:02pm UTC](https://www.ruby-forum.com/t/usrp-b210-tx-gain/242850/1 "2015-08-02T21:02:31Z")

</div>

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.

---

<div class="post-metadata">

**Author:** ![Samith\_Abeywickrama](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ad7895/32.png) [@Samith\_Abeywickrama](https://www.ruby-forum.com/u/Samith_Abeywickrama)\
**Post date:** [August 2, 2015, 9:10pm UTC](https://www.ruby-forum.com/t/usrp-b210-tx-gain/242850/2 "2015-08-02T21:10:42Z")

</div>

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

> 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.

---

<div class="post-metadata">

**Author:** ![Samith\_Abeywickrama](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ad7895/32.png) [@Samith\_Abeywickrama](https://www.ruby-forum.com/u/Samith_Abeywickrama)\
**Post date:** [August 3, 2015, 9:23am UTC](https://www.ruby-forum.com/t/usrp-b210-tx-gain/242850/3 "2015-08-03T09:23:54Z")

</div>

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