# Carrier frequency mismatching?

**URL:** <https://www.ruby-forum.com/t/carrier-frequency-mismatching/199354>\
**Category:** GNU Radio\
**Created:** [November 19, 2010, 9:19am UTC](https://www.ruby-forum.com/t/carrier-frequency-mismatching/199354 "2010-11-19T09:19:19Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Dobai-Pataky\_BSSSSl](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/77aa72/32.png) [@Dobai-Pataky\_BSSSSl](https://www.ruby-forum.com/u/Dobai-Pataky_BSSSSl)\
**Post date:** [November 19, 2010, 9:19am UTC](https://www.ruby-forum.com/t/carrier-frequency-mismatching/199354/1 "2010-11-19T09:19:19Z")

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# I use USRP sink and source and set frequency with 2.6 GHz When I run a flow graph, I see like below:

A: Flex 2400 Tx MIMO B  
r.baseband\_frequency = 2604000000.0  
r.dxc\_frequency = -4000000.0  
r.residual\_frequency = 0.0  
r.inverted = False

# A: Flex 2400 Rx MIMO B r.baseband\_frequency = 2596000000.0 r.dxc\_frequency = -4000000.0 r.residual\_frequency = 0.0 r.inverted = False

Both of baseband frequency are near 2.6 G, but they do not match up  
Can it be problmeatic for communicating each other?

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**Author:** ![Songsong\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/49beb7/32.png) [@Songsong\_G](https://www.ruby-forum.com/u/Songsong_G)\
**Post date:** [November 19, 2010, 10:02am UTC](https://www.ruby-forum.com/t/carrier-frequency-mismatching/199354/2 "2010-11-19T10:02:02Z")

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On Fri, Nov 19, 2010 at 05:17:53PM +0900, Songsong G. wrote:

> ```
> r.baseband_frequency = 2596000000.0
> r.dxc_frequency = -4000000.0
> r.residual_frequency = 0.0
> r.inverted = False
> 
> ```
> 
> ===============================================  
> Both of baseband frequency are near 2.6 G, but they do not match up  
> Can it be problmeatic for communicating each other?

The actual frequencies on the air will be 2.6 G +/- oscillator  
tolerance.

In the Tx case, the LO is set 4MHz higher than the target, and the  
DUC is used to adjust the baseband -4MHz.

In the Rx case the LO is set 4MHz lower than the target, and the DDC  
is used to move the digitized IF signal 4MHz lower prior to decimating.

This is easy to confirm with a siggen and a spectrum analyzer.

Eric

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**Author:** ![Songsong\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/49beb7/32.png) [@Songsong\_G](https://www.ruby-forum.com/u/Songsong_G)\
**Post date:** [November 19, 2010, 1:20pm UTC](https://www.ruby-forum.com/t/carrier-frequency-mismatching/199354/3 "2010-11-19T13:20:25Z")

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I understan what you told me…  
Then… however, why they have same sign?  
One is for up conversion, and the other is for down conversion  
they might have opposite signs.  
one is MINUS 4 MHz, and the other is PLUS 4 MHz

and one another…  
They meet in frequency by DXC,  
but, in my opinion, they are basically different in baseband  
frequency…  
I really really cannot understand this magic ☹

2010/11/19 Eric B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)

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**Author:** ![Songsong\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/49beb7/32.png) [@Songsong\_G](https://www.ruby-forum.com/u/Songsong_G)\
**Post date:** [November 19, 2010, 5:49pm UTC](https://www.ruby-forum.com/t/carrier-frequency-mismatching/199354/4 "2010-11-19T17:49:17Z")

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On Fri, Nov 19, 2010 at 09:19:04PM +0900, Songsong G. wrote:

> I understan what you told me…  
> Then… however, why they have same sign?  
> One is for up conversion, and the other is for down conversion  
> they might have opposite signs.  
> one is MINUS 4 MHz, and the other is PLUS 4 MHz
> 
> and one another…  
> They meet in frequency by DXC,  
> but, in my opinion, they are basically different in baseband frequency…  
> I really really cannot understand this magic ☹

Try drawing a picture…

Eric