Re: [Discuss-gnuradio] frequency synthesis

[Josh Blum]

> I got two peaks at
> 5.4995GHz and 5.5005GHz
> when the tx_rate is 1M samples per second.
> http://www.cse.msu.edu/~huangpe3/usrp/1Mtx_05Msine.png
> I thought it is right, sampling frequency is twice the sine frequency.
> 

Its under-sampled, thats why it looks so bad.

> However, when the tx_rate is 2MSps, I got one peak at 5.5005GHz.
> http://www.cse.msu.edu/~huangpe3/usrp/2Mtx_05Msine.png

Now that that looks better.

> Then I am confused. Where does the lower frequency part go?
> Or the first one is actually an aliasing because we need some degree of 
> oversampling in practice.
> 
> Also I'd like to know where do we remove the lower frequency part (freq(A-B)) 
> in uhd.
> 

tx waveforms is generating cos(t) + i*sin(t) *not* cos(t). I suggest
reading about baseband representation and experimenting in simulation
with GRC.

-josh