[Top][All Lists]

## Re: [Discuss-gnuradio] atsc_cpll finally works

 From: Achilleas Anastasopoulos Subject: Re: [Discuss-gnuradio] atsc_cpll finally works Date: Thu, 22 May 2008 22:53:12 -0400 User-agent: Mozilla Thunderbird 1.0.2 (Windows/20050317)

```Chuck,

```
I have a question regarding the comment you made in an earlier email on the subject:
```
You said:
-------------
Now my question: Is it possible to tune the usrp so the carrier is at
+.31 Mhz ? (band center at 3, from -.2 to 6.2Mhz?)  Then we could run
the cpll at 6.4 or 8Msps and get another big performance boost, maybe.
Right now the pll at 19.2 Mhz (6.4 * 3) is an expensive part. Eric, I
guess the bit timing loop would work at 24Mhz (8 * 3).
-------------

I don't see where the 19.2 MHz figure comes about...

```
My understanding is that when the USRP is tuned to the center of the 6MHz band, the signal that goes through the USB is
```u(t) = z(t) exp(-j fsym/4 +j phi)
where fsym = 1/Tsym = 10.76 MHz (so fsym/4=2.69MHz), and
z(t)= x(t)+j x(t) * g(t)
representing the complex lowpass equivalent of the VSB signal.
```
z(t) is a complex signal with non-zero frequencies in [-3+2.69,3+2.69]=[-0.31,5.69]MHz
```
```
So the signal u(t) needs to be sampled at 6 Complex-Msps, which is what you do (sampling it at 6.4 Complex-Msps, or as Eric wants at 8 complex-Msps).
```
Now my understanding is that you need to upsample u(t) by a factor of 2
(in software, ie, making it a 12.8 Complex-Msps signal) and then
run on it the complex pll where the nominal VCO frequency is
fsym/4 = 2.69MHz.
The "out" signal of the cPLL will be the real signal x(t) sampled at
```
12.8 Msps which is fine since this is the original 8-PAM signal after Nyquist sampling.
```
So, what is the 19.2 MHz frequency you are referring to?
The way I see it the cPLL works at 12.8 complex-Msps.
Am I missing something?

Thanks
Achilleas

```