Re: [Discuss-gnuradio] 8 bit samples have an odd spectrum

[Johnathan Corgan]

On Thu, Jul 10, 2008 at 10:39 PM, Chris Stankevitz
<address@hidden> wrote:

> 1. Why does b) show a lobe while a) does not?
> 2. Why do b) and c) show lobes at the same pixels (vs shifted .5 MHz)?

You *may* be seeing an artifact of the truncation of the ADC samples.
In 8-bit mode, only the upper 8-bits of the post-CIC samples are sent.
 It's possible that your analog noise floor is at or below the new,
higher LSB, and thus you get sinc-like sampling artifacts in the
frequency domain. Run usrp_fft.py with all the same parameters but in
scope mode with -S and see if the noise is just toggling the new LSB
(+- 256).  If increasing the gain makes an improvement, I'd bet this
is what it is.  In effect, you've increased the quantization noise
floor by something like 48 dB ( 20*log10(256) ), and you're seeing the
quantization noise spectrum.

-- 
Johnathan Corgan
Corgan Enterprises LLC
http://corganenterprises.com/