|
From: | Michael Dickens |
Subject: | Re: [Discuss-gnuradio] Freq_xlating filter parameter |
Date: | Thu, 20 Sep 2007 10:08:23 -0400 |
freq_xlating_center_freq_argument = current_center_freq - desired_center_freq
where "current_center_frequency" is defined by the returned structure, r = usrp.tune (..., target_frequency, ...):
current_center_frequency = r.baseband_freq + r.dxc_freq + r.residual_freq
IME, I've never seen "r.residual_freq" be significant compared to the target frequency to which to tune, so I've always used current_center_frequency = target_frequency.
Example: Suppose you want 2 RX channels centered at 440 MHz and 443 MHz, and that the USRP RX center frequency is tuned to 441 MHz. Then the "center_freq" argument for the 440 MHz channel would be 1M and for the other would be -2M.
As you've stated: These results seem to be the negative of the description provided by "gr_freq_xlating_fir_filter_XXX" ... or, more to the point: IMHO the description needs updating to be more accurate, possibly with the example above?
Hope this helps! - MLD On Sep 6, 2007, at 7:26 PM, Dan Halperin wrote:
The comment for the filters gr.freq_xlating_fir_filter_XXX says (about the parameter "center_freq"): /*!* Construct a FIR filter with the given taps and a composite frequency* translation that shifts center_freq down to zero Hz. The frequency * translation logically comes before the filtering operation. */ This implies that the frequency shift should be -center_freq, right? I think I'm observing the opposite; that is, you pass in the desired shift. All examples I can find that use this block pass in 0 as the frequency shift; does anyone else have experience with this block that can confirm or deny this?
[Prev in Thread] | Current Thread | [Next in Thread] |