On Fri, Jun 10, 2011 at 10:36 PM, George R Allen <[email protected]>wrote:
> I have calibrated my Flex to WWV as per instructions in the Knowledge Base. > On the calibration screen, there is an IF setting of 9Khz. > > > > I can change the IF; but, why would I want to do this? > Tim has given you a good answer but it is missing a critical piece of information that is useful if you are interested in measuring frequency. In order to create a 9kHz IF offset (or any other IF offset for that matter) PowerSDR needs to synthesize another local oscillator. This numerically-controlled oscillator (NCO) is implemented in software. The problem is, this NCO, like the DDS LO, is programmed in binary and does not fall precisely on decimal unit Hz boundaries. When you set a VFO PowerSDR tries to find the nearest value for both the NCO and the DDS LO that will produce the desired frequency on the "VFO" but there is always an error that is the combination of the two errors for the DDS LO and the NCO. If you want to do frequency measurements using the DDS LO locked to a high-accuracy reference, e.g. something like a rubidium atomic clock, you want to set the NCO value to zero so it introduces no extra error, leaving you only to correct for error in the DDS LO. That implies a zero-Hz IF. Now Tim is right. There is a "dead spot" in the receiver centered on the DDS LO injection frequency and extending just slightly to either side. Any signal in this "dead spot" is notched out. If you are using a 9kHz IF, that "dead spot" is outside the receiver's passband and will have no impact on any mode. If you set the IF to 0 Hz then that "dead spot" is in the center of the passband for any zero-center receiving mode, e.g. AM and FM. In the case of AM, it will wipe out the carrier and cause terrible distortion. My solution is to receive AM using either the DSB or SSB exalted-carrier technique, i.e. use DSB or SSB for receive and then zero-beat the signal. I find that I get better performance from this than either the AM or synchronous AM reception techniques. In fact, I find that suppressing the carrier on the AM signal by the "dead zone" makes it easier for the AGC to track the level in the sidebands when receiving DSB. No, it is not supposed to be used this way but I find that the performance is outstanding provided you understand what you are doing and what is happening. FWIW, I almost always run with a 0 Hz IF now. I have not experimented with using a 0-Hz IF on FM. My guess is that it won't make a difference on receive since it is unlikely that the signal you are receiving will be so precisely on-frequency such that the carrier falls in the "dead spot". Now I don't know about TX because the signals are all generated using the same master oscillator so it is possible that, on TX, the carrier might be notched. OTOH, it may be that they always use an IF offset on TX in which case this point is moot. -- Brian Lloyd, WB6RQN/J79BPL 3191 Western Dr. Cameron Park, CA 95682 [email protected] +1.767.617.1365 (Dominica) +1.931.492.6776 (USA) (+1.931.4.WB6RQN) _______________________________________________ Flexedge mailing list [email protected] http://mail.flex-radio.biz/mailman/listinfo/flexedge_flex-radio.biz This is the FlexRadio Systems e-mail Reflector called FlexEdge. It is used for posting topics related to SDR software development and experimentalist who are using beta versions of the software.
