Hi Guido and all, > Wonder if thresholding a sine wave is increasing the phase noise. When > thresholded the period jitter is soley based on a single instance of the > amplitude transition, while phase errors during for the full sine period > could average out period jitter. Or is this not the case? Phase noise is typically a problem close to the carrier but of no importance at large frequency separations.
Whenever you transform a sinewave into a squarewave you will add wideband phase noise, but under normal circumstances that is of no importance because narrowband phase noise close to the carrier is so much stronger. The narrowband phase noise corresponds to a systematic time jitter. The sine wave is phase shifted for quite some time. The original thread, why not use very low frequencies, has the conventional answer that we have to avoid the 1/f noise and the interference from 50/60 Hz hum. A good design can make these problems insignificant, but another problem is that soundcards are not DC coupled. A card like Delta 44 (which still is my favourite) does not only use a capacitor to block DC at the input. I could easily eliminate that, but the chip used in the card also has a "DC block in software," a digital high pass filter that ensures that the average of a very large number of samples always is zero. That is a high pass filter that prevents a DC offset on the input to reach the SDR software and therefore causes an infinitely deep notch at the center. With a good design the noise near the passband center can be made negligible. See for example http://www.sm5bsz.com/digdynam/practical.htm Figures 5 to 10 show the center 1/f noise of the Delta 44 becoming less significant as the input level is increased and the noise from the antenna gradually hides the "center spur" Figures 11 to 13 show that the 1/f noise is totally negligible when the system gain is adequate. There is a lot to say about the proper design of local oscillators (and soundcard modifications) to avoid the low frequency noise in direct conversion receivers. Avoiding AM sideband noise is particularly important when schottky diode mixers are used to produce I and Q. 73 Leif / SM5BSZ
