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*Subject*: RE: Silly question or is it?*From*:leif.asbrink@xxxxxxxxxxxxxxxx>*Date*: Tue, 14 Jan 2003 13:35:03 +0100

Hi Roger and All, > Yes, I just did it again. With [1] as the choice for 2nd forward FFT > routine my total delay is: 6.96 seconds and the fft2 delay is 2.73 > sseconds. > > I go into the parameter screen and change it to [2]. I change nothing > else and directly leave the parameter screens by repeatedly hitting > enter and keeping my mouse out of trouble. Now my total delay is 17.22 > sec and the fft2 delay is 10.92 seconds. > > I go back to [1] as the choice and the delay is again 6.97 seconds and > the fft delay is 2.73 seconds. > > Here are my parameters: > > First FFT bandwidth (Hz) [50] > Second FFT bandwidth factor in powers of 2 [6] > Second FFT window (power of sin) [2] OK. This is the explanation: You ask for a bandwidth of 50Hz divided by 64 = 0.78Hz. With a sine squared window the fft must span 2.5 seconds which means that the transform size has to be 246000. Now the sizes come in powers of two so you will get an fft2 size of 262144 which spans 2.730667 seconds. These transforms are interleaved so a new one arrives every 1.365 second. An fft2 delay of 10.92 seconds means that you have 8 completed transforms in the buffer. This is because you have specified delay=8 in the AFC window. There is an additional delay of 2.73 seconds to allow for the collecting of the next complete transform so there is 3.57 seconds for everything else. These settings are for coherent averaging of stable signals. I do not think the combination of narrow bandwidth and long delay time is useful for ordinary CW. Now, when you select fft2 mode 0 or 1, these algorithms are limited to max size 65536. The unscramble tables use 16 bit words to save some time and therefore the table can not go higher than 65536. With the fft2 transforms arriving four times more often the fft2 related delays will go down by a factor of four. so the eight transforms are now 2.73 seconds and the incoming one 0.68 for a total of 3.41. You observe a total of 6.97 so in this case there is 3.56 seconds for everything else. Presumably you have a baseband filter with excessive delay. The parameter select screens for fft1 and for fft2 are followed by test routines that give information about what the parameter selection resulted in. The parameters represent what you ask for, the test screen tells you what you really got. Look carefully at the test screen. There are many situations where what you actually get deviates from what you ask for. If you change to > Second FFT bandwidth factor in powers of 2 [4] you should get identical results with the different fft2 versions. By the way, First FFT amplitude [300] is an indication to me that something is wrong. Too much gain somewhere. As long as the AD converter does not saturate it does not matter but should you ever notice A/D margin=0 when running the amplitude test (A from keyboard) there is an obvious reason;) 73 LeifLINRADDARNIL