I find virtually the same results here using 50uV RF from an Elecraft signal source, and measuring using Wave Spectra. An offset of 43 to 45dB, and below LIN OUT = 5 the harmonics drop rapidly.
73 Stewart G3RXQ On Tue, 02 Sep 2008 12:28:02 -0400, Jack Smith wrote: > Brendan: > > You might wish to look at the measurements I made, with LIN OUT settings > from 2 to 100. There's an animated GIF that steps through the LIN OUT > settings so you can see the effect. The data was taken at 600 Hz. > http://www.cliftonlaboratories.com/elecraft_k3_receive_audio.htm about > half way down the page > > Like you, I see a relatively constant 45 dB or so offset between the > fundamental and 3rd harmonic for LIN OUT values of 10 and over. > > However, setting the LIN OUT at 2 results in quite a bit better odd > order harmonic suppression, on the order of nearly 70 dB. By the time > LIN OUT is set to 5, however, the odd order suppression is reduced to a > bit more than 60 dB. At LIN OUT = 10, it's 45 dB or so where it stays at > all values of LIN OUT tested between 10 and 100. > > (For some reason, the 2nd harmonic pops up at certain settings, but not > others higher or lower, but that's not of immediate concern.) > > There's a related issue with the AGC. The audio output is proportional > to RF signal level, as the AGC has a slope to it. Hence, even though the > LIN OUT might be set to, say, 2, the actual audio level and hence the > distortion, will vary as the RF signal level changes. The degree of > audio level variation with RF input changes depends on how the user has > set the K3's AGC slope and intercept parameters. > > I'll make the measurements on a couple of TTC-108 parts when they > arrive as a matter of trying to understand where the harmonic distortion > arises. To duplicate the K3's audio chain, I'll drive the TTC-108s from > an op-amp so that it will be hit with a low Z source, instead of the > standard audio generators available to me, with 50 or 600 ohm output Z. > > Jack > > > Brendan Minish wrote: >> Jack >> >> I have seen postings on the main reflector recommending settings as low >> as 3 to avoid distortion products that are being attributed to >> transformer distortion. This did not really make sense to me, especially >> at such low levels where consumer sound cards tend to get non-linear and >> nasty. >> >> I took a look at the specifications for the TTC-108 audio transformer >> used in the line out of the K3 >> >> Nominally it's specified as having an operating level of -45dBm to >> +7dBm >> THD is specified at 0.5% @0dBm 300Hz to 3.5 Khz @ 0dBm >> this would make the transformer related harmonics worst case at -46dB >> relative to the fundamental >> >> I did some measurements with an s9 CW signal zero beat at various pitch >> settings >> I used an old copy of SMAART pro that I have from my Sound engineering >> days. the sound card is a delta44 >> >> Lin = 100 Pitch 300 pitch 550 pitch 800 >> DR3 47.3dB 51.4dB 54dB >> >> Lin = 50 Pitch 300 Pitch 550 Pitch 800 >> DR3 43.1dB 47.6dB 50.6dB >> >> Lin = 10 Pitch 300 Pitch 550 Pitch 800 >> DR3 35dB 39.7dB 43.1dB >> >> Lin = 5 Pitch 300 Pitch 550 Pitch 800 >> DR3 31.9dB 37.7dB 42.7dB >> >> Lin = 3 Pitch 300 Pitch 550 Pitch 800 >> DR3 31dB 40.3db 47.6dB >> >> DR3 is the Dynamic range between the fundamental and the 3rd harmonic >> (the most dominant harmonic by a wide margin) >> >> I did a quick look at 5th and 7th orders too, again the best performance >> here was found with LIN = 100 >> >> LIN = 100 is an output level of approximately 700mv for an S9 CW sig on >> my K3 >> >> The Harmonic distortion does not increase with higher line out levels, >> it actually improves. This does not appear to be a transformer >> saturation issue >> >> With lin = 100 users will see a ghost on the waterfall and may risk >> over-driving some consumer sound cards. lowering LIN may get rid of the >> visible ghost by reducing the 3rd Harmonic to below the waterfall >> threshold (& perhaps the sound card input noise floor ) but also reduces >> the overall usable dynamic range >> >> >> 73 >> Brendan EI6IZ >> >> > _______________________________________________ > Elecraft mailing list > Post to: Elecraft@mailman.qth.net > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
