At 02:19 PM 4/19/2007, Hulen Smith wrote: >Hello Flexers, > >Some time back I saw a post from someone that explained they were getting rf >output (very low) when they were keyed up in SSB but with no mic gain or > It's almost >like a balanced modulator in an unbalanced condition. Yea yea yea, I >know.... old school. I'd appreciate any ideas. True that while in the >carrier modes this will have little consiquence, however in SSB, this won't >fly. Oh BTW it isn't oscillation. It's an on frequency signal.
It is precisely the same as the unbalanced modulator problem. The SDR1K, in transmit, is basically a programmable oscillator (the DDS), fed through a vector modulator, and the vector modulator inputs are AC coupled (C20,C5, C27, C28), with no provision for adjusting the DC offset (which is what you'd need to suppress the carrier fully). The signal is, as you've noted, quite low level, but it will be fixed. If you put some modulation on the signal (i.e. feed something into I/Q) you'll have a much larger signal to compare against, so it's easy to get the 40dB or so carrier rejection (compared to the desired signal). Most sound cards are also AC coupled (and, of course, if you're using transformers to eliminate ground loops, this is especially so). The FET mux used as the QSE is pretty good in terms of balance (because it's mostly determined by the on resistance of the FETs, and they're all on the same die, so they're pretty well matched). There might be some small imbalance in the clock signals (the DDS output filters are composed of discrete components, and they match well, but not perfectly), and there's small leakage currents in various places that all add up. This is a fundamental problem with using a SDR1K as a signal generator, by the way. It's got great dynamic range on the desired output (80-90 dB, easy), but it has this "leakage spur" that's always there, and impossible to cancel without a hardware change. If you really need low power AND low DDS leakage, you could run it through some pads. The challenge is if you need both low and high power, since there's no "adjustable attenuator" in the RF path. Jim, W6RMK _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ FlexRadio Knowledge Base: http://kb.flex-radio.com/ FlexRadio Homepage: http://www.flex-radio.com/
