Re: [Flexradio] RF output with no mic in SSB
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/
Re: [Flexradio] RF output with no mic in SSB
At 10:07 PM 4/19/2007, Ahti Aintila wrote: On 20/04/07, Hulen Smith [EMAIL PROTECTED] wrote: harder with a measure of safety, it is also very sensitive to low rf levels. Is there anyone who has figured out a way to completely eliminate this low level of rf from the SDR-1000 when no modulation is present? It's almost like a balanced modulator in an unbalanced condition. Yea yea yea, I Hulen, I have noticed that. My guess is that the carrier leakage may come due to any internal hardware unbalance of the QSE circuit (sampling pulse width, internal Ron resistance, circuit capacitance, transformer, etc. and their combinations) that cannot be completely cancelled by the software. The lack of DC coupling prevents software from totally compensating it. The power dependence seems to indicate some thermal influence to the sampling switch. I'm going to guess it's more temperature changes in the elliptic filters on the DDS output. Lots of reactive components in a fairly high order filter (there's several hundred degrees of phase shift through the filter) with an elliptic/Cauer configuration. All that means more sensitivity to value changes, particularly towards the top of the band. The FETs in the modulator are all on the same die, and their leakage will track pretty well. The other possibility might be the leakage current through the DC block capacitors on the output of the audio buffers, which could easily change with temperature. 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/
[Flexradio] RF output with no mic in SSB
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 even a mic connected. I tried hooking up an old 2M transverter last weekend and I get near full output when I key the SDR-1000. The same is true if I turn the mic gain off (0) and or unplug the mic. The level is very low, I suspect less than a milliwatt. Don't ask about my drive requirements. I had to modify the 28 mhz tx IF input using a 50 ohm swamping resistor. The original input impedence was 8k ohms. It probably wouldn't have hurt the SDR-1000 to see this impedence (for low drive levels) but I wanted to accurately measure the drive level with a WM-2. So now while I can drive it harder with a measure of safety, it is also very sensitive to low rf levels. Is there anyone who has figured out a way to completely eliminate this low level of rf from the SDR-1000 when no modulation is present? 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. TIA Hulen K5HCS ___ 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/
Re: [Flexradio] RF output with no mic in SSB
On 20/04/07, Hulen Smith [EMAIL PROTECTED] wrote: harder with a measure of safety, it is also very sensitive to low rf levels. Is there anyone who has figured out a way to completely eliminate this low level of rf from the SDR-1000 when no modulation is present? It's almost like a balanced modulator in an unbalanced condition. Yea yea yea, I Hulen, I have noticed that. My guess is that the carrier leakage may come due to any internal hardware unbalance of the QSE circuit (sampling pulse width, internal Ron resistance, circuit capacitance, transformer, etc. and their combinations) that cannot be completely cancelled by the software. The power dependence seems to indicate some thermal influence to the sampling switch. Because I use my SDR-1000s more for receiving rather than transmitting, the minor leakage doesn't disturb me. So I did not want to add any balancing components. Any other opinions, suggestions and/or findings? 73, Ahti OH2RZ ___ 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/ ___ 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/
