Hi Just to be picky:
You get 6 db for SSB to DSB when you are looking at a modulation effect (correlated sidebands) and 3 db when you are looking at random noise (un-correlated sidebands). If both oscillators contribute equally, you get a 3 db factor. The total can be anywhere from 3 db to 9 db depending on the situation. I'm sure glad I'm out of rock throwing range .... Bob On May 8, 2010, at 5:33 PM, Adrian wrote: > Minus the 6 dB L(f) conversion factor, > so we get theoretical L(f) noise floors of -176 dBc/Hz and -191 dBc/Hz. > > Adrian > > Adrian schrieb: >> Thanks to Rick's great hint, I'm now awaiting some AM123 amps to arrive. >> >> Why is it that I just don't get better than -165...-170 dBc/Hz? >> >> Let me try some simple math: >> >> At L and R levels of +7.5 dBm (that's what come out of my 10811's), >> subtract some 7.5 dB mixer conversion loss to see that 0 dBc would be >> exactly 0 dBm at the mixer output. >> (The calibration is performed with the R signal decreased by 40 db. At the >> 40 dB LNA output, I'm measuring pretty exactly 0 dBm.) >> >> At room temperature, the thermal noise is -174 dBm/Hz. >> Add some 4 dB for the LNA noise figure and LPF insertion loss, so the system >> noise floor is at -170 dBm/Hz. >> Give or take a dB, but that's pretty much about it. >> >> Now, with the calibrated carrier level of 0 dBm at the LNA input, the >> residual noise floor is -170 dBm/Hz -(+0 dBm) = -170 dBc/Hz >> >> A modern RF spectrum anylyzer has a noise figure in the 15 dB range, add 10 >> dB of 'safety' attenuation, so the analyzer noise floor is -174 dBm/Hz -(+25 >> dB) = -149 dBm/Hz. Remember that the mixer output / LNA input of -170 dBm/Hz >> is amplified by the LNA by 40 dB, so the analyzer input 'noise signal' from >> the test set is -170 dBm/Hz + 40 dB = -130 dBm/Hz, which is 19 dB above the >> analyzer's noise. >> >> If we increase the L and R levels by 15 dB, we need a +23 dBm mixer, but we >> will add 15 dB to the system dynamic range. >> The mixer conversion loss, LNA noise figure and thermal noise floor haven't >> changed, so the noise floor in dBm is still the same -170 dBm/Hz, but we're >> now refering to a carrier level of +22.5 dBm instead of the above +7.5 dBm >> (+15 dBm versus 0 dBm at the mixer output). >> That's why we can now measure down to -170 dBm/Hz -(+15 dBm) = -185 dBc/Hz. >> >> Adrian >> >> >> John Miles schrieb: >>> That sounds about right to me. I was guessing you meant 40 dB and not 30 dB >>> in the previous message, or there was something else causing about 10 dB of >>> loss. Lots of things to go wrong in this process! >>> >>> -- john, KE5FX >>> >>> >>>> -----Original Message----- >>>> From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com]on >>>> Behalf Of Adrian >>>> Sent: Friday, May 07, 2010 10:38 PM >>>> To: Discussion of precise time and frequency measurement >>>> Subject: Re: [time-nuts] HP 11729C versus 11848A >>>> >>>> >>>> Just the roughly 16 dB of insertion loss caused by the 562 ohm resistor >>>> at the input don't make that filter such a great solution ;-) >>>> So, I brewed something better together... >>>> The 10 kHz beat note is now near 0 dBm when the R input signal is >>>> decreased by 40 dB, which makes a lot more sense than before... >>>> And, I have a noise floor of some -165 to -169 dBc/Hz at +7.5 dBm input, >>>> and I'm seeing the 10811A's some 5 dB above that. >>>> >>>> Adrian >>>> >>>> >>>> John Miles schrieb: >>>> >>>>>> John, >>>>>> >>>>>> I'd say you nailed it. >>>>>> >>>>>> After some more testing, I can confirm that the limiter amp and the LPF >>>>>> are the culprit. >>>>>> I opened the box and plugged directly into the mixer LO port. >>>>>> And, for the LPF, as a quick 'n dirty solution, I connected the <1 MHz >>>>>> front panel output with the LNA input. >>>>>> Now, at 10 dBm each into the mixer ports, I'm getting a noise floor of >>>>>> <-145 dBc/Hz at 100 Hz and about -170 dBc/Hz at 10 kHz and above. >>>>>> >>>>>> >>>>> That's pretty aggressive for 10811s. The floor on those is usually >>>>> around -165 dBc/Hz. How's your calibration process -- are you >>>>> >>>> accounting >>>> >>>>> for the 600 ohm output Z of the <1 MHz output port? It'll lose >>>>> >>>> a few dB if >>>> >>>>> you try to drive 50 ohms with it, and/or the filter response won't be >>>>> correct. >>>>> >>>>> -- john, KE5FX >>>>> >>>>> >>>>> _______________________________________________ >>>>> time-nuts mailing list -- time-nuts@febo.com >>>>> To unsubscribe, go to >>>>> >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> >>>>> and follow the instructions there. >>>>> >>>>> >>>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.