Roger, Thank you so much for your thoughtful analysis. You may be interested to know that the SCU RF gain is adjustable in 0.5 dB steps from -11.5 to +40 dB. This will allow for precise optimization of system gain in conjunction with a mast mounted preamp.
73, Gerald Gerald Youngblood, K5SDR President and CEO FlexRadio Systems(TM) Email: [email protected] Web: www.flexradio.com <http://www.flex-radio.com/> Tune In Excitement (TM) PowerSDR(TM) is a trademark of FlexRadio Systems On Thu, May 24, 2012 at 8:56 PM, w3sz <[email protected]> wrote: > I agree with Gerald K5SDR. > > Taking reasonable numbers for a 2 meter system with tower-mounted preamp > [which is a must for serious 2 meter weak signal work]: > > F = F1 + (F2 -1) / G1 > > NF = 10 log F > > F = 10**(NF/10) > > So: > > If NF1 = 0.5 then F1 = 1.1220 > if NF2 = 1 then F2 = 1.2589 > > if G1dB = 25 dB then G1 = 316.2 > F = 1.1220 + (0.2589/316.2) = 1.1220 + .0008 = 1.1228 > NF = 0.503 dB > > if NF still = 0.5 then F1 still = 1.1220 > if G1dB still = 25 dB then G1 still = 316.2 > > but if NF2 = 4 then F1 = 2.5119 > then F = 1.1220 + (1.5119/316.2) = 1.1220 + 0.0047 = 1.1267 > and NF = 0.518 dB > > Degradation of system noise figure in going from 1 to 4 dB for NF2 is only > 0.015 dB > > So, I think I will take the 4 dB noise figure for the 6000 series and the > nice associated increase in dynamic range and be perfectly happy for 2 > meter weak signal work. ;) > > And when the 6000 series is used as a 2 meter IF rig for microwave work, > NF2/F2/G2 in the Friis equation are for the transverter, and the 6000 > series term becomes [+ (F3 -1) / (G1 * G2)], so I am even less concerned > about F3. > > On 144 MHz and up the key to optimal system performance is achieving > adequate gain and NF of the tower-mounted preamp, and maintaining > appropriate gain balance throughout the receive chain. Failure to achieve > optimal performance is in my experience most commonly due to inadequacy of > the tower-mounted preamp, as the Friis equation would suggest. Either > gain, or noise figure, [or dynamic range] problems at the first stage are > impossible to erase by adjusting the performance of following stages. As > long as adequate first stage gain is achieved, variations in the noise > figure of the following stages have only a very small effect on overall > system performance, as is shown above. But the dynamic range of each stage > IS important and a failure to achieve adequate dynamic range at ANY ONE > stage will seriously impact system performance. So best practice dictates > optimizing the dynamic range of later stages in preference to noise figure, > and optimizing gain, noise figure, and dynamic range of the first stage > [tower-mounted preamp] as the most important consideration of all. P.S. > the antenna is extremely important as well. It is "really" the first > stage... > > 73, > Roger Rehr > W3SZ > http://www.nitehawk.com/w3sz > > > On 5/24/2012 8:45 PM, Lee Mushel wrote: > >> Gerald, >> >> I think we have to start over. My concern with noise figure had to do >> only with 2 meter DX. I had no question about any performance on the HF >> bands or 6 meters. And I certainly know about putting the low noise >> preamp at the feed point of the antenna and I know about the switching >> associated with a practical system and I do have a sequencer. >> >> Now, I insist that we go back some 50 years to find me growing up in >> northern Wisconsin where there were a total of some four teenagers >> routinely on the cutting edge of ham radio: two meters. And this is over >> nearly one half of the state. Finding someone to talk to was a very big >> deal. And I think I can still find low noise preamps using the Western >> Electric 416b and 417a tubes in my shed that I built at that time. >> Moonbounce was only on 1296 and there was a lot of discussion on what was >> involved in calculating total system performance. >> >> Now let's go back a few days. I don't think I ever mentioned it to you >> but I do have the V/U upgrade and sitting on top of my 5000A are the >> Elecraft 144 and 432 transverters. I feed those into the RX2. >> >> If I can believe my 5000A my local noise floor a few days ago was a >> little better than -140 dBm. I would think most folks would expect that I >> would be interested in comparing the performance of the Elecraft and Flex >> front ends. And I do this frequently and I can safely say that with the >> Flex I can hear weaker signals and that never surprised me because the >> specs of my several year old Elecraft. >> >> I sincerely regret having commented on what I'm fairly sure is still an >> encouragement of mediocrity and I will not do that again. >> >> But I appreciate your taking the time to write me. >> >> 73 >> >> Lee K9WRU >> ----- Original Message ----- >> From: Gerald Youngblood >> To: Lee Mushel >> Cc: Brian Lloyd ; Ron Stockton ; [email protected] >> Sent: Thursday, May 24, 2012 3:49 PM >> Subject: Re: [FlexEdge] Flex 6000 MDS >> >> >> Lee, >> >> >> It is a very common misconception that lower noise figure is always >> better. A good article on the subject was in the June 2010 issue of QST. >> The article written by Joel Hallis is titled, "Receiver Sensitivity - Can >> you have too much?" The answer is yes. All you get is more noise and >> lower total dynamic range. What you want is for the gain to be set >> optimally for the band noise floor at your specific location. >> >> >> In fact a low noise figure may actually reduce total dynamic range for >> a given band and conditions. We could easily have put a <1 dB preamp for >> the same cost in the radio but that would have degraded total total IMD >> dynamic range. If you really care about a 0.1 dB NF preamp, it would be a >> total waste to put that inside the radio because it would ruin gain >> distribution and it would be swamped by the coax loss. >> >> >> The FLEX-6700 can give you a 4 dB NF on 20m but that would would >> ridiculous since the atmospheric noise figure equivalent in a rural area is >> probably greater than 35 dB above kTb. All you would be doing is to reduce >> the total dynamic range because you have too much gain. MDS of around -120 >> dBm is probably appropriate for most locations on 20m On 10m you probably >> need an MDS of around -130 dBm in rural areas and -122 dBm in residential >> areas. On 10m you can probably use -137 dBm (10 dB NF) only if you are in >> the quietest rural areas. >> >> >> The bottom line is if you want lowest noise figure on 2m, put the low >> noise preamp at the antenna and turn off the preamp in the radio. That >> will give you better gain distribution and will overcome the coaxial line >> loss. >> >> >> Regards, >> Gerald >> >> >> Gerald Youngblood, K5SDR >> President and CEO >> FlexRadio Systems(TM) >> Email: [email protected] >> Web: www.flexradio.com >> >> >> Tune In Excitement (TM) >> PowerSDR(TM) is a trademark of FlexRadio Systems >> >> >> >> >> >> >> >> >> On Wed, May 23, 2012 at 9:39 AM, Lee Mushel <[email protected]> >> wrote: >> >> I sure hope you are wrong about that number. You know, in 1958 I >> had a Techraft Converter that had a noise figure of 7dB and my quite >> excellent V/U upgrade is somewhere around 1 dB and my dedicated pre-amp is >> somewhere close to 0.4 dB. A noise figure of 4 in 2012 would rate >> somewhere between wretched and miserable! >> >> Lee K9WRU >> > > > ______________________________**_________________ > Flexedge mailing list > [email protected] > http://mail.flex-radio.biz/**mailman/listinfo/flexedge_**flex-radio.biz<http://mail.flex-radio.biz/mailman/listinfo/flexedge_flex-radio.biz> > This is the FlexRadio Systems e-mail Reflector called FlexEdge. It is > used for posting topics related to SDR software development and > experimentalist who are using beta versions of the software. > _______________________________________________ Flexedge mailing list [email protected] http://mail.flex-radio.biz/mailman/listinfo/flexedge_flex-radio.biz This is the FlexRadio Systems e-mail Reflector called FlexEdge. It is used for posting topics related to SDR software development and experimentalist who are using beta versions of the software.
