Re: Topband: Hi Gain Preamps, Noise Figure Comparison (KD9SV 994 x5, W1FB, W7IUV preamps)
Don, Thanks for sharing your research on the preamps. Very informative. Your study is very timely for me. I have ordered DX Engineerings new phasing box, NCC-2. Optional plug in preamps are available for the unit. I am holding off on ordering a preamp until I compare specs with other external preamps. The KD9SV unit looks pretty good ! Now to find out how it compares. Bob K6UJ On 11/8/16 1:54 PM, Don Kirk wrote: Gary (KD9SV) kindly loaned me one of his 994 x5 preamps (sometimes called KD9SV VLN (very low noise) preamp) so I could compare its Noise Figure against one of my W1FB preamps as well as 2 of my W7IUV preamps in series on 160 meters. Here is a link to a youtube video I created that demonstrates the differences in Noise Figure between the above mentioned preamps. http://www.youtube.com/watch?v=JpIiF1Hfovw The KD9SV 994 x5 preamp did in fact come out on top (best Noise Figure) and it's a commercially available preamp, whereas my W1FB and W7IUV preamps are home built. Nevertheless in my current application (pennants that are 51.6% the size of full size pennants) the W1FB and W7IUV preamp Noise Figures are adequate and I didn't notice improvement in my signal to noise ratio on 160 meters when using the KD9SV preamp. The lower Noise Figure of the KD9SV would allow me to use even smaller pennant receive antennas that would have more negative gain without harming my signal to noise ratio. I ran into a couple of interesting issues when working on this project. The biggest surprise was that the input impedance of my old HF rig (Kenwood TS-180s) was no where near 50 ohms on 160 meters (it actually measured 189 ohms). I added a transformer to the input of the Kenwood TS-180s to bring it closer to 50 ohms for my testing. Also as previously reported by Hardy (N7RT) a long time ago, the W1FB output transformer turns ratio is incorrect for a 50 ohm match, and I measured the output impedance of the W1FB preamp at 429 ohms. I subsequently added a transformer to the output to bring it closer to 50 ohms for my testing. *Noise Figure Comparison (average of two different measurement methods)* W1FB preamp: Noise Figure 3.3 dB higher than KD9SV 994 x5 preamp Two W7IUV preamps in series: Noise Figure 1.1 dB higher than KD9SV 994 x5 preamp Note 1: my W1FB preamp is not stock, as I made modifications to protect the MC1350 IC from damage when transmitting. Note 2: The KD9SV 994 x5 preamp was set at its maximum gain (41 dB) for all of my tests. *Summary* While I suspect (through my measurements) that the KD9SV preamp absolute Noise Figure value is not the very low 0.7 dB value listed on a commercial website, it's indeed a very nice adjustable high gain preamp offering a maximum gain of 41 dB and it has the best (lowest) Noise Figure when compared with my other two types of preamps. I highly recommend the KD9SV 994 x5 preamp if you're in need of a high gain preamp on 160 meters. 73, Don Kirk (wd8dsb) _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Hi Gain Preamps, Noise Figure Comparison (KD9SV 994 x5, W1FB, W7IUV preamps)
Hi JC, I did not publish any absolute Noise Figure values, my measurements that I published are all just differences in Noise Figure. It actually is much easier to measure the difference in Noise Figure between preamps that have such high gain (approximately 40dB in this case) since the noise floor generated by a 50 ohm resistor at room temperature is easily measurable (at least indirectly) after 40 dB of gain (It's -106.89 dBm for a 40 dB preamp using a bandwidth of 500 hertz if the preamp adds no additional noise). Even if you can't accurately measure -106.89 dBm level signals, you just need to be able to be repeatable in your measurements between preamps (accuracy and repeatability are not the same thing), and your system must be able to detect differences in output signal from the preamp in units of dB (easy to do with an SDR receiver at these levels using attenuators to verify changes in dB on the SDR display). In my case all of my measurements were made with my Kenwood TS-180s operating on 160 meters, and then making measurements via the wideband IF output port of the Kenwood TS-180s that's connected to an SDR receiver (the HDSDR software also allowed me to do long term averaging, and it even allows you to display power spectrum density). My second method of measurement used the S meter on my TS-180s, but just as an indicator (actually digitized the voltage supplied to the S meter). I recorded the signal strength on the S meter that the preamp generated when the preamp input was terminated with the 50 ohm resistor, then I went back and adjusted my signal generator connected directly to my TS-180s via attenuators until it provided the same S meter reading. Using the signal generator and precision attenuators I was able to calculate the voltage (and therefore power) into the TS-180s that the S meter recorded when driven by the preamp. I used this technique to compare Noise Figure between preamps, and I made numerous measurements over a 6 day period and then averaged the data to reduce my variation using this method. I did in fact test the use of #31 mix cores (10 to 14 turns) on various lines (power supply lines, 3 foot short feedline between preamp and receiver input, etc. and saw no difference in signal levels. I even eliminated the Kenwood power supply, and fed the TS-180s with a lab grade linear power supply to eliminate slight changes in supply voltage over time. P.S. the Kenwood TS-180s is one of the better radios made in regards to noise floor (measured -139 dBm by Sherwood). It would certainly be much easier to make these measurements using a spectrum analyzer, and I probably will acquire one of the lower cost Rigol spectrum analyzers in the near future. In the meantime I would love to have someone independently make measurements comparing these amplifiers that either supports or disproves my measured values. 73, Don (wd8dsb) On Tue, Nov 8, 2016 at 7:24 PM, JCwrote: > Hi Don > > Measuring noise figure below 1 db is a very complicated work. > > First you need to remove all common mode noise, your noise floor should be > better than -135 dBm at least with a 40 db preamp connected to the radio > input and a 50 ohms load. > > You need chokes everywhere and most important shield on 12V, RF in and RF > out. Check the AM band you hear carrier coming in , noise is also coming > in. > > Second the NF is very different on 1.8 MHz , measurement near 10 Mhz, does > not tell you the right NF on low bands. > > In order to have Noise Figure Uncertainty near .2db it is necessary a good > quality lab equipment, calibrated and two hours per measurement for each NF > reading. This assuming the temperature does no change during the test. > > > Your Noise Figure Uncertainty based on your video could be 5db or more. > > > 73's > JC > > N4IS > > _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Hi Gain Preamps, Noise Figure Comparison (KD9SV 994 x5, W1FB, W7IUV preamps)
Hi Don Measuring noise figure below 1 db is a very complicated work. First you need to remove all common mode noise, your noise floor should be better than -135 dBm at least with a 40 db preamp connected to the radio input and a 50 ohms load. You need chokes everywhere and most important shield on 12V, RF in and RF out. Check the AM band you hear carrier coming in , noise is also coming in. Second the NF is very different on 1.8 MHz , measurement near 10 Mhz, does not tell you the right NF on low bands. In order to have Noise Figure Uncertainty near .2db it is necessary a good quality lab equipment, calibrated and two hours per measurement for each NF reading. This assuming the temperature does no change during the test. Your Noise Figure Uncertainty based on your video could be 5db or more. 73's JC N4IS _ Topband Reflector Archives - http://www.contesting.com/_topband