Re: GLB Preamplifier > n...@... wrote: > OK, is it just me or is there NO noise figure spec. in any > of these documents?
The Philips data sheets give typical noise figures for the BF981 as 0.7 dB at 200 MHz and 0.6 dB at 600 MHz. Curves are provided for determining the source admittance necessary to obtain these optimum noise figures. This type of performance can only be obtained with a dedicated pre-amp tuned to a single frequency, for example, 144 MHz. Since minimum noise figure is dependant on a high L / low C ratio, any tunable pre-amp covering a 20 MHz bandwidth will likely have a 1.5 dB noise figure at best. > My guess is that the GLB preselector consists of the filter > followed by the preamp. Also because of the number of sections > of filtering & the overall small size of the unit, the actual > helical filter is probably quite lossy. The GLB helical layout is one helical in front of the preamp and 4 trailing helical sections. Max GLB stated gain is/was reported as 10-11dB A circuit based around the Philips BF981 Mosfet transistor, can reportedly achieve a noise figure as low as 0.7dB at 200 MHz in an optimized circuit. Realistic pre-amplifier circuits feature a more modest 1.5 dB noise figure, and a typical gain of 20dB. I personally believe the 20dB gain figure is a bit optimistic. > For comparison the 5 section helical resonator in the front > end of the G.E. Mastr II/MVP VHF HB radio has about 6 dB of > loss. Which would relatively jive (correspond) to the loss in the GLB Helical unit. The difference worth noting is the bandwidth of the GLB unit. > If you need preselection and low noise figure, I'd look here: > <http://anglelinear.com/custom/custom.html> Compare the skirt displays (pictures) for both the Angle Network analyzer plot 440 MHz, 20 MHz spread unit and the GLB P-450 unit... the GLB skirts appear to be notably sharper than the Angle device. Relative speaking "Preamp gain is easy for all"... but quality pre-selection skirts are the dirty (or not so... ) details. > or do what I do & look at the hamfests or eBay for one or > two 1/4 wave bottles followed by the preamp of your choice. A key question... what about selective circuits after the preamplifier? Nothing..? direct preamp output to the receiver? > Pass cavities are typically 0.5 to 1 dB loss. The Angle Linear > cans are even lower - I measured 0.25 dB on one of his 220 MHz > filters. > Bob NO6B The trade is insertion loss for selectivity... When you have a lot of gain... tis good to trade a fair amount of it to run at least modestly tight ship. s.
