High linearity performance? Enhancement Mode Technology? Low noise figure? Excellent uniformity in product specifications? Enhancement mode technology requires positive Vgs, thereby eliminating the need for the negative gate voltage associated with conventional depletion mode devices. Typical I-V Curves.
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Selection of transistor was relatively simple: due to accessibility, price and performance evaluation has won well-known ATF , which is possible to buy from many distributors. It turn me to the second issue, input reflections.
So - we considered this LNA as fully debuged design. Well, based on the above consideration, I did not speculated more abt. As you may see from these pics, I really tried to do it as precise as possible.
As I became older and my design inventiveness is going down, but i prioritize perfect work. My motivation for this was simple - why to lost a time, if such LNA design was hundreds time well-proven! The maximum gain has been adjusted in average it was some 1dB below OZ1PIF results and input reflections were measured.
Everything works well, no troubles, all works more-less perfect. Simply good design, which works in first setup. BTW: requested copper wire for coil with 2,5mm diameter you will find as central conductor in H cable, hi. I install a bit older, borrowed noise figure meter by HP, and when the machine was heated up, I completed calibration and start measurment of these LNAs.
First result was shock: 2,4 dB! The next was 1,63 dB, third 2,75 dB Because morning is wiser than evening I go to the bed. After that I was busy for 2 days. But did not forget it and consider more hypothesis, like bad PCB material but it was refused immediately, because one of LNAs was made on original OZ1PIF plate , bad transistors however they were from different suppliers , bad SMA connectors Chinese but with teflon and wheni nterconnected, the loss was below 0,1dB , bad capacitors trimmers but the same in the old LNA resulted with 0,45dB NF.
So no clear mistake were observed. For final, as usual, help me the mathematics. Result of my primitive analysis of the input circuit was relatively clear - when setup the computer modeling and put there value of each component include loss, the most important influence has blocking capacitor C4 on the "cold side" of the input coil L1.
So the improvement of the circuit was needed ;- The best RF blocking capacitor is no blocking capacitor. Galvanic direct interconnection has much lower loss, parazitic serial resistance would be quite low. The next reason for reconstruction of LNA input is the issue of resistance against lightning. Not bad design of transistor could be blamed for it - bad design of LNA is the reason.! I will not bother you by too long story - the modified LNA design is clearly visible on these pics: The input resonance curcuit is on the "cold" side grounded and from the "hot" side is FET Gate connected via added coupling capacitor - by empiric testing of 5 LNAs has been setup the optimum value about 5.
Bias voltage is to the FET Gate connected through air choke. However few pics will be better, than long description: Noise factor of such LNA drop down to abt. Several LNAs were even better up to 0,45dB or less. However within measurment of such low NF is very important on each connector adaptor - not only within measurment, but as well as within unit calibration - and particularly this could be be reason, why some of our collegues may kindly reach even values close to 0,2 dB, hi!
However in practice we are not the listening to the Noise factor or to IP3 , but by ears. And any LNA must be connected somewhere. When optimized by retune of the capacitor trimmer to best reflections at least dB will be NF degraded to cca 0,7 dB.
It is logical. This PHEMT device will have "naturally" minimum noise always in the other point, than power matching, maybe with a bit better situation on 23 and 13cm bands, were would be possible to arrange closer power and noise matching by relatively broadband input circuit. But such input circuit is not suitable for lower bands. Some contesters already know well such issue, hi. What is better for 2m and 70cm matching? Noise of impedance one?
Optimization is a result of LNA position and in practise could be the result of total noise of reception chain even better with better impedance and worse noise matching, than opposite. BTW check this table of loss due to missmatching and you have what thinking of, hi.
If you want to construct such LNA in your shack and no noise figure meter is available, do not be sad. Impedance matchin is the same as LNA adjustment for best gain. Optimized Noise figure you will get when the input circuit is detuned to the higher frequency until the gain on MHz has decreased by 0.
It was very same for all 70cm LNAs, which I made. Results of my measurments: LNA sample.
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