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Originally Posted by Sev
I assume both of the BFG components are on the back side of the board?
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Yes
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Do you know of any alternative componets that would lower the noise figure. Or based on the design it is baked in?
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The 1st UHF transistor (BFG67) could be replaced with a BFG520.
It might lower noise figure a couple of tenths of a dB.
Prrobably need to retune.
The transistor substitution works fine, did that when replacing balun.
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…I have come across a few B.T. 300ohm dual inputs with the 75ohm output but passed no them because there did not appear to be much love for them due to their advertised noise figures.
Any idea if those figures are actually accurate or not?
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Based on the circuit design, the components, and comparative testing:
The older 0064/0264 have about the same noise figure as the BT Galaxy series.
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To clarify a statement back on post #14 you stated you removed the ferrite bead and replaced it with a half-wave loop on a Spartan?
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Image of CM0064 with half-wave balun below:
The coax is RG-180 (95 Ohm). The impedance is not ideal, but in some ways better than 75 Ohm coax.
It could be better if the impedance for the loop was 150 Ohms.
But, such cable would have to be custom made and it would probably be too large to fit in the case.
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OTAFAN posted an interesting idea concerning replacing the polarized caps with non-polarized.
Thoughts?
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Doesn’t apply to RF amplifiers.
Some audio circuit paths require passage of bipolar signals (plus & minus).
Audio signals are very low frequency relative to RF (long wavelength, calculate wavelength for say 20 Hertz),
therefore large capacitors are needed.
All bipolar signals in an RF amplifier pass through small ceramic capacitors, they are essentially nonpolar.
The polarized capacitors in RF amplifiers are for filtering the DC (not bipolar) supply voltages.
The noise that is bothersome in preamplifiers is usually in the same frequency range as the desired signals (MHz).
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