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Old 22nd Mar 2022, 1:24 pm   #5
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: 6-gang FM stereo tuner heads

Thanks. Ok I was confused by the terminology of "limiting amplifier", not knowing it is the same thing as limiters in FM IF stages. I read a whole Rider book on the subject of FM limiters. I tried both ratio and discriminator detectors in homebrews. The ratio detector is good for low-cost radios. For most high-end stereotuners, discriminators combined with two to three progressive limiting amps are common. Interestingly, the much-hyped gated beam valve 6BN6 was rarely found in FM tuners as a standalone limiter. In fact, I have never seen them used without additional limiters. I guess it is to do with the fact that the 6BN6 has to be driven at fairly high signal voltage.

The muting function is commonly found in FM IC chips. I have never seen them to used in FM tuners using discreet components.

I am trying to find a way of measuring the "relative or indicative" noise of the antenna tuned circuit using a cheap £20 diode noise generator or my Rigol DSS signal generator with the TinySA. Some people suggested it is possible to tune a 2m pre-amp front end by ears to find the best noise figure. I doubt it is doable...

Looking at the data sheet of BF981, the noise figure is 0.7 or noise temperature of 51k at 100MHz. However, any component placed in the circuit between the antenna and the input of the first Rf amplifier will cause a reduction in noise figure. it is a juggling act of balancing three things: the optimum noise figure, the best impedance match for power transfer and attainment of the required bandwidth for the antenna tuned circuit.

I often ponder the pros and cons of using doubled vs single-sided copper clad boards in VHF prototype of bandpass filters? (using either Manhatten islands or deadline bug techniques). If double-sided copper clad board is used, connecting ground wires must be used by drilling holes in every mm distance to minimise any ground loop. In the context of the VHF bandpass filters, I read somewhere that double-sided clad boards give steeper skirts of the filter response curves. However, I cannot see the logic behind it. Others claimed single-sided copper clad board is better for bandpass filter. So far I have only tried single-sided prototyping board, so far so good, not having any issue with tuning and skirt shape.
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