[regenfreak]

Thanks David. Some terminology mentioned in your post are new to me. This is the first time i have heard of the limiting amplifier. Due to my ignorance on the subject, I dont understand fully lots of terms described in 1965 IEE paper. As a beginner, I cannot afford the cheapest spectrum anlayzer. A noise figure meter would be what they used by manufacturers to optimise the tuned stages.

Attached is the comparison between simulation and actual "cold" frequency sweep of the double tuned filter with the filter Q = 60. They match very closely.(note i cannot calibrate my cheapo sweep generator properly).

The 2nd and 3rd pictures are my test rig for "hot" frequency sweep of double tuning+ single tuning with a dual gated mosfet BF981. The overall gain is 19db and bandpass Q = 159. The "cold" Q (no amplifier) of the double tune stage is about 85 and single tune stage Q is 30.

The VHF measurement techniques of unloaded Q and filter Q is based on the work of Wes Hayward. The hardest part of project is getting the input and output impedance matching of the filter stages. The alignment and fine tuning will be quite challenging. The coupling capacitor, the input and output tappings will determine the bandwidth of the bandpass filter. According to Wes Hayward, the inductance of coil is not proportional to the square of turn number when the length to coil diameter ratio is bigger than one due to leakage flux. Therefore the tapping positions have to be deduced experimentally. I avoid the use of gimmick coupling capacitor( they are just two wires and mechanically unstable). Instead I use DIY capacitor or piston trimmers in series to get very precise value down to accuracy of 0.01pF which is the limit of my DER EE LCR meter.

In the Pioneer TX8500 service manual, it stated that "the noise figure is determined by the insertion loss of the antenna input tune circuit",

insertion loss = ( 1 - QL/Qu)**2

In my measurement Qu = 100, QL = 30, IL = 0.49db