[regenfreak]

nemo_07The first part you can find at the antenna inputs in some "kitchen-class" FM radios. These are hybrids with four pins, vertical, coated with a sealant mass mostly of brown colour.
Winding an IF filter goes as follows:
Suppose you need a 100pF||2.2uH LC tank for 250kHz 3dB bandwidth, and the AL value of your TOKO cores is unknown.
Wind 20 turns (n=20), assemble the whole, drive the tuning slug to the approximate middle position of tuning range, resonate it with a 100pF and note the resonance frequency. From the well known formula you'll get the inductance value, L_20t. Let it happen to be 2.5uH.
From L = AL*n² we get AL = 2500nH/(20t)² = 6.25 nH/t².
The number required for 2.2uH primary: n1 = sqr(L/AL) = sqr(2200/6.25) = 18.8, we take n1 = 19.
For the required bandwidth we get (loaded) Ql = 10,7/0.25 = 42.8
Given XL = 2*Pi*F*L = 148 Ohm we get the final resonance impedance being Zr = Rp = Ql*Xl = 6.3 kOhm.
From the transformer equation we have
Rp/R2 = n1²/n2² --> n2 = n1*sqr(R2/Rp) = 19*sqr(50/6300) = 1.7, we have to wind something like 1 + 3/4 turns secondary.
Important is to wind it over the "cold" end of primary.

Cheers. But I was thinking more about the design of the Pi matching network for the RF two-tone at gate 1 of the mosfet 3N201 in the schematic of post #357 from the datsheet.

From the datasheet, it is possible to read off useful parameters ; input admittance |Yfs|, input admittance gis+jbis and output admittance gos+jbos.
For some reason, the reverse transfer admittance is often omitted in datasheets. By working out the complex input impedance Z = 1/Y at the center frequency of the two tones, we can design the Pi network using the Smith chart (or SimSmith) or an online calculator. With a tone spacing of 1MHz and center frequency of 100Mhz, assume the bandwidth is 4Mhz for pi-network,Q = 100/4 = 25. I don't know. It is just a rough quest for Q. If I were to make Q smaller, I may have to connect a parallel resistor across the inductor. The Pi network also works a bit like a low-pass filter for the harmonics from the two-tone signal sources. The Pi network is more flexible than L -network as you can specify the Q. It is basically two back-to-back L networks with virtual resistance at the adjoining nodes. Also the Q of L-network is high for high-impedance circuit.

I think you need to be wary of the spectral purity of any commercial sig gen that you use for the LO to test a dual gate mosfet mixer. The relatively poor port to port isolation with this type of mixer could cause confusion when you try and measure the mixer noise figure. It usually doesn't really matter how much money you spend on a modern commercial synthesised RF sig gen, the noise floor at about 10MHz away from the carrier is rarely better than -150dBc/Hz. You might find some that can manage -160dBc/Hz but this is rare. Some commercial sig gens can be as poor as -140dBc/Hz.

For dual gate mosfet mixer, probably RF-to-LO port isolation is about 20db, LO-to-IF port isolation 30db. Because of the high impedance, the DGM has a very narrow bandwidth and low conversion gain for image noise. Now I think this measurement is a moot exercise purely for learning purpose. I will probably change it to a "thought experiment" without doing all the troublesome practical work. It is a lot of hassle to measure TOI with all the jazz of diplexer, Pi network, If-transformer, crystal filter and frequency source BPFs.

See below for the noise floor performance of a cheap 134MHz JFET oscillator against a couple of commercial signal generators. The Marconi 2022 and the HP 8648D. You can see that the cheap JFET oscillator is 25-30dB cleaner at a 10MHz offset.

Thanks. It is probably good news as I can easily build a BJT oscillator with a buffer amp and air variable capacitor tuning capacitor easily. The air variable capacitor tuned oscillator has higher Q and lower phase noise than varactor-tuned oscillator. With a reduction gear drive for the air capacitor, it should have an acceptable frequency drift. I have several cheapo VHF VCOs from 30Mhz to 1.1 GhZ from China and the Mini circuits POS-200P 95Mhz-120Mhz VCO. The Chinese "toy" VCOs are just for fun and reverse engineering of their circuits.The POS-200P VCO frequency drift is not acceptable on the spectrum analyzer for the critical TOI test even I power it with a regulated battery power source.