Yes, I often feel like the bad guy when I give a negative opinion on some of the magazine based projects posted on here
The designs often don't take into account the spread in device characteristics etc. This must lead to a lot of confusion and frustration for any unlucky builders who try and emulate the original circuit using parts that are slightly different in terms of tolerance etc.
I managed to find and grab some (SMD) 2N5484,5,6 JFETS at work and for a bit of fun I've tacked together a copy of the original VE3RF 5.5MHz oscillator onto some bare PCB. I've had to use some sticky pads to mount the SMD JFETs and so far I've only tried the 2N5484 parts. I used a T50-6 toroid as a basis for the main resonator coil and this ended up being around 6uH for oscillation at 5.2MHz. The Idss of these FETs is quite low. Probably only 2mA. However, the circuit does fire up at just over 5MHz and it produces a lot of negative resistance so I don't think loop gain is a problem with these particular JFETs. It seems to work about the same as the simulation. I get 11Vpkpk in the resonator with 150pF in the resonator. The loaded Q simulates to be about 45 but I haven't tried to measure this on the real circuit in open loop. With these numbers I think the resonator power is just over 1mW. The simulation predicts a decent noise figure for the system and I went with 7dB as a reasonable estimate. I guessed the flicker corner at 15kHz.
When entered into Leeson's equation these numbers deliver the phase noise prediction below.To get power out of the circuit I went for a simple L match down to 50R from the JFET drain. This is really a fudge and not ideal. But it did allow me to look at the phase noise on a spectrum analyser at a reasonable power level. My old(ish) Tek RTSA does have a signal source analyser feature built in but it isn't as good as the megabucks Agilent E5052A phase noise analysers we have at work. I can measure down to about -136dBc/Hz at very close offsets but that is the limit of the Tek RTSA. At work the E5052A is capable of measuring down past -175dBc/Hz.
Therefore, with the L match interface, the signal into the RTSA from the Franklin oscillator is at about 200mVrms. You can see in the phase noise plot below that the close in phase noise is very simular to the simulation plot. Also, 200mVrms is a similar RF level to the original requirement in the original post.
At 100Hz the noise is better than -80dBc/Hz, at 1kHz it is better than -120dBc/Hz. However by a few kHz the phase noise floor limit of the analyser is hit at around -136dBc/Hz. So I can't show the phase noise at a 10kHz offset because it is masked by the noise of the analyser. I'd expect it to be somwhere close to -150dBc/Hz when measured on an E5052A SSA but that really is just a guess based on the simulation. I think the reason the phase noise looks lumpy and uneven below 300Hz is because of vibration/microphony effects on my test bench that are making the resonator wiggle about and also because of noise pickup and and maybe some noise/ripple on the power supply.
I've not tried to measure the frequency drift but I'd expect it to be dominated by the T50-6 powdered iron toroid used in the resonator. Also, my workroom isn't very stable from a thermal point of view. I have a fancy shielded enclosure here that might help with some of the noise pickup although I'm not sure how well it works down at low frequencies. It might help with the drift as well because it will keep out any sudden changes in air temperature in the room as I move about and the test equipment gets warmer on this fine evening here in N Glos. However, because I've built the oscillator with a feeble and wobbly layout and a wobbly toroidal resonator it probably isn't worth trying out any stability tests.
I can try measuring the phase noise of the oscillator on an E5052A at work but this won't be possible until next week. The E5052A starts at 10MHz so I'll have to read the manual to see if I can feed it in as a baseband signal. I've never done this before with the E5052A as I 've only ever used it above 10MHz.