VFO: AM band Oscillator

@04 Nov 2015

AM band cover 550 khz to 1.5Mhz, tune range ratio reached about 3. it's such a wide range to be cover. For an AM oscillator, capacitor feedback style oscillator will not suitable, cause extra capacitance limited the tune range. 

JFET Hartley 

previously, i built one hartley, VCO: AM Hartley LO , years after, i rebuilt the same circuits. there is some other aspect of  this oscillator not experimented. hartley is famous by it's sure trigger, always work at first try. 

refer Fig.1, when i connect them together,  the 220 k bias resistor had been forgotten, so the circuits does not work. the reason is without bias resistor, the JFET(here, 2N5486) will be bias to a random state, might be totally conduct or just cutoff.

the most things shock me is the tank voltage, from about 470 KHz to 1.5 MHz, tank's peak to peak voltage changed from 30 Vpp to about 80 Vpp. i suspect i'm wrong, or the oscilloscope is crazy. but it's not,  using 1N4148 rectificate the tank voltage, get 54V DC voltage @1.5 Mhz. 

the feedback capacitor is small, only 3pf,  the positive peak at the jfet gate forwarding bias the jfet diode (thanks YNGZ from crystalradio.cn point me this phenomenon) , limited the peak voltage.

check the Fig.2, a diode added to limited the forwarding peak voltage. this limited the tank voltage to a small range: 20V to 40V peak to peak.