I’m building a tuned circuit tester, which is a Franklin Oscillator without the parallel tuned circuit. I based it on the schematic I found at this URL. The first amp is a JFET in common drain or source follower configuration, which then drives the 2nd stage, a PNP transistor in common base configuration, so there is no signal inversion and the common base has voltage gain. Some signal is used to drive the third emitter follower stage, and its output is used to drive a frequency counter.
The following photo shows the changes I made. Most are increased capacitor values, such as coupling and bypass, which don’t affect the circuit very much.
I used an MPF102 JFET for the first amp, and a 2SC3355 for the emitter follower. Instead of a 1N4148 I used a 1SS98 UHF diode for the JFET’s gate.
After completing the basic circuit, I double checked it and found a bias resistor that needed to be soldered. It pays to double-check! I then soldered a coil of 6 turns of 20AWG bare copper wire in parallel with a 24 pF trimmer capacitor across the ‘unknown tuned circuit’ pins. I applied 9 volts and checked with a dip meter and FM radio, and tuned the circuit to the bottom of the FM band. It oscillates just fine – I got a strong signal. I tried adjusting the trimpot, but it doesn’t do a lot, just changes the power.
I changed the tuned circuit to a 2.6 uH toroid coil in parallel with a 100 pF disk capacitor. This should resonate at 9.7 MHz. I put a short wire through the toroid and made a single turn loop by soldering the ends together. This allows me to put the coil of the dip meter inside of the loop to pickup the signal. I powered it up, but I got nothing. I thought it may be because the loop is acting as a shorted turn and stops the oscillator.
I removed the single turn loop from the toroid. I thought, what would be a good way to indicate that the circuit is oscillating? I decided that a half wave doubler and LED should be fine. I added two 1N914 diodes, a filter capacitor and red LED to the output. I checked the LED to make sure it would light up. When I powered it up, the circuit did not seem to be oscillating and the LED did not light. I decided to go back to the 6 turns and trimmer capacitor. I powered it up and adjusted the trimmer so I could hear the dead carrier in my radio. I got a strong carrier, but the LED still didn’t light up. My thoughts are the circuit is oscillating but the amplitude is not high enough to make the half wave doubler rectify and light the LED. Maybe I need an amplifier after the diodes? …
Things are turned around. Circuits typically oscillate easier at low frequencies, and harder at high frequencies, but not in this case. Weird.
