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MPF102 oscillating at 60 Hz Nov 7th 2020, 15:21 11 327 3 weeks, 5 days ago
connecting RF modules; impedance matching Jul 27th 2020, 13:02 4 405 on 3/9/20
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MPF102 oscillating at 60 Hz henryrichter 3 weeks, 5 days ago
Well, some of us unfortunately have to travel for work. Delving back into the project, I have determined that better construction techniques solves the oscillation problem. Specifically layout and better coil winding techniques- higher Q.
Using the Low Noise oscillator (figure 9.15 ARRL 2021 handbook) I now have 4 coils supplying a minimum of +15dBm from 2.7-29Mhz. Specifically on the 80-10 meter bands the circuit supplies 18dBm with a noise floor of -55dBm and 3dB bandwidth of 100K or better.

I still have the issue of band switching the coils attached to the tank capacitor. I had hoped to use pin diodes but they disrupt the tank. It seems I have only two options: use DB9 connectors to which the coils are soldered, or build 4 oscillator circuits, and switch ON-OFF the Power, Emitters ground and the hot end of L1. Otherwise the coils of the turned off circuits could act as antennas?
Also, I would be switching a coil lead with RF energy. I had hoped to avoid the extra lead length of the wire from the coil to the switch and back.
I will have the same problem on the front end, with 3 tuned circuits, each requiring3 or 4 coils.
Reading several books on design I haven't found this problem covered, or seen a circuit example. Maybe I am missing something.
Any help would be appreciated.

MPF102 oscillating at 60 Hz henryrichter on 12/11/20
A 10x probe.
I've had the probe load a circuit before. It dampens the signal but does not induce 60 cycle.
MPF102 oscillating at 60 Hz henryrichter on 11/11/20
The scope is a Textronix about 6years old in a plastic case.
The PS is an Elenco 317/337 kit with floating output ground.
I should mention that all the above 60 cycle detections were done with the positive scope probe only. Touching the ground clip virtually anywhere flatlines the output.
I have a conductive mat on my bench top connected to house earth, thus connected to scope ground. I ran a jumper to the circuit from a plug on the mat and the output again flatlines.
I have tried a twisted pair scope lead ( very noisy) and a twisted pair DC power lead to no effect. I simplified my 115V power to one power strip for the PS and scope only. I put the PS and scope on a grounded metal sheet and ran a jumper to the PS chassis. None of these efforts eliminated the 60 cycle.

I built a different oscillator, 2N4416 based, with the gate diode (EMRFD fig 4.4) I got smart and put a BNC connector on the output winding of the coil and ran a BNC cable to the scope. What do you know, it works fine.
This second oscillator has no drain or source resistor so perhaps it doesn't have a pickup point for AC currents?
These are only the second and third oscillators I've ever built, and now achieving higher tank voltages, perhaps nothing i have built before had a strong enough RF field to couple with an AC field.

I don't know where I am. If there is a systemic problem at my workbench I'd like to fix it.
I didn't know about differential probes till now; they seem to be mainly for high voltage? and at any rate are not cheap.
Or maybe the problematic circuit is just borderline unstable and thus reactive.
MPF102 oscillating at 60 Hz henryrichter on 8/11/20
In fact, removing the coil , using the circuit as a common source amplifier or even leaving the source lead free in the air, I still get 60 cycle, even touching the copper clad board.
I've turned off all the lights in the room, that didn't help.

I have a version of this circuit in a die cast box with two variable caps which works between 20-30 MHz. There is a lot of RF energy in the air as passing my hand over the open box shifts the freq and/or amplitude.

The current effort is to experiment with components of this circuit and a following 40673 amplifier to achieve +7dBm at a diode ring mixer

MPF102 oscillating at 60 Hz henryrichter on 8/11/20
LM317 regulator power supply. I thought about ripple passing thru the circuit and then used a 9 V battery and it still occurs.

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