MPF102 oscillating at 60 Hz
Displaying 1-11 of 11
| Nov 7th 2020, 15:21 | |
henryrichterTotal Topics: 0 Total Posts: 0 |
I am trying to create a simple Hartley type oscillator circuit using a MPF102 JFET. I am reading 60 Hz wherever I touch the oscilloscope probe to the circuit. I have rebuilt this circuit, changed the ground spots on the copper clad board changed the components etc. If I take the coil out of the circuit it still oscillates- it appears to be the transistor itself. For what it is worth with the coil attached the tank circuit generates 20 volts P-P of 60Hz with a 12 volt supply. So that is "working"? I am perplexed. I'd guess I have a ground loop I can't find? Thanks |
| Nov 7th 2020, 19:06 | |
WA0CBWTotal Topics: 0 Total Posts: 0 |
What are you using to power the circuit? |
| Nov 8th 2020, 08:20 | |
henryrichterTotal Topics: 0 Total Posts: 0 |
LM317 regulator power supply. I thought about ripple passing thru the circuit and then used a 9 V battery and it still occurs. . |
| Nov 8th 2020, 10:22 | |
henryrichterTotal Topics: 0 Total Posts: 0 |
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 |
| Nov 8th 2020, 18:01 | |
WA0CBWTotal Topics: 0 Total Posts: 0 |
What kind of scope are you using? Are you common grounding the scope and your circuit? Bill |
| Nov 9th 2020, 04:51 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
You can have magnetic coupling between your AC wiring and the oscilloscope. A loop of wire can act as an "antenna" on the oscilloscope end, even though you use a shielded cable and "ground" everything. The coupling can be minimized by using twisted pair. https://www.picotech.com/library/oscilloscopes/rejecting-common-mode-noise-in-oscilloscope-measurements Zak W1VT ARRL Senior Lab Engineer |
| Nov 11th 2020, 16:59 | |
henryrichterTotal Topics: 0 Total Posts: 0 |
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? Speculating 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. |
| Nov 12th 2020, 11:48 | |
WA0CBWTotal Topics: 0 Total Posts: 0 |
Is the probe "loading" the circuit? Are you using a direct x1, or a x10 probe? |
| Nov 12th 2020, 12:48 | |
henryrichterTotal Topics: 0 Total Posts: 0 |
A 10x probe. I've had the probe load a circuit before. It dampens the signal but does not induce 60 cycle. |
| Nov 13th 2020, 15:43 | |
WA0CBWTotal Topics: 0 Total Posts: 0 |
Thinking outside the box....... Is the scope's 3rd wire safety ground intact and connected to a 3-wire grounded receptacle? Have you measured any AC voltage between the scope ground lead and the circuit ground? |
| Dec 22nd 2020, 10:56 | |
henryrichterTotal Topics: 0 Total Posts: 0 |
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. Thanks |
Displaying 1-11 of 11
