Even if the OP put the highest quality 430nH inductor in the World in this circuit it would be severley damped by the 1k collector resistor that is effectively in parallel with it.
The oscillator is still an epic fail despite all the posts offered to the OP.
I gave a revised circuit that would oscillate in an earlier post but maybe the goal is to try and make the original circuit oscillate despite it being 'wrong' for so many reasons...
Nevertheless the challenge (my challenge) was to find out why your original circuit was not working, not to propose an alternative one.
I think the basic circuit (as in post#24) can be made to oscillate strongly (eg if the emitter resistor is reduced) but I don't see the point of pursuing it. The circuit is fundamentally flawed because it has the 1k ohm resistor in the collector. This will damp the Q of the tuned circuit resulting in limited performance in terms of noise and output power.
If the 1k collector resistor is made smaller this problem will get worse.
If it gets made bigger then there will be a poor DC operating point set for the transistor.
That's why I modified the circuit to put the inductor in place of the 1k resistor. It cures this problem and also means the 100nF capacitor is removed from the loop as well.
I believe my minor circuit changes are more valid than trying to improve the original oscillator design by changing component values. The original oscillator topology is very poor.
The version I proposed uses the same LC components but less the 1k resistor and the 100nF capacitor. So it is simpler...
Zeller,
It is a petty not to see your oscillator's display.
I have simulated it (using Micro Cap) and it seems to work properly. Nevertheless, the following items have to be considered:
1) The output oscillation range is no more than 2Vpp, taken at the COLLECTOR. This low range is due to the high value of R3, which raises too much the Collector voltage of the transistor during saturation. The current value of R3 does not avoid oscillations to happen but the output is greatly reduced.
2) You are taking your measure directly from the EMITTER. Even if this is possible, you should consider that it is the input pin and its voltage range will be much less than at the output pin (COLLECTOR). In my simulation, the voltage range for the EMITTER is about 0.5Vpp. You should require a ZOOM to see it correctly at the oscilloscope.
In addition, your are measuring directly at the EMITTER, which has a DC value of about 4VDC due to the transistor bias. So you have a 0.5Vpp signal over a 4VDC !
For that reason I propose to measure only AC on the oscilloscope and make a zoom.
3) I suppose you are using an oscilloscope probe. Usually they come with a X10 switch on it, allowing to decouple the circuit you are measuring from the oscilloscope input. But if it used, the signal reaching the oscilloscope is reduced of 10 times. Be sure you are not using the X10 switch on the probe itself, otherwise you should be displaying a 0.5/10 = 50mV signal.
As a conclusion, the oscillator should be working correctly but you should take the measure from the COLLECTOR pin. Also you should limit the measure to AC and make a ZOOM on the signal.
Performances are not very good because you have an output of about 2Vpp while using a 9VDC power supply. To increase performances, you should effectively reduce the R3 value OR increase R2.
As support, you can find attached some shots of my simulation
Obviously you should also consider GOHZU's proposal as an alternative. Nevertheless the challenge (my challenge) was to find out why your original circuit was not working, not to propose an alternative one.
Note: I assume that the self is correctly manufactured. is it a bought one or a hand-made one?
Note: any photo of your circuit already mounted ?
For 27 MHz, I would expect the circuit to work as is, without a ground plane. The connection length mainly causes additional inductance, possibly a slightly reduced resonance frequency, but not much extra losses. The coil Q should be fine.You have built it on vero board and this is only suitable for low frequency.
You should build using a groundplane and also your connections are too far apart. the components should be connected closer.
For 27 MHz, I would expect the circuit to work as is, without a ground plane. The connection length mainly causes additional inductance, possibly a slightly reduced resonance frequency, but not much extra losses. The coil Q should be fine.
The circuit is apparently the modified original one, with 1k collector resistance.
P.S.: Your "baking sheet" design is of course superiour for RF circuits, particular for those that are intended as amplifiers rather than oscillators, and required for higher frequencies in any case.
I agree it could be made to work on vero board but without seeing the underside I'm uncertain as to how good the connections are. You don't need much series inductance with the 560pF cap (470pF in my second circuit provided) to get resonance problems that will kill the bode plot and I don't know how directly this is connected on the underside. If he has modified the underside to mimic my first circuit this could have a thin connection several cm long.
Also, I quickly retested my oscillator and turned down the supply volts until it stopped oscillating. Then I turned them back up until it started again. It restarted with only 2.5V at the collector.
Does the vero board version still have the 10k in the collector and the 100nF in the feedback? These aren't needed any more.
Also, I can't see for sure that there is any RF decoupling on the 9V rail. Is this on the underside?
Is this picture really showing the revised schematic with the inductor now in the collector?
It doesn't look right to me unless the underside has been hacked a lot.
You have built it on vero board and this is only suitable for low frequency. (although I could probably rebuild it on veroboard and make it work at 27MHz by making the foil side of the vero board more suitable for RF)
You should really build using a groundplane and also your connections are too far apart. the components should be connected closer.
I'll take a picture of my version of the oscillator and post it up. My version looks ugly but it looks pretty to RF
The built contruction method I have used will support RF circuits to about 100MHz.
Note that I built this very quickly and used basic parts. Ideally, it should be built using SMD parts on a proper PCB but I built this to prove the circuit works fine.
Also, ignore the second transistor at the top of the circuit. I added this in the PSU feed to provide an active filter on the PSU rail to reduce noise so I could measure the phase nnoise on an analyser without the PSU noise spoiling the result.
View attachment 50507
Also, your picture doesn't look like it follows my schematic. The vero layout you have posted up looks like the original circuit although without seeing your PCB underside I can't be sure.
Have you simply cut and linked the underside to make the old circuit appear like my schematic?
If you only have vero board then try building it in the centre of the PCB with tight connections and try and gang together three adjacent vero rails above AND below the circuit to act as your ground.
Gang the ground rails together using lots of connections so it resembles a grid.
Also isolate the sides and create another thick ground grid up both sides so your oscillator is surrounded by a thick 'moat' of ground tracks. Isolate all redundant vero traces that are not ground and it will probably oscillate although you should really build it like I have or on a proper PCB with a decent ground foil area and tight connections.
Looking sharp at the new circuit bottomside, there's no feedback connected to the emitter node. It can't oscillate.
And the suggested supply decoupler still isn't there.
Yes, indeed.I think the goal here shouldn't be just to achieve oscillation, I think there should be a learning exercise in how to connect up RF components wisely on a PCB layout.
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