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Need help interpreting an old circuit diagram: Transistor and RF Transformer


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So the writeup I am working from says to adjust the circuit such that the air/open setting on the capacitor is about 1.3 MHz.
As I went to higher frequencies and amplitudes, I found that the waveform shifted from an almost sawtooth form (rounded on the rise) to what you see here. I was able to get the frequency up to 1 MHz and the amplitude up to about 500 mV, but as I did so, the bottom of the waveform seemed to get cut off (like you see in the picture, but more extreme) and the smaller extra peak before the main peak grew in prominence. And that extra peak seemed to depend as well on the external cell capacitance, leading me to worry that the frequency counter might start picking them up as separate pulses, screwing up the measurement.
As discussed earlier, although this oscillator circuit works, it does have its limitations.

Scratching my head, I remembered a circuit for a 1MHz oscillator which I knocked up for someone a few years ago.
Look here:
It runs at 1MHz and is easily adjusted by varying the turns on a single layer coil.
The output from the emitter of the transistor is a close approximation to a sine wave with an amplitude of nearly 2.5v p-p.

To re-iterate what I said yesterday:
If I were starting from scratch, I would select an oscillator circuit which used a simple inductor (ie just one winding).
Then using a reel of wire and suitable former, it would be easy (five minute job) to make whatever inductor was required.

PS, I think that I have "oscilloscope envy".


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PS, I think that I have "oscilloscope envy".
Don't we all, when we see something better-spec'd than what is currently sat on the bench in front of us? :)
I have a cal-failed TDS220, for obvious reasons, bought from an e-waste seller for £20 around 2011, IIRC.
It was spotted on a display table at Bowlers Computer Fair, in Manchester, with CH1 BNC socket completely adrift. (I swapped the EXT-TRIG BNC socket to CH1).
He informed me that it was a signal generator and randomly-twiddled all of the knobs whilst un-powered, muttering "This is sine, square, triangle, and this is speed and other mumblings. £50 or you can make me an offer..." - so I did.
Swapped the BNC, made sure the recall had been done, reset the factory settings and ran through the self-cal, which passed fine.
It does all that I personally need so far, but it sure would be nice to have something more sexy, like a bandwidth-hacked DS1054Z to look at.
Looking at the frequency counter I ordered, I don't see specs on the voltage amplitude necessary to count as a pulse. Another site with the same product suggests that it has sensitivity "better than 60mVPP," which I presume means 60 mV peak-to-peak. Given that, should I be aiming for a cleaner waveform at the expense of frequency and total amplitude? Or am I likely to be ok with this waveform?
Thank you all for the amazing help!
The counter should have an input attenuator so you can limit the minimum input you are interested in. If it doesn't have one, is easy make.
You should aim to get the cleanest waveform you can anyway. I don't know if harmonics would actually affect your measurement, but they might, especially if they change when you adjust the cell.

(Edit) I just followed your link. Oh, one of /those/ sort! You probably do need to make an attenuator. A simple potential divider will do it.


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I've wondered about those hobby shop various coloured copper wires, are they actually copper?, I wondered if they'd be aluminium or something.
so far, the only ones i've seen are copper.
Thread starter #47
Ok, I have everything together and stable, fully shielded with coaxial cables connecting the component=s. I can trim the frequency using the internal variable capacitor, and then can use the external capacitance cell in both open and closed positions, and see different frequencies when I do. I'm still measuring the frequency with an oscilloscope, because my frequency counter hasn't come in yet, but at least things are looking like they are working relatively well. I had a devil of a time tracking down an intermittent problem that resulted from me failing to ground the housing for the RF transformer, but I seem to have all of the bugs worked out. The waveform is quite nice, and I have an amplitude of about 100 mV. My one concern is that the frequency is about an order of magnitude below what I wanted. I'm getting about 150 kHz with both capacitors open, and the lab writeup suggested we wanted 1.3 to 1.5 MHz. Of course, I have never done this experiment before, so I don't know how critical that is.

Looking at the figure in post #29, I followed that diagram pretty closely with the following modifications: (a) 222 pF capacitor between signal and ground instead of 300 pF (because that's what my stockroom guy got me a lot of for some reason), (b) a 0.1 microF capacitor in the signal line as suggested by Nigel, (c) the transistor is a 2N3904 (more on that in a moment), (d) the transformer is the 42IF106-RC suggested by JimB, and (e) the variable capacitor is actually two variable capacitors in parallel, one in the shielded case to allow frequency trimming and one external for the liquid cell. Note that I am using an NPN transistor despite following the suggestion made by a couple of folks to switch to a positive bias voltage (as post #29 indicates). I did that because I had a whole bag of them my stockroom guy got me, and I didn't have any pnps, so I thought I would just try it and see what happens. And it worked, so I assume that I misunderstood something, and I would love to learn what I misunderstood.

Any thoughts?


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My one concern is that the frequency is about an order of magnitude below what I wanted. I'm getting about 150 kHz with both capacitors open, and the lab writeup suggested we wanted 1.3 to 1.5 MHz.
The original circuit suggested a coil with an inductance of 0.6mH.
After various experiments etc, you said that you would prefer to buy a coil (transformer) so I suggested the 42IF106-RC from Mouser, which according to the datasheet has an inductance of 680uH (0.68mH), near enough to 0.6mH

The datasheet also states that the transformer ( 42IF106-RC ) is for use at 455kHz using a 180pF parallel tuning capacitor.

The circuit that you are working to has a 0.6mH inductor and a 300pF capacitor plus whatever capacitance of the test jig, the frequency can only go down when adding capacitance. (0.6mH and 300pF will give 377kHz before considering all the usual circuit stray capacitance etc).
Which is why I suggested an alternative oscillator circuit here:
A circuit which uses a single layer coil, which is easily wound to give the inductance you require and hence the frequency that you require.



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A capacitor marked 222 is 2200pF ie 2.2nF, not 222pF.
A good point.
But that 300pF cap across the 2.2k resistor is an odd one, it is effectively decoupling the output to the frequency counter, not what you really want.


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