Valve Crystal Oscillator Grid Current

[John M0GLN]

As Lawrence says, an interesting experiment, the circuit itself is very simple to build and I've got a few FT-243 crystals, but unfortunately I've no means of measuring the r.f currents or voltages around the grid circuit, I've got a Hitachi 20MHz scope but from past experience when I go anywhere near an oscillator with the probe it just stops oscillating.

Al, if you do get around to building one and need anymore info' from the J Douglas Fortune book please let me know and I'll scan the relevant pages.

John

[pwdrive]

John, as a matter of interest is the transmitters output frequency the same as the xtal's frequency?
Any chance that the component values etc can be posted on here?

EDIT: BTW I've posted the xtal current question on another forum, I'll see what turns up.

Lawrence.

[Skywave]

Hmm, yes: the Q. in your last paragraph passed through my mind too.

Perhaps like many others, over the years, I too have built valve-based crystal oscillators - usually from published designs. But I've never had cause - or the inclination - to investigate all the details of their operation: an output was obtained which met the design requirement √. So hopefully this thread will motivate me into redressing that omission.

Al.

[pwdrive]

Me neither Al, In a basic sense I know what a xtal does and what to expect of the circuit it's used in, that's about it.

Lawrence.

[Skywave]

Very succinctly put √. And that has inspired me to produce another one-liner for the Quip of the Week (or should that be 'the weak'? )

Al.

[John M0GLN]

John, as a matter of interest is the transmitters output frequency the same as the xtal's frequency?
Any chance that the component values etc can be posted on here?

EDIT: BTW I've posted the xtal current question on another forum, I'll see what turns up.

Lawrence.

Yes for the first question, he states 'Crystal, select for the desired amateur band', so it seems he is just using the fundamental frequency.

I've attached the whole circuit again, this time with the component values, also attached are coil details for L1, L2 is the same except for a centre tap, but I don't suppose anyone will want to build the second stage.

It will be interesting to see if anyone from another forum can shed some light on this subject.

John

   
   
   

[pwdrive]

Yes, thanks for the details John, as said, I posted the question on another forum (not UKVRR)

Lawrence.

[Skywave]

Ref. post 33.
Experimental crystal oscillator built and measurements made.

The crystal itself is an ex-WD 500 Kc/s unit: it's oldest one I could find in my stock. I have no idea of the cut of this crystal, but I doubt very much that it is a AT or BT cut: this crystal is too old for those types of cuts; it might be an X-cut. Here is a close-up of that crystal unit:

       

First attempts were with a triode oscillator: screen grid strapped to anode. But I could not make that arrangement stable without the crystal installed. So I configured it as a tetrode with a by-passed 270 Ω resistor in the cathode: this resistor is simply there to protect the valve if the cct. fails to oscillate. This tetrode configuration was unconditionally stable - so measurements were made - see attached drawing.
When the 'scope probe was attached to the crystal current sensing resistor, it was necessary to slightly adjust the anode tuning capacitor to compensate - this is to be expected, of course.

   

The 10 Ω resistor in series with the crystal is only for the purposes of indirectly measuring the r.f. current through the crystal.

As for the assembly - it'll win no prizes in a constructor's competition - but it's not meant to! This is obviously a 'one-off':

           

The brown knob (seen in picture 1 above) is simply there as a convenient way to adjust the anode tuning capacitor.

For the HT @ 200-v., the r.f. voltage across the 10 Ω crystal current sensing resistor was approx. 26 mv. p-p (9.3 mv. rms, approx.). Hence, the r.f. current through the crystal is in the region 1 mA. (Note: the r.f. resistance of the crystal alone will be in the region of 100 ~ 200 Ω: that's why a value of 10 Ω was chosen for the sensing resistor).

All r.f. waveforms were measured with a 300 MHz Tektronix 2465 'scope plus low capacitance X10 probe (7 pF); all d.c. voltage were made with a Fluke 77 D.M.M.

   

Al. / Sept. 7, 2014 //

[John M0GLN]

That'a excellent Al, thank you very much for going to the trouble of setting it all up, but of course the result is probably what you would have thought it should be before I posted my enquiry, the very low mA range, the puzzle continues.

Thanks

John

[AlanBeckett]

That's splendid young Al, and in many ways a relief.
Having 40mA of anything through the Crystal just didn't seen 'right'.

Alan