xtal tank circuit - working out values? - UK Vintage Radio Repair and Restoration Discussion Forum

Al (astral highway) · 10th Jan 2008, 5:41 pm

Hi,

A vintage xtal oscillator in a siggy has a trimmer cap and inductance in parallel its anode circuit: a tank circuit, in other words.

This is a manufacturer's mod. since the original design (but still a vintage mod.)

I know it greatly improves selectivity if this circuit is tuned to the resonant frequency of the crystal (and so the tuneable inductance/ cap combo needs to broadly match the crystal fundamental if that's swapped for one much lower or higher) but can someone talk me through the benefit of having this refinement in a very low power xtal oscillator?

The manufacturer obviously thought it worthwhile. I'm just trying to catch up with their thinking

Sean Williams · 10th Jan 2008, 6:41 pm

It is probably more likeley that a tuned anode circuit is used to pick a certain frequency - either fundamental, or harmonic, you also get a much higher output at fundamental frequency by using a tuned anode load, as opposed to an aperiodic design.

Cheers
Sean

kalee20 · 11th Jan 2008, 12:22 pm

I'm assuming the crystal is in the grid circuit if the tuned LC circuit is in the anode. Strictly, the anode circuit has to be slightly off-tune (!) for the circuit to oscillate - a bit higher than the oscillation frequency, so that at the operating frequency it behaves like an inductive reactance. The mistuning is only of the nature of 1% or so.

When you adjust the anode circuit, you get a really weird effect. Assuming the circuit is oscillating, as you bring the anode circuit in tune, the amplitude smoothly increases and abruptly stops. There is no defined maximum! I made a crystal oscillator using an 11MHz crystal and an ECC82 triode with a tuned anode, and I've seen it!

The best setting is not too near the cut-off point, so that small changes, aging, temperature etc do not take the alignment past the cut-off point.

And yes, you can use this circuit to oscillate at a crystal overtone frequency - although I've never done that.

Al (astral highway) · 11th Jan 2008, 1:35 pm

Quote:

I'm assuming the crystal is in the grid circuit if the tuned LC circuit is in the anode.

Yes, that's right.

Quote:

The best setting is not too near the cut-off point, so that small changes, aging, temperature etc do not take the alignment past the cut-off point.

Thanks -- this is really helpful

10th Jan 2008, 5:41 pm	#1
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	xtal tank circuit mfrs mod -- why?? Hi, A vintage xtal oscillator in a siggy has a trimmer cap and inductance in parallel its anode circuit: a tank circuit, in other words. This is a manufacturer's mod. since the original design (but still a vintage mod.) I know it greatly improves selectivity if this circuit is tuned to the resonant frequency of the crystal (and so the tuneable inductance/ cap combo needs to broadly match the crystal fundamental if that's swapped for one much lower or higher) but can someone talk me through the benefit of having this refinement in a very low power xtal oscillator? The manufacturer obviously thought it worthwhile. I'm just trying to catch up with their thinking __________________ Al Last edited by Al (astral highway); 10th Jan 2008 at 5:55 pm.

10th Jan 2008, 6:41 pm	#2
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Re: xtal tank circuit - working out values? It is probably more likeley that a tuned anode circuit is used to pick a certain frequency - either fundamental, or harmonic, you also get a much higher output at fundamental frequency by using a tuned anode load, as opposed to an aperiodic design. Cheers Sean __________________ Engineers make things work and have spare bits when finished

11th Jan 2008, 12:22 pm	#3
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: xtal tank circuit - working out values? I'm assuming the crystal is in the grid circuit if the tuned LC circuit is in the anode. Strictly, the anode circuit has to be slightly off-tune (!) for the circuit to oscillate - a bit higher than the oscillation frequency, so that at the operating frequency it behaves like an inductive reactance. The mistuning is only of the nature of 1% or so. When you adjust the anode circuit, you get a really weird effect. Assuming the circuit is oscillating, as you bring the anode circuit in tune, the amplitude smoothly increases and abruptly stops. There is no defined maximum! I made a crystal oscillator using an 11MHz crystal and an ECC82 triode with a tuned anode, and I've seen it! The best setting is not too near the cut-off point, so that small changes, aging, temperature etc do not take the alignment past the cut-off point. And yes, you can use this circuit to oscillate at a crystal overtone frequency - although I've never done that.