Using Vintage Quartz Crystals in Transistor Circuits - Page 1
Forum section: electronics / rf-microwave

author: StuartA
author_rank: Regular Contributor
timestamp: September 27, 2024, 12:10:14 am
content: There was a British WWII wavemeter which contained 1MHz and a 100kHz crystal together in a single unit and naturally worked with a tube oscillator. I’ve been trying to get one of these to work in a transistor circuit (actually, a JFET), but have had not luck so far. However, WWII FT243’s work very well, and naturally, modern HC6U components are no problem. I also have a 1950's 100kHz crystal in a B7G envelope and that won't work with the transistor. As yet, I've not tried building a tube oscillator to try these items out, but I know the dual crystal was working when the unit was scrapped.Does any one have any experience of getting old crystals to start up in transistor circuits?

author: Xena E
author_rank: Frequent Contributor
timestamp: September 27, 2024, 12:27:38 pm
content: The xtals can just be bad.It seems that the schematic that you intended to post of your test circuit isnt showing. (Hint)Regards,X

author: StuartA
author_rank: Regular Contributor
timestamp: September 27, 2024, 10:49:00 pm
content: The main circuit I've been using was actually published in an ARRL handbook as part of universal transistor tester using a crystal oscillator circuit, I built it originally to test JFETs. So, it's the JFET circuit based around J2 in the attached schematic. I'm looking at other circuits but I'm not greatly optimistic.Yes, I appreciate the variability of old crystals; it might well be the case that If I had 100 of the dual units, then a handful might work, or none.

author: bd139
author_rank: Super Contributor
timestamp: September 27, 2024, 10:59:33 pm
content: Urgh yeah that's not going to work.Firstly the crystals work differently at that frequency. They are more "tuning forks". They require a lot less drive than that circuit will kick out. May have blown them up! I only ever got one circuit to work with a 100KHz one and that was a colpitts with very little feedback and a small series capacitor.Also as you noted it's difficult to work out if it's your circuit or the crystal that is knackered

author: StuartA
author_rank: Regular Contributor
timestamp: September 28, 2024, 06:36:22 am
content: Could you post that Colpitts?

author: RoGeorge
author_rank: Super Contributor
timestamp: September 28, 2024, 09:49:28 am
content: I don't know how the old 100kHz Quartz were made, but if you look for a very low power oscillator used in the nowadays 32876Hz tuning fork for Quartz clocks, I've just posted in another topic a link to an oscillator from Maxim:https://www.eevblog.com/forum/projects/looking-for-transistor-based-32khz-quartz-test-circuit/msg5652271/#msg5652271

author: G0HZU
author_rank: Super Contributor
timestamp: September 28, 2024, 05:31:38 pm
content: I assume this is for some form of crystal calibrator. If you want a simple, low parts count oscillator, then it should be possible to use a classic (process 50) JFET down at 100kHz or even 32.768kHz.It can be configured as a classic Colpitts oscillator (common drain) and ideally, you would need to know the motional resistance of the crystal (Rm). For a 32.768kHz crystal the Rm might be 40k ohm for example.Using a few basic equations it should be possible to predict the circuit values required in order for the circuit to start up. By adding a temporary sense resistor, it should also be possible to estimate the crystal drive current and drive power as long as you know Rm. This circuit can be arranged to deliver a comb of harmonics which would be useful if you want to use it as a crystal calibrator for a wavemeter.However, do you want to use the same oscillator circuit and simply switch between the 1MHz and 100kHz crystals? Or are you going to make two complete oscillator circuits up and switch between them?

author: edavid
author_rank: Super Contributor
timestamp: September 28, 2024, 05:56:14 pm
content: Quote from: bd139 on September 27, 2024, 10:59:33 pmThis is incorrect. Modern LF crystals are tuning forks as you describe, but vintage 100kHz crystals are AT cut and need high drive. (You can recognize them by their large physical size.)OP, you probably just need to increase the values of C1 and C2. Try making them 100X larger.QuoteNot if you have a network analyzer

author: bd139
author_rank: Super Contributor
timestamp: September 28, 2024, 06:27:58 pm
content: My bad. I stand corrected.

author: StuartA
author_rank: Regular Contributor
timestamp: September 28, 2024, 11:25:51 pm
content: @ G0HZU, I’d prefer just one oscillator, but if we are talking about just one transistor per, then I could live with two.@edavid, thanks, I’ll try increasing the value of those caps.

author: G0HZU
author_rank: Super Contributor
timestamp: September 29, 2024, 02:10:58 pm
content: I don't know if this helps, but if you have access to a decent signal generator that can tune through 100kHz in 1 Hz (or less) steps then I think you can do a quick and dirty health check on your 100kHz crystal.Assuming that your sig gen has a 50R source impedance then fit a series sense resistor of 4.7k at the output. Then set the level quite low to an EMF of maybe 20mV. Then connect the crystal as a shunt component as in the diagram below. Monitor the voltage across the crystal using a scope with a x10 probe. (10 meg ohm).If the crystal is healthy, you should see a slight dip in the voltage when the sig gen is tuned to the resonant frequency. The deeper the dip, the lower the ESR of the crystal. You need small step sizes if you want to find the deepest point. 1 Hz step sizes might be small enough. You may also find that the RF level will appear to bounce or 'ring' up and down as you find resonance. You will have to allow a second or so to let it settle in the null.A 20k ohm ESR gives a dip of just under 2dB as shown in the example below. I've had to guess the equivalent circuit of your 100kHz crystal, but the important thing is that the crystal should give a dip somewhere near 100kHz and the dip should probably be deeper than 1dB if the crystal has a healthy ESR. I'm guessing that a dead crystal will give no dip or a very shallow dip?Even if the ESR is higher than this, it should be possible to design an oscillator circuit to make it oscillate at 100kHz.

author: G0HZU
author_rank: Super Contributor
timestamp: September 29, 2024, 09:05:19 pm
content: I tried putting the 100kHz crystal model into a basic Pierce oscillator using a process 50 JFET. See the attached schematic and simulation below.The JFET could be a 2N3819, 2N5485, BF256B or a 2N4416(A) for example. These are all process 50 JFETs.See below for the schematic. This is powered from an 8V pulse that lasts for two seconds it and certainly seems to start up OK and is quite tolerant of value changes to R2 and C2. Your original circuit used a 2.2mH choke in the drain but this might not have enough inductance to act as a choke at 100kHz.So I throttled back the gm of the JFET by adding the 4700 ohm resistor and the 100nF cap at the source. This reduces the drain current to less than 0.5mA allowing a 3900R resistor to replace the choke.The 27pF cap could be replaced by a classic yellow 70pF trimmer cap and this could be adjusted to trim the frequency closer to 100kHz if it is a few Hz off frequency. I don't know if this circuit will work with a real 100kHz crystal, but I think it's going to be closer to a working circuit than the circuit you currently have. Your current circuit is optimised for operation up at 20MHz and not 100kHz.If the circuit below does work, then try playing with R2 and C2 to see if you can get it to work with both a 100kHz crystal and a 1MHz crystal.

author: StuartA
author_rank: Regular Contributor
timestamp: September 30, 2024, 08:34:08 am
content: @G0HZU Many thanks for that assessment; let me get back to you when I've tried it

author: StuartA
author_rank: Regular Contributor
timestamp: October 01, 2024, 05:53:03 am
content: Ok, with G0HZU's 'refined JFET oscillator' I now have the following xtals working;>> 1950's Marconi B7G 100kHz not previously working>> WWII 500kHz not previously working>> 2MHz HC6U working as beforeNot WorkingClass D wavemeter xtals.That's real progress. Now, the "WWII 500kHz" unit looks as if it could have come out of the same design house, and same factory as the Class D unit. When the Class D unit did not work last week, I opened and cleaned it as best I could (it's a bakelite housing sealed with cork gasket and wax and held together with 4 brass screws, but then FT243's are not much better and they seem to be happy old crystals!). I think we have to regard that unit as suspect now?Thanks J, will PM you later.