EMRFD Message Archive 9390

Message Date From Subject
9390 2013-11-19 11:13:20 mm0gyx About fundamental xtals
I built a quick test circuit using a 602 as the oscillator and connected some 20MHz xtals I acquired. I don't know if they are fundamental or not, so after picking a couple of capacitors (33pf) I tuned the main rig to 20MHz, and there it was, or thereabouts.

I'm learning as I go but I think given my simple set up these must be fundamental xtals, I'd expect to see a much lower frequency if they were overtones, what do you think?

73,

Ian mm0gyx
9391 2013-11-19 11:18:57 William Carver Re: About fundamental xtals
You got it right, Ian.
W7AAZ

On Mon, 2013-11-18 at 12:46 -0800, ian@michaelwatson.freeserve.co.uk
wrote:
>
> I built a quick test circuit using a 602 as the oscillator and
> connected some 20MHz xtals I acquired. I don't know if they are
> fundamental or not, so after picking a couple of capacitors (33pf) I
> tuned the main rig to 20MHz, and there it was, or thereabouts.
>
> I'm learning as I go but I think given my simple set up these must be
> fundamental xtals, I'd expect to see a much lower frequency if they
> were overtones, what do you think?
>
> 73,
>
> Ian mm0gyx
>
>
>
9392 2013-11-19 17:31:25 Ashhar Farhan Re: About fundamental xtals
I have been playing with 20 mhz crystals too. They are fundamental
mode devices. I got them from the local market. The ones marked KDS
are pretty inferior to those marked MEC in terms of their accuracy and
Q.
The amazing part about them is their swing. On almost all of them I
was able to pull them from 20.010 to 19.985 - 35 KHz. The circuit used
a 3uH coil and a 22pf in series with the crystal.
I will have to check the motional parameters from the lab notes. I can
post them later if needed. But a QER SSB filter built with works with
50 ohms termination. I have used this in my new transciever.
- farhan

On 11/20/13, William Carver <bcarver@safelink.net> wrote:
> You got it right, Ian.
> W7AAZ
>
> On Mon, 2013-11-18 at 12:46 -0800, ian@michaelwatson.freeserve.co.uk
> wrote:
>>
>> I built a quick test circuit using a 602 as the oscillator and
>> connected some 20MHz xtals I acquired. I don't know if they are
>> fundamental or not, so after picking a couple of capacitors (33pf) I
>> tuned the main rig to 20MHz, and there it was, or thereabouts.
>>
>> I'm learning as I go but I think given my simple set up these must be
>> fundamental xtals, I'd expect to see a much lower frequency if they
>> were overtones, what do you think?
>>
>> 73,
>>
>> Ian mm0gyx
>>
>>
>>
>
>
>

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9394 2013-11-19 20:10:40 ektpuah Re: About fundamental xtals
Dear Farhan, Can you please provide more details? best regards James Puah 9V1JP
9395 2013-11-19 20:11:28 mm0gyx Re: About fundamental xtals

Oh, great! Thanks William. I didn't know what value capacitors to use for the divider, tried 350pf, didn't hear anything, tried 33pf and heard it. I don't know why though, so that's my next little thing to work out.


I think a 33pf capacitor yields about 240 ohms capacitive reactance, so I guess the capacitors represent a potential divider between base and emitter with a tap between the 2 to re-energise the crystal. When I used the 350pf the reactance was 22 ohms, maybe this value's too small. There must be a rule of thumb for the values of the 2 based

9396 2013-11-19 21:01:36 William Carver Re: About fundamental xtals
When the capacitors get too big, their reactance is too small for the
oscillator to work.

An oscillator designed for 20 MHz would have divider capacitors about
1/3 as big as a 6.7 MHz oscillator. 1/4 as big as a 5 MHz oscillator,
etc. So one could take a magazine/book oscillator with a specified
frequency and proportion its capacitors for 20 MHz.

One might well find an oscillator that's made for mid-HF use....5-9 MHz
or something like that. In which case I'd go for 1/3 sized capacitors.

W7AAZ




9398 2013-11-20 21:31:18 mm0gyx Re: About fundamental xtals

 OK farhan, interesting. I am working towards a 12m cw rig for sota, I'm planning

9399 2013-11-20 23:41:38 Ashhar Farhan Re: About fundamental xtals
here are not lab notes:
20 MHz xtal characterisation
freq shift  ~ 4 KHz @ 20 Mc Fc
Cm = 22 pf x 2 x (4/20,000) = 0.008pF = 8 fF
Lm = 7923uH = 0.007H

Using ARRL Handbook 2011, section 11.23 to design a QER filter >
C = (fo . Cm)/(Bw * K23)
K12 = 0.7394, K23 = 0.5346, Order is 8, q = 1.2532
C = 28 pf for 3 KHz bandwidth
Rt = 70 !! (the exclamations are there in the hand written notes)

QER, measured loss = 2.8db, 6db measured bandwidth = 3.4KHz at 50 ohms termination


9400 2013-11-21 06:50:16 William Carver Re: About fundamental xtals
On Thu, 2013-11-21 at 13:11 +0530, Ashhar Farhan wrote:
>
> here are not lab notes:
> 20 MHz xtal characterisation
> freq shift ~ 4 KHz @ 20 Mc Fc
> Cm = 22 pf x 2 x (4/20,000) = 0.008pF = 8 fF
> Lm = 7923uH = 0.007H

Hi Farhan,

Lm = .008H ?????

W7AAZ
9402 2013-11-21 17:12:33 Ashhar Farhan Re: About fundamental xtals
Bill,
That's right. But it is an error from my lab notes, not a typo. I
guess I will have to move down the caps...
- farhan

On 11/21/13, William Carver <bcarver@safelink.net> wrote:
>
>
> On Thu, 2013-11-21 at 13:11 +0530, Ashhar Farhan wrote:
>>
>> here are not lab notes:
>> 20 MHz xtal characterisation
>> freq shift ~ 4 KHz @ 20 Mc Fc
>> Cm = 22 pf x 2 x (4/20,000) = 0.008pF = 8 fF
>> Lm = 7923uH = 0.007H
>
> Hi Farhan,
>
> Lm = .008H ?????
>
> W7AAZ
>
>
>
>

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