EMRFD Message Archive 4137

Message Date From Subject
4137 2010-02-01 11:49:43 Robert Cerreto Crystal Test Set Designed by Chris Trask
I need some help,
 
I built the above Crystal Test set designed by Chris Trask. The only thing different were the transistors. I used 2N4401's for Q1 &Q2 and 2N4403's for Q4, Q5 and Q6. T1 is a T-50-43 core with 14t/7t windings. I cannot get the oscillator to start with any crystal. If you watch the oscillator output with a o'scope you can see that it tries to start, but, oscillations quickly die out.
 
Would any of youse have an idea what may be wrong? You can find this test set on Chris Trask's website.
 
TNKS ES 73, Bob WA!FXT




[Non-text portions of this message have been removed]
4138 2010-02-01 13:10:41 Chris Trask Re: Crystal Test Set Designed by Chris Trask
>
> I need some help,
>
> I built the above Crystal Test set designed by Chris Trask. The only thing
> different were the transistors. I used 2N4401's for Q1 &Q2 and 2N4403's
for
> Q4, Q5 and Q6. T1 is a T-50-43 core with 14t/7t windings. I cannot get the
> oscillator to start with any crystal. If you watch the oscillator output
> with a o'scope you can see that it tries to start, but, oscillations
quickly
> die out.
>
> Would any of youse have an idea what may be wrong? You can find this test
set
> on Chris Trask's website.
>

Let's see if we can somehow diagnose what the problem is:

1. With no crystal in the test socket, measure the voltage across R7.

2. If there is no voltage present, then the problem is with the AGC
circuit.

3. If voltage is present, hook a signal generator to the R12 side of
the crystal socket. Test V1 to make sure that the generator
signal
is present.

4. Test V2 and compare the level with V1. If it is lower, then there
is insufficient gain in the open loop.

Chris

,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
_
4141 2010-02-01 16:49:15 Phil Sittner Re: Crystal Test Set Designed by Chris Trask
Robert-

I'm curious as to how you resolve this for I also built (twice just to make sure I didn't have a layout problem) and had identical results. Chris's response is very similar to the one I received and perhaps the culprit is the gain of the transistors used(?). In any event, I'm interested in how you get it up and running.

regards,
Phil kd6rm
----- Original Message -----
4143 2010-02-02 02:13:54 drmail377 Re: Crystal Test Set Designed by Chris Trask
So Phil, did you ever get you're build of the test set working in the end?

David

4144 2010-02-02 02:26:45 Weddig, Henning-C... Re: Crystal Test Set Designed by Chris Trask
Chris and other builders,

I also ran into difficulties when building the x-tal tester. The reason
was a too low gain! Therefore I modified the value of load
resistor/choke at the output of the differential pair.
In order to achieve a "good" phase shift I am using RF transistors i.e.
BF254 for the differential pair.
What I still should do on a version 2 of my design is to simulate the
phase shift of the active part and even measure it using a (calibrated)
VNA.

I have put series resistors to the base of each buffer stage.
It is commonly known (and can be shown by calculation, this reminds me
to a course in my university student life!) that a capacitively loaded
emitter follower can oscillate due to its negative input impedance.
I have uploaded my schematic to the files folder.

Regards

Henning Weddig
DK5LV
4145 2010-02-02 05:59:52 Phil Sittner Re: Crystal Test Set Designed by Chris Trask
David-

Close, but no cigar. The best performance I was able to achieve was the oscillator starting, and then I would watch the level sag, and then the wave would dampen. The oscillator would then again start up and repeat the process. Frankly, I was a little embarrassed by not being able to get it to run yet it now appears that I have company. What about you, have you been able to get it running properly?

regards,
Phil kd6rm
----- Original Message -----
4146 2010-02-02 06:25:49 Chris Trask Re: Crystal Test Set Designed by Chris Trask
>
>I also ran into difficulties when building the x-tal tester. The reason
>was a too low gain! Therefore I modified the value of load
>resistor/choke at the output of the differential pair.
>

I suspected that might be the case when this problem first occurred, however the one fellow here tells me that the AGC voltage is clamped low, which says that there is a problem with the AGC loop and not the open-loop gain. Adding a resistor to force the AGC loop integrator to go high did not remedy the situation, so we're looking at a few other potential sources of trouble.

>
>In order to achieve a "good" phase shift I am using RF transistors i.e.
>BF254 for the differential pair.
>

I have suggested to others that they use the MPS6521 and MPS6523 in place of the present transistors. They have twice the gain while retaining most of the other parameters of the 2N4123 and 2N4125.

>
>What I still should do on a version 2 of my design is to simulate the
>phase shift of the active part and even measure it using a (calibrated)
>VNA.
>

I would like to see what you end up with as it would be nice to extend the performance to more easily accomodate 20+ MHz fundamental crystals. I didn't pursue this as what I came up with suited my immediate needs for designing crystal filters.

>
>I have put series resistors to the base of each buffer stage.
>It is commonly known (and can be shown by calculation, this reminds me
>to a course in my university student life!) that a capacitively loaded
>emitter follower can oscillate due to its negative input impedance.
>I have uploaded my schematic to the files folder.
>

That's a very worthwhile addition to the basic circuit.


Chris Trask
N7ZWY / WDX3HLB
Senior Member IEEE
http://www.home.earthlink.net/~christrask/
4147 2010-02-02 06:42:47 Chris Trask Re: Crystal Test Set Designed by Chris Trask
>
> Close, but no cigar. The best performance I was able to achieve
> was the oscillator starting, and then I would watch the level
> sag, and then the wave would dampen. The oscillator would then
> again start up and repeat the process. Frankly, I was a little
> embarrassed by not being able to get it to run yet it now appears
> that I have company. What about you, have you been able to get
> it running properly?
>

You hadn't told me about that part of the problem, or perhaps I missed it while focused on the other aspects. It's motorboating, which means that the AGC time constant is either too low or too high. Check the value of R2 and C3, then decrease one of them to see if the rate of the blocking oscillation changes.


Chris Trask
N7ZWY / WDX3HLB
Senior Member IEEE
http://www.home.earthlink.net/~christrask/
4148 2010-02-02 07:11:04 k5nwa Re: Crystal Test Set Designed by Chris Trask
At 08:25 AM 2/2/2010, you wrote:
>
>
>I have suggested to others that they use the MPS6521 and MPS6523 in
>place of the present transistors. They have twice the gain while
>retaining most of the other parameters of the 2N4123 and 2N4125.
>
>
>Chris Trask
>N7ZWY / WDX3HLB
>Senior Member IEEE

I'm always looking for good devices to add to my stock, so what if
anything makes this transistors special compared to others garden
variety such as 2N4401/03, BC550/560?

Their high gain, linearity, Ft?

Thanks


Cecil
K5NWA
www.softrockradio.org www.qrpradio.com

"Blessed are the cracked, for they shall let in the light."
4149 2010-02-02 07:21:43 Chris Trask Re: Crystal Test Set Designed by Chris Trask
>
>At 08:25 AM 2/2/2010, you wrote:
>>
>>
>>I have suggested to others that they use the MPS6521 and MPS6523 in
>>place of the present transistors. They have twice the gain while
>>retaining most of the other parameters of the 2N4123 and 2N4125.
>>
>>
>>Chris Trask
>>N7ZWY / WDX3HLB
>>Senior Member IEEE
>
>I'm always looking for good devices to add to my stock, so what if
>anything makes this transistors special compared to others garden
>variety such as 2N4401/03, BC550/560?
>
>Their high gain, linearity, Ft?
>

Higher gain alternative for the 2N2222/2N2907 and 2N4401/2N4403, but with a lower fT and higher NF. They would increase the gain margin of the test set.


Chris Trask
N7ZWY / WDX3HLB
Senior Member IEEE
http://www.home.earthlink.net/~christrask/
4160 2010-02-02 17:03:43 Phil Sittner Re: Crystal Test Set Designed by Chris Trask
Chris-

I'm anxious to revisit this issue and see if I can get it running properly. Thanks for the tip and I'll also check the details you provided to wa1fxt.

Phil
----- Original Message -----
4162 2010-02-02 18:41:57 Phil Sittner Re: Crystal Test Set Designed by Chris Trask
Henning-

Comparing your revisions to Chris's original circuit leads me to ask a
couple of questions only because I'm trying to understand why I couldn't get
the circuit to maintain oscillation.
(I'll list Chris's component designation followed by yours in parenthesis)

1. R8 (R7) change from 100 ohm to 330 ohm. Choke seems to have remained the
same. Is this the principal reason for increased gain?
2. R4 (R8) change from 1.0k to 10 k. What does this do?
3. C10 (C8 + C9) do you now make three measurements instead of two? If so
how have you modified the equations?
4. Q5,Q6 (T5,T6) You explained the addition of base resistors to suppress
parasitic oscillations. Is that why you added the coupling capacitors and
bias networks?
5. Q1,Q2,Q3 (T1,T2,T4) Is the BF254 a hotter transistor than the 2N4123
specified?

There is so much to learn and so little time. This is an interesting
exercise and I'm anxious to compare the results of Chris's circuit with some
others. My experience in attempting to measure crystal parameters has been
most frustrating because of poor correlation between a variety of methods.
I hope the questions aren't too big a burdon.

regards,
Phil kd6rm
----- Original Message -----
4165 2010-02-02 23:02:57 drmail377 Re: Crystal Test Set Designed by Chris Trask
I haven't tried yet. Watching the progress of others (or lack of) far more advanced than me leads one to believe the effort may prove to be a waste of time.

I'll keep watching though.

4168 2010-02-03 05:36:34 Chris Trask Re: Crystal Test Set Designed by Chris Trask
>
> I haven't tried yet. Watching the progress of others (or lack of)
> far more advanced than me leads one to believe the effort may
> prove to be a waste of time.
>

Not necessarily. I've heard from dozens of people who have built one with no trouble at all.


Chris Trask
N7ZWY / WDX3HLB
Senior Member IEEE
http://www.home.earthlink.net/~christrask/
4207 2010-02-05 19:58:59 Phil Sittner Re: Crystal Test Set Designed by Chris Trask
Chris-

I found the cause of my problems with the test set-a poor choice of transistors. I revisited my breadboard and used a CA3086 array and your circuit performed flawlessly. It's pretty obvious to me that there just wasn't sufficient gain for the transistors I tried. The issue I had with the circuit starting and then dying out is consistent as well. The CA3046 is available from Jameco for less than a buck , the CA3086 is lesss than $2, and either seems to be a good choice. Thanks again for the support you gave me throughout this effort and keep up the good work.

Phil kd6rm
----- Original Message -----