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Old 25th February 2002, 08:16 PM   #1
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Unhappy ECL oscillator just won't oscillate

I have been trying to build an ECL oscillator similar do the circuits described in the IQD crystal data book, by Winser and by Unruh (the latter two articles were on the ACG pages when they were still in service - what happenend to them?).
Basically, in this circuit the crystal is used in series resonant mode, it is connected from the non-inverting output to the non-inverting input, which in turn has a 15R load to AC ground. The rest is just for getting the DC setpoints right. The input is shorted to ground by the 15 R resistor for all frequencies except the series resonant mode where the crystal has nothing but its real impedance, which is around 10-20 R for HC-49 case devices.

As the 10216 ECL line receiver is not easily available, I used the single gate. 10H16 PECL line receiver. Apart from being faster (0.5 ns prop delay) which wouldn't hurt in this application, the chip is just the same.

Initially, the circuit would sometimes not start up, sometimes start up at the desired frequency, sometime start up in a spurious oscillation aroung 300-600 MHz. I tried various crystals, load resistors beween 10 and 39 R, loutput pull - downs between 100 and 200 R. No change.

The I found that the returns from the input and output loads had different vias to the ground plane just half an inch apart. I provided another wire bridge. The spurious oscillations were gone, but so was the fundamental oscillation. Even injecting a square wave signal couldn't humor the circuit into oscillation. Any ideas?


Frustated,

Eric
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Old 25th February 2002, 08:40 PM   #2
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Question ECL oscillator

Hi Eric,
First a important question : what is the frequency of the crystal you are using?
From about 20 Mhz and up crystals operate in the third overtone mode and from about 80 MHz in the fifth overtone mode. It also depends on the manufacturer. Especially the overtone crystals are very temperamental to put it mildly.
Personally I don't have experience with these ECL oscillators, but collected some 10116's in the shoebox.
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Old 26th February 2002, 07:52 AM   #3
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Hi Elso,

those were all fundamental mode crystals, I tried various specimen from 12 to 20 MHz. All worked nicely when inserted in the classic 74HCU04 circuit.

I followed your link to the Jung-like regulators. I think I have seen something similar in some AD app note, only that they took some great care to avoid oscillation.

Regards,

Eric
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Old 26th February 2002, 08:38 PM   #4
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Wink found the solution

Redid the circuit on a spare PCB, not having soldered in the ECL-to-TTL translator helped, too.

1) I removed the two low pass filters that are used in the IQD, Winser and Unruh circuits to derive the DC setpoint. Instead, I feed the inverting input directly from the V_BB pin, with a 100 nF to ground. That is the nice thing about using a line receiver gate, it actually provides the DC reference. Not sure that this is critical, but it sure reducec component count by two resistors and two capacitors.

2) Soldered the load resistor across from non-inverting to inverting input instead to ground. Should be the same, as the inverting input is also grounded, but sure removes a couple of mm worth of copper which will have some impedance.

After these alterations, the circuit worked fine with one of the more benign crystals, even with a 39 R input load resistor.

Having implemented these changes into the final board (which has the translator soldered in), it worked most of the time, but sometimes I got the spurios oscillation (150 to 400 MHz). Lowering the input load to 10 R helped. On start-up, the spurious oscillation lingered, but it would always give way to the crystal fundamental mode after a second.

I put in the 16.9344 crystal salvaged from a Sony player. Now I could trigger the fundamental mode only when I played with the scope probe. I lowered the load to 8.7 R - fundamental mode kicked in somewhat more easily. This gave me the clue: the circuit still had too much gain aroung 150 MHz. A 100 pF in parallel to the input load rolled of gain by 3 dB at 150 MHz. Now the fundamental is there right from the start

Will post the circuit some day when I get to draw it...

Eric
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Old 26th February 2002, 08:52 PM   #5
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Smile ECL oscillator

Hi Eric,
Glad to hear it is working now.
Don't use the three leaded crystal from the Sony. It is more like a crystal <B>filter</B> Use a "normal" 16.9344 Mhz crystal.
The tendency of your circuit to oscillate at a much higher frequency reminds me of my early attemps to build a oscillator with the LT1016. Too much drive on the crystal.
The KWAK-CLOCK uses gentle, minimal drive to the crystal resulting in much lower jitter and absolutely no spurious oscillations.
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Old 27th February 2002, 08:18 AM   #6
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Default three leads

Dear Elso,

in the three leaded crystals provided by IQD/C-MAC, the third lead is simply the case. This saves you having to solder a wire from the ground plane to the case.

There are three leaded little blue things, too. These are ceramic resonators which have much higher internal damping than crystals. The Q of a resonator is on the order of 1000 where a 16 MHz crytal in the large HC49 case will have about 80000. The center lead is also ground, but in addition there are two lag capacitors to each active pin of the resonator.

My ECL circuit uses the crystal in series resonance where its impedance becomes equal to its series resistance which is typically about 10-15 R. You want to provide a low impedance drive and load to keep the Q of the circuit. The drive into the crystal is about 900 mV with a square wave. On the load side, I get a beautiful 400 mV sine. In first order approximation, I am dissipating about 20 mW in the crystal. This may be to much as most crystals are specified at 1- 5 mW. That is why I am trying to find the article on the Winser clock implemented into a TEAC that was on the ACG pages. The author (Werdin?) did line out how to measure and calculate the crystal drive. If I don't find it, I'll have to do the math myself...

Your Quak clock appears to use parallel resonance. Here the resonator impedance becomes infinite. To preserve Q, you want to load the circuit as little as possible. The 10 M resistor looks fine. However, I am a little puzzled whether the 1 k source resistor won't introduce too much damping. What do you think? Also, what does the 10 pF in series with the crystal do? I assume it corrects the crystal frequency but this is really a circuit you would expect to find in a series resonant circuit.

What do you think?

Regards,

Eric
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Old 27th February 2002, 11:28 AM   #7
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Hi Eric,

Found in my archives the paper by Wedin. As I only got a paper version, I copy the whole paragraph.

Here we go

"The current through the crystal (and the load resistor) is the square root of the crystal's maximum permissible power dissipation divided by its internal series resistance, I=Pxs/Rxs. With values inserted, 0.5mW and 13 ohms, we get 6.2 mA. The voltage drop accross the crystal is the square root of the maximum permissible power dissipation multiplied by the crystal's internal series resistance, U=Pxs*Rxs. With values inserted we get 0.081 Vrms. The ECL-receiver (0.8Vpp or 0.28Vrms), Url=Uecl-Uxs. With values inserted we get 0.2 Vrms. Finally we get the value of the load resistance as the voltage drop of the load resistor divided by the current through it, Rl=Url/I. With values inserted we get 32 Ohms."

N.B. : 0.5 mW is the value of the max dissipation of Wedin's crystal, and 13 Ohms is the internal series resistance he had get, taking two thirds of the manufacturer's value (20 Ohms)

If you're interested, drop me an e-mail with your address, and I'll send a paper copy of the whole paper.

Elso,

I'm about to test your clock with both the fast comparator and the mosfet we talked about. I have to finish PCB design, but in a week or two, I should have some results to post here.

Cheers
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Old 27th February 2002, 12:43 PM   #8
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Default thanxxx!

Hi Francois,

thanks for copying that paragraph. That will certainly help me. I think I still have a paper copy, but it is still in one of countless not yet unpacked boxes (moved two months ago).

From the literature I have found on oscillators, I guess the subject of how to make a very high Q oscillator has not been delved into. Some authors appear even to be confused whether they use parallel or series resonance.

Where is Limoges? I think I have been there.

Greetings,

Eric
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Old 27th February 2002, 01:22 PM   #9
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Default did the math

Well, I did a few quick calculations. I am dissipating about 2 mW, the crystal is presumably specified for 1 mW.

However, I am not sure that that part is entirely correct. First of all, it assumes everything to be in phase. I guess, in series resonance this is a pretty accurate guess. I will check, though...

However, I am not happy with the treatment of the dissipation. After all, the crystal is driven by a square wave (at least with a fast ECL unit), and it outputs a sine. I thought he commented on that, I think. Do you recall what the article was called?

I am beginning to think about some improvements. For example, the resistive load might be replaced by an LC series resonant circuit. Also, the ECL line receiver could be built in a discrete circuit. This way, it would be possible to limit the drive and still maintain a low impedance load.

Greetings,

Eric
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Old 27th February 2002, 02:25 PM   #10
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Default If it can help...

Hi back,

I dunno if you already know it, but a lot of interesting informations can be found at

http://www.telequarz.de/info/info_q.html

(this is the direct link, but can be reached in a regular way to see all other docs : http://www.telequarz.de/index_e.html )

And, lucky guy, there's a nice book on quartz and oscillators, and IT'S IN GERMAN !!

Most instersting chapters (IMHO, and if I may guess from my poooooooor german) are chapter 2 (mostly theoretical) and chapter 6, with oscillators analysis.

Chapter 2 is summed up in the first pdf than can be downloaded.

Wedin's article is called "Re-clocking TEAC VRDS-T1 and TEAC VRDS-7"

And, oh, Limoges is 300km south from Paris, near the center of France
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