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Norcal 40a (testing 4.9MHz xtals)

 

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Hi Ram,

Do you have an oscilloscope? If so look at the waveform from your crystal test oscillator--I bet it's more of a square wave than a sine wave and if so your counter is looking at the stronger 3rd harmonic component and reading that instead of the fundamental.

73,

Steve AA7U

On 11/11/2019 9:52 AM, Ramakrishnan Muthukrishnan wrote:

I am very early into the project. As a first step, I thought of building a G3UUR crystal tester to select crystals for the filter and the oscillators. I have got about 100 4.9152 MHz crystals that I got from aliexpress. Last weekend I built the G3UUR Colpitts Oscillator (straight out of the EMRFD book and also described in many other webpages). I also have a counter with a 1 Hz resolution, again obtained from Aliexpress specifically to hook up into the Oscillator.

Unfortunately, when I plug in the 4.9152 MHz crystal, I get a reading of 14.742475. Looks like it is 3x the frequency of the crystal. I tested many from the lot of 100 crystals, which came in lots of 50 from two different vendors and they all read the same. I had a 28.04 MHz crystal which seem to be oscillating at the correct frequency. I don't know what to make out of these readings. Is the crystal, a 3rd overtone crystal? Or is it that the colpitts oscillator is designed to operate at a higher frequency and the 3rd harmonic is where the tuned circuit that comprise the crystal (which is inductive?) and the two capacitors are getting tuned?

73
--
Ram VU3RDD
 

I've sometimes seen counters display harmonics. It could be a matter of overloading the counter. Try attenuating the signal to the counter a bit, possibly just be using a very small coupling capacitor.

73-

Nick, WA5BDU

On Mon, Nov 11, 2019 at 11:19 AM Steve Ratzlaff <ratzlaffsteve@...> wrote:

Hi Ram,

Do you have an oscilloscope? If so look at the waveform from your crystal test oscillator--I bet it's more of a square wave than a sine wave and if so your counter is looking at the stronger 3rd harmonic component and reading that instead of the fundamental.

73,

Steve AA7U

On 11/11/2019 9:52 AM, Ramakrishnan Muthukrishnan wrote:
I am very early into the project. As a first step, I thought of building a G3UUR crystal tester to select crystals for the filter and the oscillators. I have got about 100 4.9152 MHz crystals that I got from aliexpress. Last weekend I built the G3UUR Colpitts Oscillator (straight out of the EMRFD book and also described in many other webpages). I also have a counter with a 1 Hz resolution, again obtained from Aliexpress specifically to hook up into the Oscillator.

Unfortunately, when I plug in the 4.9152 MHz crystal, I get a reading of 14.742475. Looks like it is 3x the frequency of the crystal. I tested many from the lot of 100 crystals, which came in lots of 50 from two different vendors and they all read the same. I had a 28.04 MHz crystal which seem to be oscillating at the correct frequency. I don't know what to make out of these readings. Is the crystal, a 3rd overtone crystal? Or is it that the colpitts oscillator is designed to operate at a higher frequency and the 3rd harmonic is where the tuned circuit that comprise the crystal (which is inductive?) and the two capacitors are getting tuned?

73
--
Ram VU3RDD
 

Hi Steve,

I will have a look at the waveform this weekend and report back. Thanks.

73
Ram VU3RDD

On Mon, Nov 11, 2019, at 10:49 PM, Steve Ratzlaff wrote:

Hi Ram,

Do you have an oscilloscope? If so look at the waveform from your crystal test oscillator--I bet it's more of a square wave than a sine wave and if so your counter is looking at the stronger 3rd harmonic component and reading that instead of the fundamental.

73,

Steve AA7U
 

That was a possibility I totally overlooked, Nick. Let me try that this weekend. Thanks.

Ram

On Tue, Nov 12, 2019, at 5:04 AM, Nick Kennedy wrote:
I've sometimes seen counters display harmonics. It could be a matter of overloading the counter. Try attenuating the signal to the counter a bit, possibly just be using a very small coupling capacitor.

73-

Nick, WA5BDU

On Mon, Nov 11, 2019 at 11:19 AM Steve Ratzlaff <ratzlaffsteve@...> wrote:

Hi Ram,

Do you have an oscilloscope? If so look at the waveform from your crystal test oscillator--I bet it's more of a square wave than a sine wave and if so your counter is looking at the stronger 3rd harmonic component and reading that instead of the fundamental.

73,

Steve AA7U

On 11/11/2019 9:52 AM, Ramakrishnan Muthukrishnan wrote:
I am very early into the project. As a first step, I thought of building a G3UUR crystal tester to select crystals for the filter and the oscillators. I have got about 100 4.9152 MHz crystals that I got from aliexpress. Last weekend I built the G3UUR Colpitts Oscillator (straight out of the EMRFD book and also described in many other webpages). I also have a counter with a 1 Hz resolution, again obtained from Aliexpress specifically to hook up into the Oscillator.

Unfortunately, when I plug in the 4.9152 MHz crystal, I get a reading of 14.742475. Looks like it is 3x the frequency of the crystal. I tested many from the lot of 100 crystals, which came in lots of 50 from two different vendors and they all read the same. I had a 28.04 MHz crystal which seem to be oscillating at the correct frequency. I don't know what to make out of these readings. Is the crystal, a 3rd overtone crystal? Or is it that the colpitts oscillator is designed to operate at a higher frequency and the 3rd harmonic is where the tuned circuit that comprise the crystal (which is inductive?) and the two capacitors are getting tuned?

73
--
Ram VU3RDD

--
Ramakrishnan
 


On Tue, Nov 12, 2019, at 5:04 AM, Nick Kennedy wrote:
I've sometimes seen counters display harmonics. It could be a matter of overloading the counter. Try attenuating the signal to the counter a bit, possibly just be using a very small coupling capacitor.

Turned out that the berg connector that the counter vendor supplied had a faulty connector. A friend of mine had the same counter and with that counter the oscillator worked just fine.

Another observation with the scope is that when the counter is connected, the output waveform as shown in the oscilloscope is severely distorted. Perhaps the counter does have a low input impedance?

In any case, I can start the process of measuring the motion parameters of the crystals now for the Norcal 40a build.

73
Ram VU3RDD
 

It's good that you've found the problem. The issue with the counter affecting the source reminded me of what I saw in a counter I got from eBay. From my notes:

"Hey, I noticed this after connecting it to my 10.000000 TXCO from HamCom 6/19/15:? The output of my oscillator dropped from 10 dBm to 9 dBm, so the counter is loading it slightly.? Also, I turned on the scope and the waveform was somewhat distorted.? When I unhooked the counter, power returned to 10 dBm and the sine wave was clean.? To monitor without disturbing, an isolation circuit might be needed."

Also, from the technical information in the eBay ad:

"2 Measurement channels (channels are low impedance)"

So some kind of buffer on the input, presenting a high impedance to the tested circuit might be a good?idea.

73-

Nick, WA5BDU


On Tue, Nov 26, 2019 at 11:50 PM Ramakrishnan Muthukrishnan <ram@...> wrote:

On Tue, Nov 12, 2019, at 5:04 AM, Nick Kennedy wrote:
I've sometimes seen counters display harmonics. It could be a matter of overloading the counter. Try attenuating the signal to the counter a bit, possibly just be using a very small coupling capacitor.

Turned out that the berg connector that the counter vendor supplied had a faulty connector. A friend of mine had the same counter and with that counter the oscillator worked just fine.

Another observation with the scope is that when the counter is connected, the output waveform as shown in the oscilloscope is severely distorted. Perhaps the counter does have a low input impedance?

In any case, I can start the process of measuring the motion parameters of the crystals now for the Norcal 40a build.

73
Ram VU3RDD

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