SI5351 as an oscillator

Thread Starter

8dm7bz

Joined Jul 21, 2020
199
Hello,
I try to use the si5351 as an oscillator with the sa612 (I attached the circuit below). I got myself a breakout board from adafruit for the si5351. IIf you have a look at the attached schematic you can see (on the left hand side) a 0V to 3.3V square wave voltage at 20kHz. That goes into a 220k resistor, because the sa612 expects a 200-300 mVpp signal.

Now I have tried two different implementations of V3 (signal source in the schematic). First I use a signal generator (Siglent SDG 1032X) with a high z output configured. I attached the waveform that appears after C1, called generator.png. Finally I read the analog signal with an ADS1115 breakout board into Arduino. That is shown in the picture called generator_reading.png. There is a curve, because I lifted the transmitter antenna. Anyways, thats the favoured output I want. That's the goal. Now I want to change the signal source, away from a signal generator, to a chip.
So I used the si5351 as a clock generator. I programmed it to generate a 20kHz square wave (0V - 3.3V). The same as the signal generator was set up to. The attached picture called si5351 is the signal after C1 (just as above). There is a difference in signal strength, which might or might not matter for the actual problem.
The real problem is the reading I get. The picture called si5351_reading.png shows part of a sine wave which is present even though I hold the transmitter rather steady. The sine wave has a period of 6 to 7 seconds, and doesn't change much to my movement.

My question is: Where does the sine wave come from ?, and how do I get rid of it ? I essentially want to mirror the behaviour of my function generator with this chip.

Note: If you have another chip recommendation, keep in mind that it would be favourable to produce 3 different independant signals (which can range between 20kHz and 100kHz) cheap. This chip can do 3 independant clock frequencies. I don't want to lose that capability, so I would love to hear suggestions on how to change the circuitry around it.

-8

EDIT: the s5351_reading.png is not the full picture. I couldn't capture the full wave, so it's only part of it. But is does get back up and down and so on.
EDIT2: The opamp used is an AD8615, that's what I had on my hands.
 

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bloguetronica

Joined Apr 27, 2007
1,467
I don't have much detail to go on, but what you are seeing is not a sine wave, but rather a square wave that was fed through an RC low pass filter. I would suspect C2 coupled with R2.

Also, what kind of op-amp is U1? I'm guessing that you are attempting to have unity gain with an op-amp that is not very stable at that configuration, and so you used C3 and R5 to increase its phase margin? Of course, probably the problem is not there, as I would point to C2 as the culprit, but I'm curious about your implementation.
 

Thread Starter

8dm7bz

Joined Jul 21, 2020
199
If you want to see something specific let me know. I edited my post on the bottom saying that one can't see the whole sine wave because I couldn't capture it. Sorry about that.

But anyways, let's say the problem is because of C2 and R2. How would that explain that it works with an external function generator and not with the si5351 ?

About the implementation: the opamp used is an AD8615, because that is what I had already available. And a lot of the rest of the implementation is discussed in this thred. I would like to keep this thread on topic please.

And to make it clear again: The circuit provided does work like I want it to, but only with an external function generator (as V3 in the schematic) and not with the si5351.
 

Thread Starter

8dm7bz

Joined Jul 21, 2020
199
I found the mistake. The problem is a small difference in frequency between the oscillator (sa612 - pin 6) and the received signal (sa612- pins 1, 2).
I simulated this and found that a frequency difference of 0.16 Hz would explain the about 6 seconds period of the sine wave. And I could also imagine that (although the si5351 and my function generator are both set to 20kHz) this difference is possible between those two sources. And my oscilloscope couldn't catch those low frequency differences either.

Thanks, case closed :D
 

Wolframore

Joined Jan 21, 2019
2,411
I've scanned the datasheet for si5351... did I read that correctly... synthesize up to 200 MHz with 0ppm error ? How is that possible or am I misunderstanding this value.
 

upand_at_them

Joined May 15, 2010
766
I think the "0ppm error" is a "calculation error", meaning that you can configure it for your exact frequency. In my brief use of these I have observed that they have some fixed error (that can be corrected for), they of course have temperature error (it uses a standard crystal), and there is jitter at high frequencies.
 
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