25MHz crystal oscillator output quality - is this typical? - Page 1
Forum section: electronics / projects

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 16, 2021, 01:53:18 am
content: Hi,I'm putting an AD9959 4-channel DDS board through its paces and have a question about the output quality of the onboard 25MHz crystal oscillator. The attached image shows the quality. Is this typical for what I suspect is a very cheap non-brand Chinese component and could I replace it with something better? I'm seeing a lot of noise from the clock on the DDS outputs.Thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: February 16, 2021, 02:13:25 am
content: The ripple on the clock may be coming from the AD9959, possibly from its PLL. Put the AD9959 into full powerdown mode, disconnect jumper W1 and see if the on-board 25MHz oscillator output still looks sh!tty.

author: srb1954
author_rank: Super Contributor
timestamp: February 16, 2021, 06:10:36 am
content: Quote from: JohnnyMalaria on February 16, 2021, 01:53:18 amOutput waveform doesn't look too bad. I wouldn't be concerned with a clock waveform like this.However, there may be other aberrations in the waveform that your scope is not seeing due to its limited bandwidth. You really need to get a higher bandwidth scope when you are working with digital signals at this frequency.

author: fourfathom
author_rank: Super Contributor
timestamp: February 16, 2021, 06:41:19 am
content: This looks a lot like transmission line termination mismatch effects. This can be due to trace stubs on the board, a high-z input impedance at the end of a trace (or transmission line) causing a reflection that isn't soaked up by an appropriate series resistor at the low-Z driver output, or similar transmission-line-type effects. If this is the case, the waveshape you are seeing isn't really a problem, as long as the reflections don't cause the waveform to get near the logic threshold (yours don't appear to be nearly that bad).Where are you probing, and how long is the trace between the clock output and the DDS clock input? Is there an interconnect cable (I see a SMA connector?) A typical FR4 PCB trace will have a propagation delay of about 170ps/in (or six inches per ns), and the reflection generally has to make a round-trip to show up as a level-shift step. I see several steps, the worst showing perhaps about 10ns prop delay, which probably means a coax cable interconnect (or a *very* long trace). So look at your terminations.Or, as has been suggested, it might be power supply noise caused by clocked logic, especially if there is a PLL somewhere that is multiplying that 25 MHz clock to a higher frequency.

author: gnuarm
author_rank: Super Contributor
timestamp: February 16, 2021, 09:27:34 am
content: I'm not seeing anything obviously wrong with the clock waveform. The small ripple in the high and low voltages have no effect on the clock. What you need to be worried about is impacts on the edges, noise that affects the timing of the threshold crossing. Try scoping the output pin on the chip, but also (and more importantly) the input pin on the DDS. That's the waveform you care about.When you say you see "noise" on the DDS output, how are you measuring that? There may be details in this aspect that is important to finding your problem.

author: Benta
author_rank: Super Contributor
timestamp: February 16, 2021, 12:17:09 pm
content: Quote from: fourfathom on February 16, 2021, 06:41:19 am+1

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 16, 2021, 02:18:27 pm
content: Thank you all for your guidance- there are some very useful ideas I will try today.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 16, 2021, 04:58:14 pm
content: Here's the clock signal.Top-left: At REFCLK input pin of AD9959Bottom-left: At oscillator output pin with AD9959 powered upTop-right: At oscillator output pin with AD9959 fully powered down and jumper W1 in placeBottom-right: At oscillator output pin with AD9959 fully powered down and jumper W1 removed

author: jmelson
author_rank: Super Contributor
timestamp: February 16, 2021, 05:16:28 pm
content: How are you measuring this? Without great care on the ground connection of the scope probe, you will get plenty of noise and abberations.Even the scope probe capacitance could affect it, either making it better or worse. What you want is an extremely short wire from the closest ground reference to the probe's ground ring.Jon

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 16, 2021, 05:55:00 pm
content: Quote from: jmelson on February 16, 2021, 05:16:28 pmThere's a photo of the probe in my first post. I removed the spring-loaded clip and grounding clip and put on the small grounding pin coil. It's as close as I can get.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 16, 2021, 06:14:47 pm
content: Quote from: edavid on February 16, 2021, 06:06:35 pmWell, I've stumbled across something odd. Even with the jumper removed, the AD9959 still works. The way the board appears to be configured and based on the spec sheet, it should be using the external clock. The AD9959 has its own internal oscillator but requires an external crystal and for one of the pins to be connected to 1.8V. The board has it connected to GND so it should disable the internal oscillator.So my next immediate step is to take some photos with my microscope and see if the board actually agrees with the circuit diagram. This board is weird because the silk screen shows the values and not references.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 17, 2021, 12:09:18 am
content: This AD9959 is operating even without any signal on the REFCLK inputI've confirmed that pin 24 (CLK_MODE_SEL) is connected to ground and the REFCLK is ac coupled per the datasheet.With the W1 jumper disconnected and no clock signal visible at REFCLK, the chip still runs and the SYNC_CLK output is exactly 1/4 of the oscillator's frequency.The 25MHz oscillator is being driven by 3.3V and its output is also about 3.3V. I understand from another thread that this is too high.Looking at the power rails, there's a lot of noise at 25MHz. There's similar noise on the DAC outputs, too.Is it possible that driving the oscillator this hard is polluting the power rails/overloading the REFCLK buffer such that the internal circuitry reacts as if there were an external clock signal?Should I remove the oscillator and/or connect it to the onboard 1.8V instead of 3.3V?Thanks.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 20, 2021, 10:10:55 pm
content: I like the shape.Do you want to achieve a pure meander like from a calibrator?

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 26, 2021, 03:41:03 pm
content: Hi - I have an updateI have removed the on-board oscillator and now use a clock signal from Arty Z7 Zynq 7000 board. I have programmed the Z7 to control the AD9959 board via SPI.For my application, I create four signals. One at 40MHz, one at 40+delta MHz, and two at delta MHz as a quadrature pair. In the photo, I feed the two RF signals into an HP balanced mixer. The output should be at delta MHz - which is exactly what I get. The signals are very clean.I won't be using my RPI for this now that I have figured out how to do it with a Zynq 7000 system.

author: mawyatt
author_rank: Super Contributor
timestamp: February 26, 2021, 04:09:55 pm
content: Did using the clock from the Z7 board improve the AD9959 output sinusoid waveform quality? Specifically the phase noise?Best,

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 26, 2021, 04:22:09 pm
content: It seems to have done so. With the on-board oscillator, the output from the balanced mixer (yellow trace) was somewhat jittery compared to the other traces when using one of the others as the trigger source. I don't see any evidence of that using the Z7 as the clock source. Of course, all of the outputs may have some jitter but my application can tolerate them as long as I'm using the same clock source for all the signals.I also suspect using short SMA-BNC cables instead of the scope probes helps, too