Electronic – Issue for TCXO through frequency multiplier (PLL) for STM32 I2S signal

pllstm32tcxo

I try to use TCXO (KT7050A24576KAW33TAD) at 24.576 MHz through frequency multiplier (PLL) x4 (NB3N511DG) to clock the I2S bus (SAI domain) at 98.304 MHz on a MCU (STM32L4R5VIT6). Signals are not what I expected. Can someone help me to understand what I miss ?

1 – SCHEMATIC

(I remove R15)

2 – IMPLEMENTATION

3 – TCXO OUTPUT SIGNAL

Take on pin 5 of KT7050A24576KAW33TAD probe x1 with short ground spring

I expect a "CMOS" signal from KT7050A24576KAW33TAD but it look like more than "Clipped Sine Wave". Page 77 of the Kyocera Catalog I expect the following signal

4 – PLL/MULTIPLIER OUTPUT SIGNAL

Take on pin 5 of NB3N511DG probe x1 with short ground spring

I use CubeMX for quick test, I think STM32 pin "CLK_SAI" is well put in alternate function as "SAI Ext Clock". I expect a "CMOS" signal too on NB3N511DG output (square with lower min and higher max). (CLK_S0 and CLK_S1 are pulled low from the STM32)

5 – LACK OF UNDERSTANDING THE DATASHEET

When i bought the TCXO I see it's a CMOS output from supplier :

But friend of mine caught my attention on KT7050A24576KAW33TAD datasheet and suggest it can be use in both mode :

But I don't understand how to change mode from "Clipped Sine Wav" to "CMOS" mode…

This is the first time I play with TCXO and multiplier, I probably don't understand something (or more), can someone help me to find my issue ?

EDIT 1 : MEASUREMENT PROBE

I use 1x probe with short ground spring like this :

EDIT 2 : NEW MEASUREMENT PROBE x10
Many BIG THANKS to Justme and Tom Carpenter

Ok, with x10 probe (set on probe and oscilloscope) i got :

Output of TCXO :

It's look like a CMOS signal 🙂

Output of PLL :

This is a sinwave 🙁 I

So the input of PLL seem good now, but the output is not what I expected. The NB3N511DG datasheet say :

Phase−Locked−Loop (PLL)design techniques are used to produce a
low jitter, TTL level clockoutput up to 200 MHz with a 50% duty
cycle.

TTL/CMOS Output with 25 mA TTL Level Drive

The logic diagram show a TTL/CMOS output buffer

And caracheteristics tell me :

Does CMOS not implied square signal ?
Can someone help me to understand why I don't get a almost square signal at NB3N511DG PLL output ?

EDIT 3 : NEW MEASUREMENT PROBE x10 @50MHz
Many BIG THANKS to Justme

Pff… Don't forget Shannon-Niquist theorem… Don't try to look at 100MHz signal width 100Mhz bandwidth oscilloscop…

So I change the PLL multiplication coefficient to x2 to go lower than FS/2.
I can go to 49,152 MHz, here is the measurement :

It's look like what I expected !

Best Answer

The problem is the 1x probe mode. It does not have enough bandwidth in 1x mode.

Set it to 10x for more bandwidth.

Always use 10x probes, unless you know you must use 1x mode, and that does not happen very often.

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Electronic – input a square wave LO into a mixer

Yes, a square wave can be fine. When you mix, you get the sum and the difference of the frequency components from each input. The square wave consists of the fundamental and all the odd harmonics, so you need to do some filtering somewhere, of course.

The Softrock Ensamble RXTX is a simple direct conversion HF transceiver which makes a good example. There's a schematic on that page (a pretty bad one, but hey, you get what you pay for). U3 generates a square wave of a programmable frequency. U5 splits this into two quadrature clocks, QSD CLK 1 and 2. These find their way to U10, which is the mixer. It's not much more than a couple of analog switches. The output of the mixer goes to a couple of simple amplifiers and then to the audio input of a computer.

It's notable that this design wouldn't work with anything but square waves for the LO. U10 is an analog switch, with digital inputs to determine the state of the switches. It's not an ideal multiplier. If you fed it with a sine wave, the gain of the input transistors in U10 would make it a square wave anyway. This isn't true of all mixers, but here it is.

The filtering on this radio happens between the antenna and the mixer. The filters strip out all the harmonics above the band the kit was built for. Were this not done, then the LO frequency, plus all of its odd harmonics, would be aliased down to baseband. However, all those odd harmonics don't exist in the signal coming from the antenna after the filter, so there's no problem¹.

You can also use this to your advantage. There's another kit in the same family, the Softrock RX Lite II, which when built for 20m or 30m, samples at a lower harmonic. That is, the LO is actually 1/3rd of what it would otherwise be, and it's the harmonic of that square wave input to the LO that actually mixes the signal down. Again, filtering removes out-of-band signals between the antenna and the mixer.

Given your difficulty in finding a suitable crystal, maybe this is good for you. If you can find one that is at 1/3rd of the desired frequency, you could make that work, with appropriate filtering.

^{1: provided, of course, that you don't have some really strong signal near one of these harmonics that can find its way in despite the filtering. It could be an issue if you lived right next to a broadcast station or you had to deploy this radio in a high-RF environment. It's an inexpensive hobbyist kit, not professional equipment.}

Electronic – Implementing dynamic frequency scaling on PIC32 by changing PLL multiplier/divider

Yes, you can switch the clock speed of the PIC32MX dynamically.

The easiest way is to use the OSCConfig call in the PIC32 Peripheral Libraries, since it switches the clock temporarily to the FRC mode before switching back to one of the other clock modes, including PLL. So you don't have to explicitly switch to FRC mode first, like you would have to do if you were writing directly to the clock mode fields. You do need to disable interrupts around the call though.

Here is an example:

    #include <p32xxxx.h>
    #include <plib.h>
       .
       .
       .
    INTDisableInterrupts();
    OSCConfig(OSC_POSC_PLL, OSC_PLL_MULT_15, OSC_PLL_POST_1, 0);
    INTEnableInterrupts();

The various constants are listed in the documentation for the OSCConfig call.

Danger, Will Robinson! The code example Microchip shows in their documentation is wrong, so if you cut and paste from the documentation, it won't compile! The example uses OscConfig instead of OSCConfig. The latter is correctly used in the rest of their documentation.

I found the documentation for the PIC32 Peripheral Libraries is in this document. Oddly, I couldn't find the PDF on the Microchip site. There is a compiled help file with the same information in the Microchip\MPLAB C32 Suite\doc folder, but I like the PDF better.

You don't have to install anything to use their peripheral library, as it comes with the PIC32 compiler. NOTE: I am using an older compiler, pic32-gcc.exe v2.01, so it is possible the call has changed, but I doubt it for compatibility with older code.

Best Answer

Related Solutions

Electronic – input a square wave LO into a mixer

Electronic – Implementing dynamic frequency scaling on PIC32 by changing PLL multiplier/divider

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