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Part Number: LMX2572LP
HelloI've choose LMX2572LP for my project and have met weird situation.I use frequency band - 137..175 MHz, arbitrary FSK I2S modulation with symbol rate - 9600 bps, and sampling rate - 230.4 kHz (24 samples per symbol).It works perfect if selected VCO isn't VCO4. I've made some screenshots to show the problem. Conditions are mentioned under each picture.
First - 148MHz, VCO forced to VCO3.
pic.1 NVCO=3, I2S_On, FSK_EN=1, INPIN_IGNORE=0, SPAN=500kHz
pic.2 NVCO=3, I2S_On, FSK_EN=1, INPIN_IGNORE=0, SPAN=100kHz
pic.3 NVCO=3, I2S_Off, FSK_EN=0, INPIN_IGNORE=1, SPAN=500kHz
pic.4 NVCO=3, I2S_On, FSK_EN=0, INPIN_IGNORE=1, SPAN=500kHz
Second four pictures - the same condition, 148 MHz, but VCO auto calibrated to VCO4
pic.5 NVCO=4, I2S_On, FSK_EN=1, INPIN_IGNORE=0, SPAN=500kHz
pic.6 NVCO=4, I2S_On, FSK_EN=1, INPIN_IGNORE=0, SPAN=100kHz
pic.7 NVCO=4, I2S_Off, FSK_EN=0, INPIN_IGNORE=1, SPAN=500kHz
pic.8 NVCO=4, I2S_On, FSK_EN=0, INPIN_IGNORE=1, SPAN=500kHz
As you can see - if VCO4 is used - there are very high sampling rate spurs and common noise floor, even if FSK disabled, but host I2S pins remain active,
And these noise and spurs disappear, only if host I2S pins set inactive.
Sadly most of my frequency band falls into VCO4.
So the question is - how can I realise my FSK I2S in my frequency band?
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In reply to Dean Banerjee:
Thank you for your answer.
I've tried your suggestion about CAL_CLK_DIV=3, but there are no effect.
Also I've checked other frequencies from my frequency range. All frequencies that has fallen into VCO4 (145 ... 162 MHz) has the same high spurs and noise floor. I can use other VCO's (VCO3 and VCO5 which work perfectly) near the borders of the VCO4 range, however wide number of frequency channels can be synthesized with VCO4 only.
Maybe you have another ideas which I could try?
And please notice, that the same picture can be seen even if FSK_EN = 0 while host I2S remains active (see the pic. 8 in my first post).
It seems like the VCO4 cannot work properly with I2S FSK.
Can you check this, please?
With sinserely respect
In reply to Boris Krotkov:
From lmx2572 spur characterization, the spurs between VCO3 and VCO4 are very similar; within 2 dB.
Are you using TICSPro by chance? If so, could you save the file and post so that we an try to replicate this?
If not, could you give your input reference frequency and register settings (preferrably hex format)?
Yes, I use TICSPro, the file is attached.
But I have to mention, that this weird VCO4 behavior can be noticed only when FSK I2S is used.
If I disconnect or shut down host I2S pins - noise picture looks perfect with each VCO used, includes VCO4.
But if those I2S pins are connected and active - noise and spurs become just awful (only when VCO4 is used, regardless FSK_EN bit).
I use TICSPro from the file with modulation parameters: one bit per symbol, symbol rate = 9600, sampling rate = 230.4 kHz (24 samples per symbol), deviation = 3.6kHz, data pattern = PN9 with Gaussian shaping (BT=0.45).
P.S. I've attached .ZIP file as the board doesn't allow to attach TICSPro file (.TCS) :)
You can see the difference between VCO3 and VCO4 if you just use signal generator for generating square wave on I2S pin FS (SysRefReq).
I've attached picture with this difference, had found in my experiments (I used square wave about 192 kHz, 2.6V p-p to I2S pin FS)
FSK_EN bit doesn't affects these spurs.
Actually, you can connect this signal to any I2S pin for the same effect.
With sincerely respect
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