Author Topic: LMX2322 prescaler  (Read 2938 times)

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Offline smbakerTopic starter

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LMX2322 prescaler
« on: December 19, 2015, 09:51:27 am »
Working on a frequency counter project and trying to use an LMX2322, datasheet at http://datasheet.eeworld.com.cn/pdf/193228,TI,LMX2322.pdf, and don't understand how I'm programming it wrong. There's a handful of projects on the web that use the LMX2322 as a prescaler, and that's where I got the idea.

According to the datasheet:

Quote
The N divider counts such that it divides the VCO RF frequency by (P+1) A times, and then divides by P(B-A) times.

P is fixed at 32. I've set A to 0 and B to 8. According to the datasheet, these are valid values for A and B.

By my calculations, it should be dividing the input signal by 33 zero times and then dividing by 32 eight times. This would be an overall division by 256. However, when I actually prototype the circuit out, I'm seeing the signal divided by 8. A 1 Mhz input signal is yielding a 125 Khz output signal. What am I misunderstanding about the data sheet? As far as I can tell, divide-by-8 isn't even a valid configuration of the chip.

Thanks,
Scott
 

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Re: LMX2322 prescaler
« Reply #1 on: December 19, 2015, 10:51:10 am »
According to the datasheet (page 4), the minimal RFIn frequency is 700MHz, and the oscillator 5MHz. Where are you putting this 1MHz in?
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Offline smbakerTopic starter

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Re: LMX2322 prescaler
« Reply #2 on: December 19, 2015, 04:41:08 pm »
Well that may explain it... That's the sentence in the datasheet I must have missed!

I'm applying the 1 Mhz to the Fin pin via a 1nf capacitor. The circuit I've constructed is very similar to the one described here: http://hem.bredband.net/noda/freqmanual.pdf. I'm using a different CPU (a parallax propeller) and I'm driving it from 3.3V. This particular project claims to work down to 5 Mhz. I have tried injecting signals as high as 16 Mhz into mine, and still get the divide-by-8 behavior. I don't have the cp_out line connected to a microcontroller, but rather have it going to my scope and/or frequency counter

I suspect they're not using the chip for it's intended behavior and maybe this is how they're operating it out of the datasheet's stated range -- they're not using the oscillator circuit at all, and are using it only as a prescaler based on the N counter. Code that I found in github suggests they operate the device in test mode (TEST=1, CP_TRI=1, PD_POL=1). 

The output when observing with the scope is noisy. I'm wondering if shoddy construction technique in my prototype is to blame, and input noise is being detected and triggering the counter.

Scott
 

Offline smbakerTopic starter

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Re: LMX2322 prescaler
« Reply #3 on: December 20, 2015, 12:46:54 am »
Turns out it was poor construction technique. I rebuilt the circuit, and it's working fine, dividing by 256 with input signals as low as 5 MHz. Below 5 Mhz, it starts to get kinda wonky.
 


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