PLL spurious ouput?

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7rots51

Joined: 17 May 2002
Posts: 1427
Helped: 14

23 Oct 2006 20:08 pll spurious

Hello friends

I designed a PLL+VCO circuit at 160MHZ with LMX2316 and minicircuit VCOs and 25MHZ crystal osc as ref.

Output is stable (Fcom=20KHZ,loop filter BW = 1KHZ),but as shown below have spurs at output near 160MHz carrier.Please guide me to remove the spurs.

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rancohen_2000

Joined: 24 Aug 2006
Posts: 129
Helped: 25

23 Oct 2006 21:09 lmx2316

hi 7rots51,

for good answer i need the schematics. if you can please show only the high side of the signal with span of 2 MHz?

toonafishy

Joined: 18 Jun 2002
Posts: 461
Helped: 25
Location: Earth

23 Oct 2006 21:12 lmx2316 pll

What is the delta frequency between the 160MHz and the spur?
Is your charge pump comparison frequency 20 kHz?
Are you using a switching power supply? Power supply noise will appear as a spur on the PLL.
Do you have the extra RC network on the output of the loop filter (R3/C3) or are using just the two caps/1resistor network (R1, C2/R2)?

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biff44

Joined: 24 Dec 2004
Posts: 1834
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Location: New England, USA

23 Oct 2006 21:48 spurious ouput

I am going to bet you are using a .714286 MHz phase detector frequency, and this is just that spur (R=35, N=224). Try:

1) Using lower divisor numbers (higher phase detector frequency), such as R=15 and N=96. This will give you a higher phase detector frequency of 1 2/3 MHz

2) Add an RC lowpass filter at the VCO tune port to filter out phase detector spikes at 1 2/3 MHz. I would put in a lowpass with a 3 dB corner at around 20 KHz, and see if you like it better. (Whenever you add an additional lowpass filter you compromise the phase margin of the control loop, so you want to keep the lpf corner frequency say 20X the loop bandwidth, which I assume you correctly state as 1 KHz).

radiohead

Joined: 13 May 2004
Posts: 412
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Location: Heart of Europe

23 Oct 2006 22:29 Re: PLL spurious ouput?

I agree, it is probably your detector frequency which is too low, or your loop filter does not remove the spurs enough. You can consider selecting a higher order loop or simulate the filtering performance at the phase detection frequency.

7rots51

Joined: 17 May 2002
Posts: 1427
Helped: 14

25 Oct 2006 17:20 Re: PLL spurious ouput?

By adding a 10uf tan cap to VCO supply (there was only 1u ,100n ) and changing channel spacing to 25khz and loop filter to 250Hz ,I got this spectrum,Is it ok?

the phase noise is -90dBc ,is it enough?

the sch of circuit is in pdf file(simulation with analog device PLL software)

bye

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khouly

Joined: 20 Oct 2003
Posts: 2479
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Location: EGYPT

25 Oct 2006 17:25 Re: PLL spurious ouput?

hi , it is a good results the spurs are disappeared , and it is great

about the phase noise u should u know is it good or band from the specs u need to have
and also u forget to mention the offset u measure the phase noise for

khouly