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Part Number: LMX2592
Could you kindly help to check the below schematic.
LMX2592 on board outputs 3GHz and 6GHz for mixer local oscillation.
However, it is found that there are 5/5.2/5.4/5.6/5.8/6.2/6.4/6.6/6.8/7 GHz spurious around the 6GHz frequency of LMX2592 output, and the spurious amplitude is about - 70dBc.
This kind of spurious is very similar to phase discrimination spurious, because our phase discrimination frequency is 200 MHz. Our charge pump current is about 12 mA and the loop bandwidth is about 100 kHz. The loop filter is shown in the red box below.
These spurious and 6GHz main signals in the mixer intermodulate 200/400/600/800/1000MHz low-frequency signals just into our intermediate frequency bandwidth. So how to reduce the phase discrimination spurious, whether it can pass through modify the loop filter parameters or LMX2592 configuration parameters to obtain, mainly reducing 10 dB is enough.
Pls. kindly help to give some advice!
Thanks a lot!
Assume fosc=100MHz, fpd=200MHz, I verified the performance with an EVM, I also got spurs at multiple of 200MHz offset but the worst one is better than 82dBc.
Charge pump current is 12.1875mA; loop bandwidth is greater than 200kHz.
In general, if the spurs are due to fpd, then reducing the charge pump current will also reduce these spurs. You could also reduce the loop bandwidth in order to reduce fpd spurs.
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In reply to Noel Fung:
On customer's board, there are some PD spurs around Lmx2592 output 6GHz signal, especially at 5.2GHz/5.4GHz/6.8GHz, the worst value is about -67dBc.
To lower the PD spurs, we have changed the Icp to 6.25mA or lower , but that spurs changed a little.
Then we used a narrow loop filter as below, nothing seemed to help.
Please help us to check lmx2592 register value, if changing some value can get help. We really need to lower these spurs 10dB at least.
Hope your reply,
In reply to Lian Wu:
Register setting is fine.
In the layout, where is C1? It is placed closed to CPout pin or not?
Customer has known the reason.
When they change frequency from 3GHz to 6GHz, they only set the 0x30 register and there are higher spurs.
If set the 0x1F register to 0201 same time , the spurs will be lower.
Do we need refresh all registers when only changing frequency?
Please see datasheet section 7.3.9 for the details. Basically, we should turn OFF unused buffers in order to save current as well as reduce crosstalk.
Thanks for your support.
The issue is closed.
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