Part Number: LMX2572
Hello,
We intend to use LMX2572 in our VHF and UHF DMR projects. Now, we are trying to optimize the phase noise and spur performance of LMX2572. Reference frequency is a 48 MHz CMOS TCXO and other circuit elements are as you recommended. We have observed some phase noise and spur issues as you may see in Figure 1.1.
Figure 1.1
In Figure 1.1 carrier frequency is 351.275 MHz. We measured two different phase noise plots at that frequency.
-TICS Pro black trace paramaters are shown Figure 1.2 below.
Figure 1.2
As shown in the Figure 1.2, Fractional Denominator is set its maximum value (2^32-1). For this reason, There is an little offset at RF Output and VCO Frequency. As a result, Phase Noise is degraded.
- TICS Pro green trace Parameters are shown in the Figure 1.3 below.
Figure 1.3
In this case, Fractional Denominator is set 3000000000, so there is no offset at the RF Output Frequency and VCO Frequency. Eventually, phase noise is not degraded. But, there are lots of spurs in this situation.
However, at some frequencies, we measure a good phase noise without any spurs.
As a conclusion, why does a small offset in VCO frequency cause a drastic effect in the phase noise, and how can we solve spur issues without any phase noise degradation at output frequency?
Best Regards,
Gündüz.
