Dear Analog Engineering Community,
I am using the ADF4113HV to generate a phase locked signal on 7.784-7.786GHz from a 48MHz crystal reference.
The simplified loop circuit is
The settings are as shown:
The current setting resistor is 4.7Kohm.
The resulting signal is contaminated with a travelling spur much like that seen in the fractional N PLL synthesizers. I tried adding a 470k-ohm negative bleed resistor, but the spurs were still observed with no chance. I notice that the spurs are often strongest at power up and move away with time. However, if I am unlucky, the spurs move close to the carrier and stay there, causing severe phase disturbance. Shifting the reference frequency slightly causes them to move about. What is going on here?
Is there something about my settings that is causing this or is it an intrinsic problem with the phase detector of the 4113HV chip. I tried running on battery power, and the results were the same. Help!!!!!!! I am doing some sensitive precision timing measurements and this is really leaving me in a bind!
Thanks and best regards,
Here is a brief video of the spectrum of the synthesizer. I have seen this before in the ADF4157 fractional N PLL, but I find this very strange behavior in an integer-N PLL. The reference clock seems clean when observed on the spectrum analyzer (i.e. no switching power supply hash or leakage).
There is no traveling spur from the ADF4113HV core - like you said, the traveling spur is a fractional-N problem. Your spur is probably coming from your reference, from the gain block, or from the VCO itself. Can you try isolating/bypassing these blocks to see if the spur goes away?
Thanks for your response.
I found the source of the wandering spur. It is a subtle temperature dependent interaction between a voltage regulator and an 3rd overtone voltage variable crystal oscillator in a phase-locked 48MHz reference. Apparently, slight shifts in supply voltage would intermittently induce the excitation of a spurious mode of oscillation that would interfere with the desired mode. Since the frequency multiplication is so high, the phenomenon was not visible on the reference signal spectrum. The problem was only confirmed by using a known clean reference for the PLL. By changing a couple of capacitors in the crystal oscillator feedback network, the problem went away.
At the end of that day, the ADF4113HV was working perfectly. I only initially suspected the digital dividers because of having seen the exact same effect with the fractional-N PLLs. Since this is an integer N PLL, the frequency dividers cannot have this behavior.
We are also using ADF4113HV in our several applications. Our phase frequency detector frequency is 6.25 KHz. So, there are very high spurs at 6.25 KHz and at its coefficients. The current setting resistor is 4.7 Kohm. The loop BW is quite low, 3 dB corner frequency is 100 Hz.
We couldn't solve the problem yet. We'll be pleased if you could help to solve or reduce the spurs.
Using a low value PFD will result in worse spurs, if you could increase the PFD frequency you will reduce this spur level...
you can model the spur in ADIsimPLL as 5nA of leakage current to see how close you are to the expected level.