PLL FAQ
This is based on the assumption that you are troubleshooting a frequency synthesizer based on a National LMX series part, or an Analog Devices AD4*** Series part. These are highly integrated frequency synthesizers, requiring the addition of an external loop filter and VCO to build a frequency synthesizer.I have based these questions on the ones that have appeared in the National semiconductor discussion web page. Link. Analog Devices doesn't have a web page for support, but they do have questions posted at the Applied Radio Lab's Forum. Link.
Phase Noise
The first question is: do the closed loop simulations meet specification? Before you build a PLL, you should have an idea of the theoretical performance you should achieve. Far out, a decade of so past the loop filter corner frequency, the phase noise is dominated by the VCO, so the phase noise should be close to the VCO phase noise. I've got a plot here, which I generated from PLLDesign.xls, showing the total phase noise, VCO contribution, and the Reference Oscillator noise contribution.
For this plot, the loop filter corner frequency is set to 10 kHz. You can see that below about 20 kHz, the total phase noise is dominated by the VCO, while below 5 kHz or so, the total phase noise is equal to the reference oscillator noise. Moving to a higher order loop filter design will include more noise from the resistors, but that doesn't change the overall conclusions.
Power Supply
Power Supply noise can directly modulate circuits in the synthesizer, causing RF noise to appear. To check for this, you need to make sure that the Voltage supply is as quiet as possible. I'm a big fan of building a battery box to supply the circuit with a clean DC supply. Stack the batteries in order to get the supply voltage that you need. If powering the synthesizer from batteries improves the phase noise, then you know that you need to look at making the supply quieter. When you are doing this test, make sure that you turn off everything else on the board. If you need to, use the vendor's software to program the PLL directly.
Reference Oscillator Noise
what are you using as a reference oscillator? Close in, under the loop filter bandwidth, the reference oscillator contributes noise to the system. If you are using a noisy signal generator as a source then your measured phase noise could be artificially high
VCO Performance
Are you using a packaged VCO or one that you did yourself? When you did the calculations in easypll, did you use measured performance of the VCO?
Board Coupling
Are there any other noise sources on the board? I'm talking about things like microprocessors, DSP's, or FPGA? They could be injecting noise into the synthesizer. That is why I asked about powering the synthesizer with batteries.
Loop Filter Components
Did you put the right values in the right locations? The loop filter can't tolerate having components swapped. I've made this mistake several times, and it's silly.
Environmental Do you have this in a shielded box? Depending on the frequency that you are working at, you could be measuring not just phase noise but also a local cell phone transmitter or TV station
Test Equipment
How are you measuring this? Is your test equipment capable of measuring phase noise at the frequency offset you are dealing with? You need some margin below the noise floor and the measurement you are trying to make.
Loop Doesn't Lock
Input Power
Is there enough power at the input to the Phase Frequency Detector, both from the VCO feedback and the Reference oscillator.
Problems locking at Band Edges
If so, does the tuning range output from the PLL match the tuning range requirements of the VCO? For example, if you are powering the PLL IC from 3.3 Vdc, and the VCO tuning port call's for 0 to 15 Volt tuning range, then unless you have an active filter, you won't be able to tune over the entire range.
Charge Pump Polarity
Most PLL's have a setting to accommodate a positive or negative Kv.If you have the polarity wrong, then the PLL will rail at one end or the other.
Not enough Output Power
This is primarily due to the VCO, so check the VCO output power.
FM Modulation
There are multiple options for FM/PM of a PLL. Modulating the Reference Path provides modulation within the loop bandwidth. Modulation the VCO directly will add modulation outside the loop filter Bandwidth, and doing both together will perform "Dual Port" modulation, which provides a nominally flat modulation response.