Project update 26 of 27
ERASynth uses triple loop synthesizer architecture to provide low phase noise and low spurious together with fine tuning resolution. The first loop is a very narrow bandwidth loop that locks an ultra-low phase noise 100 MHz VCXO to a high stability 10MHz time-base (TCXO or OCXO). The second loop generates a low phase noise and fine-tunable reference signal around 133 MHz. The second loop operates in fractional-N mode and is realized with an LMX2594. The third loop provides the final output signal and operates in integer-N mode. The third loop is also realized with an LMX2594. At the firmware level these 3 loops are controlled with a smart algorithm to provide a spurious-free output signal. Aside from extra cost associated with using two LMX2594s, there is also a phase noise trade-off (several dBs) with this architecture. You could see that ERASynth’s phase noise at 1 GHz (10 kHz offset) is around -120dBc. TI’s datasheet spec for LMX2594’s phase noise at the same frequency is around -128dBc/Hz. As per LMX2594’s eval board guide, TI measured this spec using a very low phase noise Wenzel OCXO and they used only one LMX2594 without any post amplification. In order to provide a similar phase noise performance, we decided to add a low phase noise mode to ERASynth. In this new mode, second loop is turned off and third loop operates as a frac-N PLL directly driven by the ultra-low phase noise VCXO. We also extended the loop bandwidth by maximizing the CP current setting. This resulted in several dBs of phase noise reduction at some offsets. Please see the figure below for a comparison.
ERASynth++ Low Spurious vs Low Phase Noise Mode
As you can see there is more than 10dB improvement at 50kHz offset. What is the tradeoff? Mainly elevated spurious levels at some frequencies. Since main PLL is now in frac-N mode, it will exhibits severe spurious signals at frequencies close to integer boundaries. But for most frequencies, the new low phase noise mode will do just fine.
Now that ERASynth has a low phase noise mode where main PLL works in fractional mode, it also has the added benefit of phase shifting. When LMX2594 operates in frac-N mode, the phase of the output signal can be adjusted by changing the phase accumulator value. Previously phase shifting was not possible because ERASynth only had low-spurious mode of operation where main PLL was always integer-N. As explained above, in low phase noise mode, ERASynth’s tunable reference PLL is bypassed and main loop operates as a fractional PLL. So it is possible to have sub-degree phase shifts now. Note that there are some restrictions on the phase shift. Both the amount and speed of the phase shift depend on the output frequency.
Users can now change frequency and amplitude with specific steps. If you are increasing/decreasing the frequency or the amplitude in a linear fashion, you no longer have to enter a bunch of numbers each time. You can just set the increment/decrement value and click the plus or minus signs located right next to frequency and amplitude fields. Both the Windows and WEB GUI have this new improvement.
We have made a nice addition to our measurement arsenal: APPH20G. It is a 26 GHz phase noise analyzer from the Swiss company AnaPico. It costed us a great sum of money (~50k USD) but Adnan just had to buy it :-) We now have the capability to measure almost any practical phase noise. Besides phase noise, APPH20G is able to do transient measurements. So it will enable us to design much faster and much lower phase noise synthesizers in the future.
ERASynth+ and ERASynth++ units are in stock and available at crowdsupply.com and mouser.com. This was a small 50 unit run, so please order now while units are readily available.
The ERA Team