DAC20¶
Project description¶
General¶
100MHzInTheLoop Version 1.0 originally used a Morion MV137 OCXO which has a frequency pulling range of 5x10-7. This translates into +/- 50 Hz offset for a 100MHz OCXO, = 100 Hz over 0 - 5V tune. For a 16 bit DAC this means 1.5 uHz / DAC-bit.
From version 1.0 we learned that he Morion MV137 has a phase noise bump at 40 Hz. Therefor focus shifted to using another OCXO, i.e., the KVG O-40-ULPN-100M on the next version of 100MHzInTheLoop (Version 2.0). The KVG has a frequency pulling range of 2.5 ppm (= 2.5x10-6). This translates into +/- 250 Hz offset for a 100MHz OCXO, = 500 Hz over 0 - 10V tune. For a 16 bit DAC this means 7.6 uHz / DAC-bit.
The sensitivity of the KVG is 5 times higher compared to the Morion. This results in an increase of phase noise over the region where the DAC is controlling the oscillator frequency (DAC update rate = 3.8 KHz) as can be seen in Figure 1 below.
Figure 1: KVG Phase Noise, Free-run and in TRACK-PHASE
It is clear that for the wider KVG frequency pulling range the granularity of the DAC needs to be increased. Tests were performed with a 1 K ohm potentiometer and a 1 K ohm resistor that superimposes the 16 bit DAC range, with a 5 times higher granularity around the coarse center Vtune range that is set by the potentiometer. The phase noise resulting from a 5x better granularity is shown in Figure 1 (green trace).
It is decided to replace the 16 bit DAC for the MAX5719A 20 bit DAC that results in 4 bit (or 16x) better granularity.
Figure 2: DAC20 PCB
A small DAC20 PCB (see figure 2) is designed to fit the location of U9 (16 bit DAC) on the 100MHzInTheLoop Version 2.0 (see figure 3).
Figure 3: DAC20 PCB fitted on U9 of the 100MHzInTheLoop V2.0
Design details¶
- DAC20 Schematics (Note: Design created with Mentor Graphics using a Xpedition).
- Manufacturing files 11500.04.02 100MHzInTheLoop V2.0 (Note: Design created with Mentor Graphics using a Xpedition).
General questions about project¶
- Peter Jansweijer - Nikhef
- Guido Visser -Nikhef
Status¶
| Date | Event |
|---|---|
| 28-11-2023 | Added DAC20 info |
28 November 2023
