Abstract: LO phase noise on the MAX2309 is improved at 100MHz. PLL bandwidth, reference noise, and VCO components are changed to allow ~-90dBc/Hz phase noise at a 10kHz offset. The on-chip VCO is set to operate at 200MHz, with a comparison frequency of 1MHz. Three sets of PLL components are installed and the performance is documented on spectral and phase noise plots for each rendition.
This application note describes a method to achieve local oscillator (LO) phase noise performance of nearly -90dBc/Hz (at 10kHz offset) using the MAX2309 I/Q demodulator. A complete 100MHz intermediate frequency (IF) design solution is offered including voltage controlled oscillator (VCO), loop filter components, and performance plots.
Objective
Optimize the MAX2309 evaluation kit for LO phase noise as measured at the LO buffer output.
Procedure
A standard MAX2309 evaluation kit was obtained and the VCO tank components were re-configured, allowing for oscillation at 200MHz (twice the IF Frequency) with KVCO = 6.6MHz/V. Please see the MAX2309 evaluation kit schematic for reference designator locations. The component values are shown in Table 1.
Table 1. VCO Component Changes
Reference Designator
New Value (200MHz)
Part Number
Manufacturer
L5
82nH
0805CS-820XKBC
Coilcraft
C61
3.9pF
COG capacitor
Murata
C4, C6
27pF
COG capacitor
Murata
D3, D5
Varactor
SMV1763-079
Alpha-Industries
Loop Filter Design 1
The loop filter component values are shown in Table 2. This loop is designed for a unity gain frequency of 11.6kHz with 50° phase margin, and a 425µA charge pump current.
Table 2. Loop Filter #1
Reference Designator
Filter Value
C30
560pF
R23
5.1kΩ
C29
5.6nF
R10
0Ω
C31
Open
Figure 1. 100MHz LO signal at LO buffer output Icp = 425mA, HP8561E.
Figure 2. Phase noise of 100MHz LO signal, HP8561E.
Important Note: Originally phase noise was measured at approximately -80dBc/Hz, however it was determined that the 13MHz reference source possessed less than acceptable noise output itself. The source was changed to a KSS VC-TCXO-208C-13.0 and phase noise improved by approximately 6dB.
Table 3. Key Test Parameters
Parameter
Value
Units
Vcc
3.0
V
FIF
100
MHz
Fref
13
MHz
Fosc
200
MHz
Fcomp
1
MHz
Kvco
6.6
MHz/V
Target Phase Noise @ 10kHz
-90
dBc/Hz
Ta
+25
°C
Loop Filter Design 2
After measuring phase noise with the modified VCO and loop filter, a second loop filter was evaluated. This filter was designed for a narrower loop bandwidth with a 9kHz unity gain frequency, 53° phase margin, and 425µA charge pump current. The values are shown in Table 4.
Table 4. Loop Filter #2
Reference Designator
Filter Value
C30
1nF
R23
3.9kΩ
C29
10nF
R10
0Ω
C31
Open
The phase noise was measured and the results are shown in Figure 3 and Figure 4.
Figure 3. 100MHz LO signal at LO buffer output Icp = 425mA, HP8561E.
Figure 4. Phase noise of 100MHz LO signal, HP8561E.
Finally, a 210µA charge pump current was selected to further reduce the loop bandwidth. This resulted in much improved phase noise at 10kHz offset at the expense of tuning speed. This loop has a unity gain frequency of 5kHz and still has very good phase margin of 44°. The final results are shown in Figure 5 and Figure 6.
Figure 5. 100MHz LO signal at LO buffer output ICP = 210µA, HP8561E.
Figure 6. Phase noise of 100MHz LO signal, HP8561E.
For Larger Image Figure 7. The MAX2309 Evaluation Board, 100MHz LO, Phase-noise optimized.
Figure 8. The MAX2309 Evaluation board.
Figure 9. The MAX2309 Evaluation board (cont.).
Conclusion
The MAX2309 is capable of achieving approximately -90dBc/Hz phase noise at 10kHz offset. While quite dependent on the reference oscillator spectral purity, loop filter design, and charge pump current, the end result is ultimately achievable once optimized.
Automatic Updates
Would you like to be automatically notified when new application notes are published in your areas of interest? Sign up for EE-Mail™.
Download, PDF Format (106kB)
AN1744,
AN 1744,
APP1744,
Appnote1744,
Appnote 1744
Download, PDF Format
