LMX2492EVM: Wide bandwidth chirp

Jay Shenoy

Prodigy 30 points

Part Number: LMX2492EVM
Other Parts Discussed in Thread: LMX2492

Hi,

I'm looking to generate a 9.6Ghz - 11 Ghz chirp signal (linear frequency ramp) for an FMCW radar application. The chirp

seems to be working fine at lower bandwidths but I seem to getting strange signal

artifacts that do not resemble a chirp when trying to generate 9.6-11Ghz. I'm using

the onboard clock with TICS Pro currently. Are there any issues with this sort of bandwidth

on the board? I also plan to down convert this chirp with a mixer.

Thanks

over 2 years ago

0 Timothy T over 2 years ago

TI__Mastermind 28530 points

Hello Jay,

Would it be possible to share the TCS file you are using to generate the chirp?

73,
Timothy

0 Jay Shenoy over 2 years ago in reply to Timothy T

Prodigy 30 points

Hi Timothy,

I'm essentially just setting the LMX2492 PLL. Setting up the default configuration. And changing the values in the ram calculator from 9600Mhz to 11000 Mhz while enabling ramp and RST.

0488.HexRegisterValues.txt

R141	0x008D00
R140	0x008C00
R139	0x008B00
R138	0x008A00
R137	0x008900
R136	0x008800
R135	0x008700
R134	0x008600
R133	0x008500
R132	0x008400
R131	0x008300
R130	0x008200
R129	0x008100
R128	0x008000
R127	0x007F00
R126	0x007E00
R125	0x007D00
R124	0x007C00
R123	0x007B00
R122	0x007A00
R121	0x007900
R120	0x007800
R119	0x007700
R118	0x007600
R117	0x007500
R116	0x007400
R115	0x007300
R114	0x007200
R113	0x007100
R112	0x007000
R111	0x006F00
R110	0x006E00
R109	0x006D00
R108	0x006C00
R107	0x006B00
R106	0x006A00
R105	0x006900
R104	0x006800
R103	0x006700
R102	0x006600
R101	0x006500
R100	0x006400
R99	0x006300
R98	0x006200
R97	0x006100
R96	0x006000
R95	0x005F00
R94	0x005E00
R93	0x005D00
R92	0x005C04
R91	0x005B27
R90	0x005A10
R89	0x005900
R88	0x005800
R87	0x00575B
R86	0x0056C0
R85	0x005500
R84	0x005400
R83	0x005300
R82	0x005218
R81	0x005100
R80	0x005000
R79	0x004F00
R78	0x004E7A
R77	0x004D00
R76	0x004C00
R75	0x004B00
R74	0x004A00
R73	0x004900
R72	0x004800
R71	0x004700
R70	0x004600
R69	0x004500
R68	0x00447E
R67	0x004300
R66	0x004200
R65	0x004100
R64	0x004000
R63	0x003F02
R62	0x003E00
R61	0x003D00
R60	0x003C00
R59	0x003B00
R58	0x003A01
R57	0x003900
R45	0x002D00
R44	0x002C00
R43	0x002B00
R42	0x002A00
R41	0x002900
R40	0x002800
R39	0x002752
R38	0x002618
R37	0x002510
R36	0x002408
R35	0x002341
R34	0x002204
R33	0x002120
R32	0x002000
R31	0x001F32
R30	0x001E0A
R29	0x001D00
R28	0x001C1F
R27	0x001B08
R26	0x001A00
R25	0x001901
R24	0x001800
R23	0x001703
R22	0x0016E8
R21	0x001500
R20	0x001400
R19	0x001300
R18	0x00122C
R17	0x001100
R16	0x001060
R15	0x000F00
R14	0x000E00
R13	0x000D00
R12	0x000C00
R11	0x000B00
R10	0x000A00
R9	0x000900
R8	0x000800
R7	0x000700
R6	0x000600
R5	0x000500
R4	0x000400
R3	0x000300
R2	0x000201
R1	0x000100
R0	0x000018

0 Noel Fung over 2 years ago in reply to Jay Shenoy

TI__Guru* 81380 points

Hi Jay,

The on-board VCO requires >7V tuning voltage in order to output 10.5GHz. You have to modify the EVM to use active filter to support >5V tuning voltage. Check EVM user's guide for the EVM schematic.

0 Jay Shenoy over 2 years ago in reply to Noel Fung

Prodigy 30 points

Hi Noel,

Thank you for the answer and I was wondering if you could provide some more details on how to get the tuning voltage up to 7volts.

0 Noel Fung over 2 years ago in reply to Jay Shenoy

TI__Guru* 81380 points

Hi Jay,

Please read Chapter 41 of this book for details of active loop filter. https://www.ti.com.cn/cn/lit/ml/snaa106c/snaa106c.pdf

You may use PLL Sim to do an active filter design. https://www.ti.com/tool/PLLATINUMSIM-SW

0 Jay Shenoy over 2 years ago in reply to Noel Fung

Prodigy 30 points

Hi Noel,

Thank you for the information. I actually need to generate a waveform from 5.5-7Ghz. I was wondering if I still need to modify the tuning voltage to achieve this. If not, what would I need to change in the TICS Pro to achieve this. Is there any way I can generate a 1 or 1.5Ghz bandwidth chirp without using the active filter design? Thanks

0 Noel Fung over 2 years ago in reply to Jay Shenoy

TI__Guru* 81380 points

Hi Jay,

From the VCO datasheet, we might be able to do a 1 GHz chrip (9GHz to 10GHz output) with passive loop filter.

0 Jay Shenoy over 2 years ago in reply to Noel Fung

Prodigy 30 points

This sounds great. Since by default the VCO ramp starts at 9.6Ghz. Is there a recommended configuration to change the ramp start to 9Ghz?

0 Jay Shenoy over 2 years ago in reply to Jay Shenoy

Prodigy 30 points

Also, another question is that my chirp that I'm currently using from 9.6-9.7Ghz only appears on the RFout/2 port and not the RFout port. I'm wondering why this is happening.

0 Noel Fung over 2 years ago in reply to Jay Shenoy

TI__Guru* 81380 points

Hi Jay,

Both RF and RF/2 output are active, make sure you are not measuring the VCO fundamental signal from RF/2 port.

To get ramp started at 9GHz, put 9000 in the VCO frequency box in PLL page.

0 Jay Shenoy over 2 years ago in reply to Noel Fung

Prodigy 30 points

Hi Noel,

This seems to make sense. However, I seem to be having a small issue. I'm currently generating a 9-10Ghz chirp for an FMCW application. I'm currently sending everything through a wire and am mixing the chirp signal with itself. The output I'm getting should look sinusoidal; however, I'm getting the result below. Some of the leakage at the edges makes sense due to PLL reset but I'm wondering why the signal isn't sinusoidal. I'm wondering if this is an issue with the chirp.

0 Noel Fung over 2 years ago in reply to Jay Shenoy

TI__Guru* 81380 points

Hi Jay,

I do not understand what is "sending everything through a wire and am mixing the chirp signal with itself", can you share the connection diagram?

BTW, did you get correct chirp directly from the EVM (without mixing)?

0 Jay Shenoy over 2 years ago in reply to Noel Fung

Prodigy 30 points

Hi Noel,

I'm not entirely sure if the chirp is correct since I cannot directly sample the waveform's high sampling frequency. This is the block diagram of the setup I'm currently using. I'm sampling the down converted signal which should be a sinusoid if the chirp is fully generating properly.

0 Jay Shenoy over 2 years ago in reply to Jay Shenoy

Prodigy 30 points

Hi Noel,
The chirp looks like it is working thank you for your help. One last question I have is if I want to feed a 30Mhz external reference clock to the chirp generator instead of using the onboard 100Mhz clock, will I have any issues generating this same 9-10Ghz FMCW waveform?

0 Noel Fung over 2 years ago in reply to Jay Shenoy

TI__Guru* 81380 points

Hi Jay,

If you use 30MHz reference clock, the max. fpd you can use is 60MHz. Without modifying the loop filter, the loop bandwidth will be smaller than the EVM default, depending on how fast you sweep the frequency, lower fpd may or may not have any impact to the ramp.

Clock & timing

Clock & timing forum

LMX2492EVM: Wide bandwidth chirp