Amplitude modulation in the signal from the ADF4351 - Page 1
Forum section: electronics / rf-microwave

author: pulsarvictor
author_rank: Newbie
timestamp: July 31, 2023, 11:37:51 am
content: Hello all.I found amplitude modulation in the signal from the ADF4351 installed on chinese board:This is the sheme of this board:ADF4351_board.pngI applied power supply to board from accumulator 12V + 7805.Both output of ADF4351 were loaded on 50 Ohm.I did following experiment:Frefin = 10 MHz for ADF4351 was taken from the external 10 MHz output of the Agilent E4422B TXCO.I made samplind of the signal from the ADF4351 output (F = 87.5 MHz) using the ADC ADS5541.External clock for ADC (Fs=70 MHz) was taken from of Agilent E4422B main output.So both signals were coherent.Frequency ratio = Fin/Fs=5/4,Thus I had 4 points for "period" of input periodical signal.After processing of the discrete implementation of the signal, I saw periodic amplitude modulation.The envelope of the low-frequency amplitude modulation can be seen here:ADF4351_Fin=87_5MHz_Fs=70MHz_Coher_16M__Signal.pngThe period of this envelope is approximately 304 uS.Features of the chinese board and scheme that I paid attention to:R20 is not installed on the board.Pin 5 of U2 is unconnected.In the scheme:C17 = 1200 pF;C18 = 2700 pF;C15 = 39 nFR6 = 360 Ohm;R5 = 680 Ohm;As a reference desing I used:Evaluation Board User Guide ADF4351__UG-435.pdfFigure 22 shows following values:C(17) = 2700 pF;C(18) = 1200 pF;C(15) = 47 nFR(6) = 360 Ohm;R(5) = 1 KOhm;After this I removed the C17 and C18 from the chinese boardand measured them with Tek DM253:C17 = 1000 pFC18 = 3300 pFThen I rearranged these capasitors on the board.Now there are the following values:C17 = 3300 pFC18 = 1000 pF,C15 = 39 nFR6 = 360 Ohm;R5 = 680 Ohm;As I think it is enought close to Evaluation Board scheme.I had a suspicion about the quality of the chip installed on the board.Then I bought an ADF4350 from an AD distributor and installed it on the board instead of ADF4351.This is possible because these chips are compatible.Then, I did following experiment:Frefin = 10 MHz for ADF4350 was taken from the external 10 MHz output of the Agilent E4422B TXCO.Next, I made samplind of the signal from the ADF4350 output (F = 157.5 MHz) using the ADC ADS5541.External clock for ADC (Fs=70 MHz) was taken from of Agilent E4422B main output.So both signals were coherent.Frequency ratio = Fin/Fs=9/4,Thus I had 4 points for "period" of input periodical signal.After processing the discrete implementation of the signal, I saw periodic amplitude modulation again.The envelope of the low-frequency amplitude modulation can be seen here:ADF4350_Fin=157_5MHz_Fs=70MHz_Coher_16M__Signal.pngThe period of this envelope is approximately 1660 uS.For comparison, the signal envelope from Hittite HMC1034:HMC1034_Fin=157_5MHz_Fs=70MHz_Coher_16M__Signal.pngPlease suggest how to fix this problem.Thank you,best regards,Viktor.P.S.The reason for choosing this board is because I need many synthesizers in my projectto receive coherent signals, and my budget does not allow using many originalboards from AD.

author: Bud
author_rank: Super Contributor
timestamp: July 31, 2023, 12:10:34 pm
content: Did you program the chip?

author: pulsarvictor
author_rank: Newbie
timestamp: July 31, 2023, 04:58:42 pm
content: As following:printf(" ADF4351 \n");// Fout = 87.50 MHz; Pout = +5dBm; Fin = 10 MHz (External)Send_code_to_Synthezator_1(0x00D80005); // REG5Send_code_to_Synthezator_1(0x00D5003C); // REG4Send_code_to_Synthezator_1(0x000004B3); // REG3Send_code_to_Synthezator_1(0x00004F42); // REG2Send_code_to_Synthezator_1(0x08008011); // REG1Send_code_to_Synthezator_1(0x008C0000); // REG0printf("ADF4350 \n");// Fout = 157.5 MHz; Pout = +5dBm; Fin = 10 MHz (External)Send_code_to_Synthezator_1(0x00D80005); // REG5Send_code_to_Synthezator_1(0x00C5003C); // REG4Send_code_to_Synthezator_1(0x000004B3); // REG3Send_code_to_Synthezator_1(0x00004E42); // REG2Send_code_to_Synthezator_1(0x08008011); // REG1Send_code_to_Synthezator_1(0x007E0000); // REG0

author: RoV
author_rank: Regular Contributor
timestamp: July 31, 2023, 05:14:23 pm
content: Check Vtune signal with a scope. It should be a stable DC value if the loop is stable.Check also the power supply lines.

author: Bud
author_rank: Super Contributor
timestamp: August 01, 2023, 12:10:42 am
content: Is D1 LED turned on or off? If it is off , the chip has its PLL unlocked (in non-functional state). You will need to decode the registers programming values and map them into programming parameters as specified in the Datasheet, then look and analyze them if the parameters make sense for your clock frequency and reference frequency. You should download ADI SimPLL simulation software and learn how to use ithttps://www.analog.com/en/design-center/adisimpll.htmlThen you can enter your components values and dividers values and run the simulator to see if the PLL locks. Play with the loop filter components until it locks reliably.This is not a 1 day endeavor but unfortunately becoming a home expert in ADI PLL chip programming is the way to deal with the chinese made products. You need to understand how the chip works. In China they simply often copy one other's designs without understanding.

author: tchicago
author_rank: Regular Contributor
timestamp: August 01, 2023, 02:09:33 am
content: Quote from: RoV on July 31, 2023, 05:14:23 pmAlso, this board has a fancy LDO from LT. Sometimes, there could be "unlucky" combination of input and output capacitors that causes LDO to oscillate. Check its output with a scope.P.S. Слава Україні!

author: pulsarvictor
author_rank: Newbie
timestamp: August 07, 2023, 06:04:00 am
content: Thank you for your reply.I have been testing Vtune and VCO for several hours. The SDS1202X-E oscilloscope recorded only random noise. No regular signals.For Vtune: Urms, mean = 8mVFor VCO: Urms, mean = 8mVRegards.Viktor.

author: pulsarvictor
author_rank: Newbie
timestamp: August 07, 2023, 07:03:07 am
content: Hello BUDI have 2 boards:- with ADF4350- with ADF4351.D1 LED turned on on both boards.The output frequency measured by the SDS1202X-E oscilloscope exactly matches the programmed frequency.I used Analog Devices ADF435x Software.This soft reports about inconsistencies in the programming data if there are such inconsistencies.Obviously you are right, I need to use ADI SimPL software.Thank you for your suggests.Regards,Viktor.

author: nctnico
author_rank: Super Contributor
timestamp: August 13, 2023, 12:36:59 pm
content: Quote from: tchicago on August 01, 2023, 02:09:33 amThis might be the case as the ltc1763 is not specified to be stable with a ceramic capacitor.Also L1 & L2 are wrong in the schematic. These should be 50 Ohm for wideband operation.Last but not least: how where the signals measured? Which samplerate and was peak detect enabled? It could be aliasing as well if the samplerate is not sufficient.