ADF4351 - simple you would think? - Page 1
Forum section: electronics / rf-microwave

author: g0mgx
author_rank: Regular Contributor
timestamp: June 16, 2019, 07:33:02 am
content: Hi GangI’m trying to make myself a 100MHz reference signal using a ADF4351 – simple you would think?I’ve used these devices on SV1AFN boards (https://www.sv1afn.com/adf4351m.html) before and have some simple Arduino software (https://www.qsl.net/g/g0mgx//BlogFiles/ADF4351_Simple.ino) which programs the registers for me – I even cheat and use the Analogue Devices software to calculate the register values for me from the target frequency and the reference (seehttps://www.qsl.net/g/g0mgx//BlogFiles/ADF4351%20prog.PNG).The Arduino software is extremely simple and just programs the device registers – nothing more. My ADF4351 boards are clocked by my shack reference at 10MHz.This has always worked perfectly for me until I tried to set the frequency below about 500 MHz (ish) when the frequency being generated by the AD4351 seems to be mainly inaccurate and also jumps about somewhat.I also have an eBay sourced ADF4351 module which has been cloned to “look” like an Analogue Devices dev board and that works fine at all frequencies (including 100MHz) when programmed directly from the manufacturers software (seehttp://g0mgx.blogspot.com/2016/11/44ghz-signal-generator-really.html).Either I’m missing something for lower frequency programming of the device or there’s something with the config of the SV1AFN boards – I’m really not sure.I could rip my desk mounted ADF4351 sig gen apart and try my Arduino software directly with the eBay board in that and see if the problem remains, but before I do that I thought I would ask the experts.Any ideas guys? I’ve tried to link all the bits and bobs above.MarkG0MGX

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 16, 2019, 10:00:02 am
content: certainly you should try your arduino code on the ebay module to exclude any hardware issues on the SV1AFN board. So far I just used a chinese board, but I know SV1AFN is not selling any crap, so my guess is a software problem.maybe you can post your register calculations to let it double check.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 16, 2019, 10:24:38 am
content: Does the chip allows to read the registers after writing them?Asking because it happens to me a few times to have communication issues, some of them very tricky, e.g. the same circuit and same ribbon cable not working after changing the order of the wires inside the ribon (once the clock was near GND, the other time the clock was far from GND).In a case like yours where the chip behaves erratic, I will first try to read back the registry values for that particular less than 100MHz setting that is not working, and see if they are indeed as written.If not, put some series resistors on the SPI wires to dampen the edge ringings.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 16, 2019, 10:41:51 am
content: no, it has just a SPI input, but no data out, like all the other ADF4xxx PLLs with or without VCO do not.if the PLL on the ADF4351 doesn't lock, it's almost certainly a wrong register value; the SPI interface itself is really uncritical.

author: g0mgx
author_rank: Regular Contributor
timestamp: June 16, 2019, 12:35:50 pm
content: Well,I've ripped my desk sig gen to bits and connected the eBay sourced ADF4351 module to my Arduino and it works:https://www.qsl.net/g/g0mgx//BlogFiles/rigol100mhz.bmpConnect it back to the SV1AFN module and the output frequency seems to alternate between 90MHz and 95MHz with a fairly regular pattern.I'm flummoxed.The SV1AFN schematic is here:https://www.sv1afn.com/adf4351m.htmlThis message covers the eBay board I have:https://www.eevblog.com/forum/testgear/help-with-software-for-adf4351-signal-source/msg2462334/#msg2462334- I'm clocking mine with an external 10MHz reference. There's a guesstimate at the schematic at 7:16 into the video on the post.MarkG0MGX

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 16, 2019, 01:01:39 pm
content: can you post your calculation and the register values?still, my guess is a configuration issue rather than the hardware.

author: OwO
author_rank: Super Contributor
timestamp: June 16, 2019, 01:08:10 pm
content: If this is a 2 layer board that layout is horrendous. There is no ground plane under or around the reference clock trace, and even the RF out traces cross a broken ground plane. I have a far better 2L layout for ADF4350/1 with solid ground plane but it requires two jumper wires.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 16, 2019, 01:13:18 pm
content: you mean SV1AFN board? well, its layout is a bit weird, but I didn't hear any rumor so far that it doesn't work.it would have to be a loop filter matter to cause such an erratic behavior.

author: edigi
author_rank: Regular Contributor
timestamp: June 16, 2019, 02:58:54 pm
content: I have only the ebay variant of the board so I cannot comment about this particular board but still some random thoughts.The VCO always operates in the 2200-4400 MHz range. Anything below is a divided down version of the VCO output.In case of 100 MHz the VCO operates at 3200 MHz and the RF divider is 32.It's very unlikely that the RF divider would have an issue so if there is something wrong around 100 MHz then it means problem for a lot of other frequencies as well...One note: The frequency steps in the 2200/32-4400/32 range (where 100 MHz belongs) are REF (10 MHz)/(4096*32) that is roughly 76 Hz in case of 10 MHz reference.Since the 100 MHz means integer-N mode, some of the bits have to be set differently from fractional-N mode and I suspect that this is not taken into account in the linked register table (e.g. ABP)...The ADI datasheet is very readable, and in my view worth reading instead of trusting some random tables that can be found on the net.If you use already Arduino to send register content to ADF it's only one small step to directly fill them in the SW.Issues to be clarified:- Is there some lock indication (LED) and what does it show?- What is used to check the signal e.g. some frequency counter (that my have an issue for lower frequencies in the high frequency range and influenced by strong harmonics) or something else (SSA, DSO whatever)?

author: g0mgx
author_rank: Regular Contributor
timestamp: June 16, 2019, 06:17:31 pm
content: Let me try and answer the questions - thanks to all who have offered help.I'm measuring the output with a Rigol Spectrum Analyser - DSA 815 The output from the eBay module is here:https://www.qsl.net/g/g0mgx//BlogFiles/rigol100mhz.bmp.The register calculations are being performed by the Analogue Devices software - screen shot here:https://www.qsl.net/g0mgx//BlogFiles/ADF4351%20prog.PNGthese are the values I am using for the programming. The software I am running on the Nano is here:https://www.qsl.net/g/g0mgx//BlogFiles/ADF4351_Simple.ino.The MUXOUT pin on the board is low so that means its not locked - there is an on board LED connected which is also not lit. Register 2 starts with 1800xxxx hex - that configures the MUX out to be the digital lock (default is tri state which I dont understand!) - but with the 1800xxxx values the PLL locks over 500MHz but not belowMy current hunch is that there is some filtering of the ref clock on the eBay board which is absent on the SV1 board - I need to make a LPF for the 10MHz reference and stick that in line and see if anything changes - could harmonic content in the ref signal screw up the frequency divider?I'm still totally baffledThe only other difference is that the eBay board has an onboard voltage regulator and the SV! board doesnt - I need to tripple check the 3V3 supply I'm using for the SV1 board as I guess that could also screw things up nicely?MarkG0MGX

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 16, 2019, 06:45:48 pm
content: You should set the LDF flag to INT-N, since you're not using Frac-N mode for this frequency.there's no filtering on the ebay board for the reference signal either; I'm using a 10MHz TCXO with a clipped sine - works like a charm.the 3.3V reg on the ebay board is more a bug than feature; it's just a noisy LD1117tri-stated just means it's in high impedance not causing any issues with any possible external signals; the muxout bit fields have no influence on the PLL or the VCOah yeah, just notices - like already mentioned from the previous writer: the ABP flag - set it on 3ns

author: edigi
author_rank: Regular Contributor
timestamp: June 17, 2019, 08:32:44 am
content: Quote from: g0mgx on June 16, 2019, 06:17:31 pmBased on SA's output I don't see the issue.You get clean sine output only in the VCO's range.Below 2200MHz the output of the VCO is divided to get the output signal and that means the waveform will resemble to a square/clipped sine. That is full of harmonics what you can see in the output of the SA.Thus either you've missed this part of the chip's operation and you have false expectations or I've profaundly misunderstand this whole story.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 17, 2019, 02:25:25 pm
content: the SA's output is from the ebay module where he has no issues; so all ok theremy guess are the 2 flags mentioned - together with the differently designed loop filter on SV1AFNs board

author: chrisl
author_rank: Regular Contributor
timestamp: June 17, 2019, 08:41:05 pm
content: Sounds like it is not locking at the low freq. Did you check the LD pin for the lock status?The LD bit can sometimes give false status tho.... but usually a good place to check.

author: erikka
author_rank: Regular Contributor
timestamp: June 25, 2019, 02:26:16 pm
content: A very stupid suggestion: Did you calculate with the right reference clock?The green ebay module uses 10MHz reference XCO, the black module (which is a 4 layer design but much worse) uses 25MHz.I modified the ADI tool so it can use an arduino to set the registers of the ADF4351.Helps debugging if you can quickly change any setting.

author: edigi
author_rank: Regular Contributor
timestamp: June 26, 2019, 08:07:11 am
content: Quote from: erikka on June 25, 2019, 02:26:16 pmThat's a pretty bold statement (especially without further details) since many different variants of the black module exists (I personally own 2 totally different variants; different power plug, different TCXO with different component arrangement, same PIN header but different pins...).

author: fcb
author_rank: Super Contributor
timestamp: June 26, 2019, 10:55:44 am
content: Are you using level translators to go between the (5V?) signal lines on the arduino to the +3V3 ADF4351? Ignore this if your using a 3V3 arduino.

author: Yansi
author_rank: Super Contributor
timestamp: June 26, 2019, 11:25:08 am
content: Some of you may find as a surprise, that some of the ADF4351 sourced from China are actually a rebranded device from IDT, the 8V97051.Datahseet:https://www.idt.com/document/dst/8v97051-datasheetThis device is not fully compatible with the AD435x. Not really sure about the exact difference, just sayin.Usually you can tell by the package, as IDT uses standard square corner QFN and Analog Devices uses their specific LFCSP.

author: edigi
author_rank: Regular Contributor
timestamp: June 26, 2019, 12:29:25 pm
content: That's pretty interesting, I wish my boards were equipped with this chip as it seems to be in many ways improved version over the original ADI chip (see extended registers).With quick check it seems to me that they've at least aimed for register compatibility.

author: Yansi
author_rank: Super Contributor
timestamp: June 26, 2019, 08:27:10 pm
content: When ordering for bare chips from aliexpress, usually you can tell from the photos, whether it is QFN or LFCSP.Regarding populated boards, it seems to be a lottery what you get.Also I am not really sure the square QFNs from china are that IDT device, but I kind of think they are. Please let me know if someone can verify for sure.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 26, 2019, 11:56:16 pm
content: That's interesting; didn't know yet about that IDT chip. my only project so far uses one of the black boards with a QFN chip - labeled ADF4351 - but what does that mean?!?Programming the code I oriented 100% to the AD datasheet and had no issues with the given parameters.

author: OwO
author_rank: Super Contributor
timestamp: June 27, 2019, 05:02:13 am
content: I remember someone else on this forum mentioned that ADI themselves transitioned from the molded LFCSP to the sawed QFN some years ago, and this was a good way of telling if you are getting new old stock.

author: erikka
author_rank: Regular Contributor
timestamp: June 27, 2019, 08:34:04 am
content: Quote from: edigi on June 26, 2019, 08:07:11 amApologies.I was referring to the sub 25$ boards from eBay I own. There are more expensive black boards that I do not own and thus did not evaluate.

author: edigi
author_rank: Regular Contributor
timestamp: June 27, 2019, 02:00:10 pm
content: I happen to own only those cheap ADF4351 ebay boards so would be curious to read your experience why do you think that the black module is worse...I'm actually surprised that if you aim for low phase noise (or at least seems so from your ADF4351 related posts) at close frequencies why do you use low frequency reference as my immediate gut feeling was that the more the reference is multiplied (after all that's what PLL does), the more noisy the output will be at least from this contributing factor.Not only my gut feeling, but also this seem to be rigorously described in this document (see PFD-referred noise around slide 26 and in particular slide 28 and see also page 40!):https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-776-high-speed-communication-circuits-spring-2005/lecture-notes/lec22.pdfFor sake of completeness I link also the first part.https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-776-high-speed-communication-circuits-spring-2005/lecture-notes/lec21.pdfLook also the ADI datasheet all of those phase noise figures show phase noise with relatively high PFD (and accordingly the loop filter bandwidth is also on the higher side)...People aiming for this could be better served by ADF4355 and alike (I have also that board) even though it's more expensive (ADI e.g. mentions AD4351 as first generation synthesizer)...One of the benefits of the earlier linked IDT chip (beyond the obvious ones coming directly from extended registers) is the relatively high PFD that it can operate (and nice that not to impact stepping more bits are available for fraction).Naturally the loop filter has to be adjusted for the used PFD and I don't know if that's properly taken care of the various boards.

author: erikka
author_rank: Regular Contributor
timestamp: June 28, 2019, 07:52:52 am
content: Well, I am not an expert, just picked up this hobby after playing with vacuum tubes when I was young.Here are some phase noise plots from my own build 2GHz spectrum analyzer using two ADF4351 modules and one SI5351 module. First IF is at 2.01GHz, second IF at 10.7MHz and third (when doing narrow band FFT based resolution filters) IF at 40kHz. Above 10KHz RBW I use a log detector at 10.7MHz with 120dB range. The ADF4351 uses low spur mode setting for all measurements. The phase noise measurements are done with the ADF4351 running around 2GHz. If you use a first IF of 100MHz the same ADF4351 delivers a much better phase noise.First is with two green modules using a common 10MHz reference, second with the same green modules but using the reference doubler in the ADF4351 and third replacing the first LO green module with a black module using a 25MHz reference. The close phase noise of the black module is a bit better (-80dB versus -75dB) but the increase in spurs is obvious.Then two 50MHz wide scans, first with two green modules using the reference doubler and a 9MHz sinus input signal and second with one green module replaced with a black module and a 10MHz input signal.As you can see the 25MHz reference of the black module is a real problem as it blasts through everything and there are stronger fractional/integer spurs. The green modules has some 20MHz (and harmonics) bleeding but at a much lower level. Also there are very little fractional/integer spurs.However I am still learning so all this may be wrong.

author: erikka
author_rank: Regular Contributor
timestamp: June 28, 2019, 10:47:59 am
content: Quote from: chrisl on June 17, 2019, 08:41:05 pmUsing the Mux output you can connect a scope to various critical signals in the PLL path than can help you discover why the PLL is behaving as you observe.

author: CJay
author_rank: Super Contributor
timestamp: June 29, 2019, 04:38:52 pm
content: What happens if you tap the SPI signals from the Ebay module and feed them to the SV1AFN board?