Chinese LMX2595 board, any experience ? - Page 1
Forum section: electronics / rf-microwave

author: prutser
author_rank: Regular Contributor
timestamp: September 12, 2024, 06:39:16 pm
content: Already for some time there are PCB's with an LMX2595 on AliExpress and Ebay. In the past these cost around 300,- but recently I see them for < 140,-I was in the process of designing a PCB for an LMX2595, however the IC itself costs over 115,- in single qty, so a complete PCB for less than 140,- becomes an interesting alternative.Does anyone has some experiences wit those Chinese boards ? Especially with respect to phase noise ( I assume a decent external reference is required for some good phase noise performance)

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 13, 2024, 01:11:14 am
content: The LMX2820 is superior, but the board on aliexpress has significant issues because they used 10 MHz and not 100 MHz for the clock, while copying all the datasheet values for the loop filter (despite having a phase detector frequency of 10 MHz instead of the 200 MHz from the datasheet. This means that while the board locks, the phase noise is bad.The LMX2820 can be bought on LCSC for around 50 bucks each, and you can easily make your own breakout board for it. You could also buy a breakout board on Aliexpress, but replace the 10MHz crystal with a 100 MHz one and then write your own firmware.Here's what the phase noise looked like on my test board at 20GHz (second picture) with a properly designed loop filter, and the Chinese board (with the improperly designed loop filter). You can see that the phase noise on my board is >10dB better than the Chinese board.Here is the LCSC link to buy the chip:https://www.lcsc.com/product-detail/RF-Misc-ICs-and-Modules_Texas-Instruments-LMX2820RTCT_C2871859.htmlThey are out of stock but a month ago I asked them for a quotation and they let me buy 3 pcs for $50 despite being out of stock.The LMX2594 is also available for just over $20:https://www.lcsc.com/product-detail/Clock-Generators-PLLs-Frequency-Synthesizers_Texas-Instruments-LMX2594RHAR_C2862572.html

author: prutser
author_rank: Regular Contributor
timestamp: September 14, 2024, 08:52:36 am
content: Thanks for your reply. The LMX2820 seems they way forward for me, I am still wondering why I ignored or overlooked this part. Maybe it was an exceptional high price that time. I will give LCSC a try, price are much more reasonable than Digikey/Mouser.I saw in another post you used 'standard' stackup from JLPCB, how were the losses above 10GHz ? I have tried to find a "low cost" PSB house which does 4 layer Rogers or PTFE, but it seems impossible. Cheapest I could find was approx €400 for 3 pcs 5x5cm PCB (Rogers / FR4 hybrid). For amplifier, mixer, filter I am considering the JLPCB PTFE 2 layer, I made some test board with some grounded coplanar waveguides and SMA connectors, but the result was disappointing (reflection), I had better results with 4L JLC7628 (loss approx 3dB @ 12GHz for 5.5cm length) . 12GHz is currently the max I can measure.What PFD frequency, reference oscillator and SMA connector did you use ?

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 14, 2024, 05:28:26 pm
content: I used a PFD frequency of 200 MHz (100MHz in with the OSC_2x doubler enabled). Any 100 MHz crystal oscillator on the market should work. If you really care about stability you could probably use a OCXO (just make sure it has sharp edges since the chip works better with that). The connector used was an Amphenol 132255-12, but one of these is probably better and cheaper:https://www.newark.com/amphenol-rf/901-10511-2/rf-coaxial-sma-jack-50-ohm-pcb/dp/99Y7069Use the same values for the loop filter as in the eval board/datasheet. Use C0G or film capacitors.The loss at 10GHz is not that bad, but the loss at 20GHz is significant. Generally the rule is 0.2 dB / GHz / in, so at 10 GHz you will lose about 2 dB per inch of trace length.

author: bufffler
author_rank: Newbie
timestamp: December 28, 2024, 10:32:54 pm
content: Does anyone know where to get the chinese pc program for these devices? In typical fashion, the gui screen is shown, but there is no hint as to where to get the program. win10 recognizes presence and assigns a port, but I can't find the app anywhere...Thanks.

author: Verticon
author_rank: Contributor
timestamp: December 31, 2024, 10:31:52 pm
content: @ prutserthe prices for an LMX2820 or an LMX2595 in Europe are still high. Therefore a good solution could be to buy a chinese module and change the components for the loop filter. Because you should not underestimate the way from the naked IC to the complete board.Two years ago I built a 4-18 GHz signal generator using an LMX2595 as the main oscillator that was driven by a low phase noise 100MHz oscillator. The LMX2820 was not available at those days. The design and development of the circuit and the PCB and finally the manufacturing could be very time consuming. I used a JLPCB 4 layer FR4 stack and after many hours of optimising i ended up with a fairly good working device. The pictures below are a 6GHz SA picture, a phase noise plot at 6GHz and picture from my old HP141 SA at 18GHz.

author: rfdave#gmail.com
author_rank: Contributor
timestamp: January 01, 2025, 02:59:18 am
content: Has someone written Arduino code to control one of these?