Fiber-capable General Purpose SerDes? - Page 1
Forum section: electronics / projects

author: Echo88
author_rank: Frequent Contributor
timestamp: September 02, 2018, 08:43:00 pm
content: At the moment im using 4 optical fiber cables to establish a SPI-communication with noise sensitive ADCs. Id like to know if there are easy to use SerDes-ICs or modules, which are capable of transmitting those 4 separate SPI-signals over 1 fiber-cable and deserializing them after the transmission? Doesnt have to be high bandwidth, communication in the kHz-range is sufficient.Thanks.

author: TiN
author_rank: Super Contributor
timestamp: September 03, 2018, 12:01:00 am
content: HP and Fluke use VersaLink optical isolators with fiber. You can buy transmitters/receivers and fiber with connectors in places like Digikey. Good up to some megabits.Since it's just pure logic level interfaces, you can transfer most of interfaces without problem, including SPI.

author: Echo88
author_rank: Frequent Contributor
timestamp: September 03, 2018, 10:10:35 am
content: I already use said VersaLink Transmitters/Receivers and thought it might be possible to reduce the number of fibers from 4 to 1 with fitting multiplexing-ICs.

author: Hydron
author_rank: Super Contributor
timestamp: September 03, 2018, 11:51:42 am
content: How noise sensitive are you talking? Would a simple MCU running at a low clock speed be too much on the ADC side of the system? A UART -> SPI bridge in software sounds like it would do the job.Other ideas:- Can you could generate the CS signal on the ADC side from the activity on CLK/MOSI? (possibly with some gating on the input side) - that would drop the fibre requirement by one.- Look into Manchester encoders/decoders or a DIY programmable logic implementation of one - simple self clocking encoding scheme, used in S/PDIF amongst many other things. If you can get rid of CS then that would let you do bi-directional comms with two lines (encode CLK and MOSI on single line, MISO un-touched).I do understand the want to reduce the number of versalink parts, they are not cheap!

author: Echo88
author_rank: Frequent Contributor
timestamp: September 03, 2018, 12:51:38 pm
content: Yeah, i searched on ebay and other sources to find cheap HFBR-emitters/receivers to justify the use of fiber in my design.In the end i used a 650nm emitter and a 660nm receiver from different HFBR-series, but it works like a charm. I think i paid 5€/receiver, 2.5€/emitter and 1.25€/2m-cable.Of course i could develop the SerDes myself, but i rather spend the time on programming a GUI for my adc-project and i hoped someone already published a OSHW-design for such a SerDes.The fiber-links are used because i wanted to rule out emi noise from the µC, since i want to get the absolute lowest noise floor achievable with the ADCs i test. Also: i never did a project with fiber-links before, so it was interesting.

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 03, 2018, 01:57:59 pm
content: Quote from: Echo88 on September 03, 2018, 12:51:38 pmThat cable included the end connectors?

author: Echo88
author_rank: Frequent Contributor
timestamp: September 03, 2018, 06:26:06 pm
content: Indeed Mastertech. My source:https://www.ebay.de/itm/AVAGO-HFBR-RNS002Z-Optical-Fiber-Glasfaserkabel-2M-1-St%C3%BCck/361709394704?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649Also the user owiecc still sells receivers/emitters i think:https://www.eevblog.com/forum/buysellwanted/fs-eu-avago-1531z2531z-fiber-optic-transmitters-and-receivers/

author: owiecc
author_rank: Frequent Contributor
timestamp: September 04, 2018, 05:27:07 pm
content: I have SFH756/SFH551 €2/pc.

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2018, 11:49:41 am
content: I agree with Hydron about programming a microcontroller to operate as a UART to SPI bridge given your low data rate requirements.

author: knapik
author_rank: Regular Contributor
timestamp: September 06, 2018, 12:17:09 pm
content: Maybe you could check out some of the same connectors used with S/PDIF / TOSLINK? However, I've read that you have to be careful as the receiver response doesn't extend down to DC, but is 100kHz minimum.Something like these could be useful?https://www.digikey.com/product-detail/en/everlight-electronics-co-ltd/PLT133-T9/1080-1429-ND/2693986https://www.digikey.com/product-detail/en/everlight-electronics-co-ltd/PLR135-T/1080-1431-ND/2693959*edit*Sorry, there was a lot of talk about connectors and only after I hit post I realised you were talking about recommendation for a SerDes. I just hope I don't derail the conversation.

author: Hydron
author_rank: Super Contributor
timestamp: September 06, 2018, 01:40:37 pm
content: S/PDIF and TOSLINK stuff certainly isn't far off topic - for starters the manchester code technique used is possibly a good match for this application, and they also provide options for low cost tx/rx parts and fibre. If the OP was looking at using ADCs running at audio type frequencies (and an I2S interface is available) then maybe S/PDIF transmitters and receivers are a possible SerDes solution too?