[Dprglist] Broadcom AFBR-S50LV85D vs. ST Micro 53LCX

[John Swindle]

Paul,
Regarding Range Ambiguity: During physical layer training on high-speed interfaces such as DDR5 and PCIe where flight times exceed one or two ticks of the clock, it is possible to lock onto an alias of the training sequence and be off by one (Help me, Off-by-One Kenobi. You're our only hope.) or off by two. One trick is to use training sequences that have gaps or half-frequency sequences. If the receiver locks onto the wrong clock, the half-frequency sequence will fail predictably making it easy to immediately adjust the lock.
I suspect the rangefinder is doing the same thing, by simply failing to emit some of the pulses. If the receiver reports a result for a pulse that wasn't emitted, the receiver is off by a cycle. Easy to detect and easy to process. A single detector could handle multiple ranges ongoingly. Same thing was done in the 765 Floppy Disk Controller with intentionally suppressed clocks.
Pseudorandom sequences prevent aliasing but are harder to correct. Most high-speed training is still done with fixed, non-random patterns, though DDR5 has two LFSRs.

I suspect that my emails to DPRG list are not posted. Would someone let me know they saw this?
Later,John Swindle


-----Original Message-----
From: Paul Bouchier via DPRGlist <dprglist at lists.dprg.org>
To: DPRG <dprglist at lists.dprg.org>
Sent: Wed, Mar 23, 2022 2:57 pm
Subject: [Dprglist] Broadcom AFBR-S50LV85D vs. ST Micro 53LCX

At last night's RBNV, Doug P introduced us to the Broadcom AFBR-S50LV85D TOF range sensor. It looked interesting, with a max range of 30m, and I wondered how it compared to the ST Micro 53LCX, which I'd analyzed previously and reviewed in a DPRG list post on Dec 15 2021. That review is now on my site at https://sites.google.com/site/paulbouchier/home/analyses-blog.
I found an excellent application note here:https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiv2qCiudz2AhWGl2oFHR7yAq0QFnoECAcQAQ&url=https%3A%2F%2Fwww.avnet.com%2Fwps%2Fwcm%2Fconnect%2Fonesite%2F217f3b0e-788e-44c0-a63a-9595debc8370%2FApplication%2BNote%2BAFBR-S50%2BToF%2BBasics.pdf%3FMOD%3DAJPERES%26CVID%3DnuRdfj6%26CVID%3DnuRdfj6%26CVID%3DnuRdfj6%26CVID%3DnuRdfj6&usg=AOvVaw1D41jdve_frPH2eVGMTVF2
Like the ST 53LCX family of sensors, Broadcom offers a family of sensors under the AFBR-S50LV, and the 85D version is optimized for long range. Like the 53LCX family, they use "Inferred Time-of-Flight" (iTOF) for distance measurement, which as John Swindle observes, isn't direct time-of-flight measurement, but is phase measurement of the return of the modulated transmit signal cross-correlated with the received signal. 

Someone last night observed that the Broadcom sensors have a linear (8 x 4) array of sensors and wondered if this is a linear array sensor like the old Sharp sensors, which used parallax to measure distance as indicated by which sensor in a linear array received the reflected light. Not really. The linear array enables the sensor to detect nearby and distant objects, whose reflections would fall in different places, so it accommodates parallax, and it  looks at several factors including amplitude to choose which pixels to use for distance reporting, but it doesn't use parallax for distance measurement.
The Broadcom sensor family varies the diameter of the transmit spot in different models. The 85D is long range because it uses a very narrow beam that consequently produces a stronger reflection from distant objects than the ST senstors and has better range in bright sunlight. By contrast, the ST sensors don't seem to have such narrow beams - they illuminate the whole of the sensed region. This lets the ST sensors offer selectable areas of the array for sensing, or to sense multiple areas, producing a multi-zone sensing. It's not clear to me whether the Broadcom 85I version can also produce multiple sense zones.
Unlike the ST sensors, the Broadcom ones don't appear to offer multi-target capability (i.e. multiple returns at different objects reported as different objects within the same zone.
The broadcom application note brought up an interesting facet of phase-difference distance measurement which didn't appear in the ST documentation: Range Ambiguity. There is a range, dependent on the modulation frequency, beyond which a reflection will appear to be much closer than it is. This is because the phase angle of the return exceeds TWO_PI, so it thinks the object is a phase closer. This distance is 6m or 12m depending on the short or long range setting of the sensor. Maybe the ST sensors, being shorter range, don't suffer from this problem because they can't detect anything further away - I don't know. Broadcom has some techniques to mitigate this.
In summary, they're similar kinds of sensors, both sophisticated, and with versions optimized around various applications. Which is better is application-dependent, and ya pays ya money an' ya takes ya choice. 😁
I would have thought that for obstacle sensing for small, slow robots, a short-range multi-zone wide-angle sensor would be better, whereas for an automobile, a single-zone long-range sensor would be better. But there might be other considerations - feel free to chime in with your thoughts.
I hope this was useful.
Regards
Paul

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