HMC799 TIA oscillations? - Page 1
Forum section: electronics / rf-microwave

author: awallin
author_rank: Frequent Contributor
timestamp: July 04, 2020, 11:55:24 am
content: Hi all,I'm trying to build a moderately fast (4-700 MHz or so) photodiode frontend with HMC799https://www.analog.com/en/products/hmc799.htmlI get roughly the desired optical response (SA TG drives a VCSEL laser for the light source) as seen in the spectrum from 0 Hz to about 400 MHz, but there's an oscillation at around 1.6 GHz which is present with the detector dark also. At 1.6 GHz this is probably attenuated a bit (3-6 dB?) by the (BUF602,https://www.ti.com/product/BUF602) buffer-amplifier I am using - so measured directly at the HMC799 output this would be stronger than -27 dBm as seen on the SA. The BUF602 by itself (no input) does not oscillate.Trace A is the analyzer floor with the TIA off. Trace B is the TIA on (with 1.6 GHz oscillation), Trace C is the photodiode illuminated with the TG-modulated VCSEL.The optical response changes slightly when changing photodiodes from Thorlabs FDS015 to Thorlabs FDS025 - but the oscillation is the same.Anything obviously wrong with my schematic or PCB? (kicad sources:https://github.com/aewallin/One-Inch-Photodetector)(I may ask on the AD forum also, if I figure out how to post there..)thanks,AW

author: tggzzz
author_rank: Super Contributor
timestamp: July 04, 2020, 12:13:58 pm
content: Groundplane? Output near input?Have you read Phil Hobb's well-regarded book "Building Electro-Optical Systems: Making it all Work"https://electrooptical.net/Building_ElectroOptical_Systems/Alternatively there is a section on TIAs in TAoE 3.

author: awallin
author_rank: Frequent Contributor
timestamp: July 04, 2020, 12:26:34 pm
content: the top and bottom ground-planes look like this. I am hoping there are enough vias between them..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 04, 2020, 05:36:36 pm
content: Hmm, I don't know anything about HMC799 offhand. An input stub or feedback path would be a possibility. I'm not sure that either is present, or has the right length to be resonant.None of the HMC pins are ground? Just the pad?The PD case is grounded, yes? Through one of the pins, traditionally pin 3 I think?I would like to see less ground pour keepout. In particular, the ground return path around the BUF is quite long: top side, looping around top and right, past the output connector, to the bypass caps. Ground can be poured or connected under it directly, and also the patch above the HMC can be connected by lowering the PD keepout top edge, say to midline with, C105 is it? PD keepout right edge can be brought in to the GND pins, and bottom edge can be raised midline with C106.I wouldn't worry about nearly so many vias around the periphery; just put them where it matters, around traces on either side.Also, that polarity protection diode, the pads are very small -- I've seen some mfgs suggest high density footprints that look like this, and sometimes the part fits,but you wouldn't know that from the drawingbecause they almost always show the loose JEDEC SOD-123, or 323 or whichever this is, and not their (presumably!) tighter internal drawing. Following IPC recommendations for medium density, those pads will be much longer -- the main advantage is ease of hand soldering (in particular, a generous toe fillet dimension), and better compatibility in case you need to fit other parts or mfgs in that place.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: July 04, 2020, 06:24:57 pm
content: T3sl4co1l makes sensible points.In addition, have you followed the manufacturer's example layouts, especially ground plane, via and decoupling capacitor positions.

author: Dulus
author_rank: Contributor
timestamp: July 06, 2020, 10:46:27 pm
content: T3sl4co1l has very good suggestions.respectIn addition, you have a very very VERY bad ground loop over there :/HMC has no ground on the top layer, and you have no via's in periphery, so RF has only the bottom layer.I crudely tried to show the return path in the attachment.put the hmc and PD near each other and route the RF and return/ground paths as short as possible and "with respect to RF" as possible. and stich around with via's. Route the rest afterwards.Also you dont need the cutout around PD that much. it has to be a high impedance line though.make the RF trace as thin as possible, and 2~3x trace width clearence around the RF line, and you should be fine.As general suggestions;-Dont brake ground paths. You have a ground break at top layer in the top-left corner of the PD cutout.-Your output is close to your input. coupled with bad grounding and return path, its perfect recepy for positive feedback-passive componets on the RF path has a different width from the RF trace itself. make them the same thickness(make traces thick as component pads) and choose suitable dielectric thickness for your impedance. trace impedance is changing at those spots, and that can create reflections which may cause your oscillation.-in my opinion, you need via's wherever you can place them =)dont know if you'll make another board. if you do, please share your progress. i would be happy to help.if not, get some jumper wires and fix the ground breaks. especially on the bottom layerbest of luckDeniz

author: awallin
author_rank: Frequent Contributor
timestamp: July 09, 2020, 05:51:41 pm
content: Thanks for the comments so far.Following a suggestion on the AD forum I tried a series resistor in front of the 22nF input-coupling capacitor. This does remove the 1.6 GHz oscillation but instead produces a strong 330-340 MHz oscillation. I tried both a 49R resistor and a 499R resistor.I'm not sure if I will make another board for HMC799 - seems expensive (28eur @ digikey) and hard to use... the promise of 10 kOhm transimpedance and up to 700 MHz BW is nice - but so far not much luck..I will have to do some calculations (TIASim or TIs equivalent calculator) what could be done with the (almost) 10-fold cheaper TI opamps like OPA859, OPA858, OPA818 and similar.Maybe I'm trying this with the wrong photodiodes? (thru-hole parts with specified 0.6-0.9 pF capacitance at 5V reverse bias.)

author: Dulus
author_rank: Contributor
timestamp: July 10, 2020, 11:44:57 am
content: You're welcome.QuoteWell, it did solve your 1.6GHz oscillation problem.PD is most likely OK. Their bandwidth and responsivity can change, but from chinese no-datasheet ones to emcore best quality ones, all i've worked with did what you'd expect from them.The issue really is your layout i believe. And you cant solve that with just a resistor.You can try ~0.5pF to 2pF caps to ground throughout your stages, and cut the gain at high frequencies. this may solve your problem.HMC input node would require a lower value cap than the 50ohm lines since the impedance is much higher.you dont have to solder the PD trough the holes. bend and cut the leads as short as possible and solder them as close to the HMC input as possible. And as i said get a few jumpers and patch your ground connections.You dont need another IC, you can re-use the one you have forever with new boards:D