How to identify an RF gain block? - Page 1
Forum section: electronics / rf-microwave

author: petemate
author_rank: Regular Contributor
timestamp: November 13, 2024, 08:07:06 pm
content: Hi, I'm trying to identify an RF gain block used on a PCB that I got a hold on. The PCB belongs to one of these RF amplifiers:https://www.sonoma-instrument.com/products.htmlThe problem is that I don't know which one! There are no identifying markings on the PCB(other than Sonoma name and an internal number), so I'd expect it to be a generic PCB used across all those amplifiers.From the PCB I can only tell that there is an LM317 configured as a 62mA current source. I also know that the gain block in question is one of those 4-SMD gullwing things(also called SOT-86-4, I believe?). There are no markings on the actual gain block.When I combine the bandwidth spec of the amplifiers and the 62mA supply current, I am unable to find any component on Mouser and Digikey that matches the Sonoma specs. If they have the bandwidth, they don't have the gain - and vice versa. This leads me to believe that there is some sort of gain-bandwidth product for RF blocks, just like there is for opamps(yeah, as you can tell I don't know much about RF stuff). Is that the case?For instance, its impossible to find a 5GHz 26dB 62mA RF gain block with a noise figure of 6dB, as corresponding to the 352 amplifier. Actually, I can't find any gain block with more than ~12-16dB gain if it has to be supplied by 62-65mA, so that PCB doesn't match any of the Sonoma amplifiers listed. What gives? What more can I do to identify the gain block used?Thanks!

author: DaJMasta
author_rank: Super Contributor
timestamp: November 13, 2024, 09:11:27 pm
content: Testing its performance should tell you, though it requires a VNA or at least a CW signal source and power meter at minimum. Sometimes the package has markings on the bottom, but that's a relatively generic RF amplifier package and there are quite a few that were never listed on the main suppliers' sites.If you had a picture of the board, you could probably suss out some specifics, for example the 321 probably needs a heatsink or a large pad for the output amp stage, whereas the structures (and inductor values) will need to be smaller on the higher frequency parts. If there is a low pass filter on the output, trying to identify the passives used could give you an idea of the maximum intended output frequency, but this may not be necessary/present.Have you tried contacting Sonoma? The smaller manufacturers are much more likely to respond to unusual inquiries about their products, in my experience.

author: ftg
author_rank: Frequent Contributor
timestamp: November 14, 2024, 09:28:23 am
content: It is not very common to specify the current outside of typical and absolute maximum in RF MMIC gainblock datasheets.They are very likely using it below the maximums, so there's that.The noise figure of the part is also likely below 6dB if the whole assembly achieves nominal 6dB NF across it's operating band at that bias point.Could you post a picture of the board?

author: RadioNerd
author_rank: Regular Contributor
timestamp: November 14, 2024, 07:03:18 pm
content: Are you sure that all the gain of these amplifiers is achieved using a single MMIC gain stage?Can you post a picture of the PCB you are speaking about?

author: petemate
author_rank: Regular Contributor
timestamp: November 14, 2024, 08:21:09 pm
content: Hi all, thanks for replying!You ask for photos, so here we go:Also my traceout of the PCB.The unpopulated footprint fits with an SOT-86 package and there was a (presumably broken) SOT-86 part without markings taped to the PCB.

author: DaJMasta
author_rank: Super Contributor
timestamp: November 16, 2024, 04:39:19 am
content: The design is so basic and the main part is missing... so does it even matter which model it is?You seem to be missing another something in the biasing network (maybe a resistor), then a power supply and some DC blocking capacitors. With basically no filtering and only a single stage, that means the performance of the amplifier is entirely down to the amplifier that was used, and the remainder that you have can probably adopt a wide variety of amplifiers provided that they are compatible with the power supply.

author: petemate
author_rank: Regular Contributor
timestamp: November 16, 2024, 07:32:15 pm
content: Quote from: DaJMasta on November 16, 2024, 04:39:19 amWell, yes and no. Its true that I could basically just put in whatever MMIC, but I'd like to at least match the general parameters of the units. e.g. ~20dB gain, 3Ghz BW, with a reasonably low noise level. And I do need that 62mA current unless I also want to change those resistors. Which gain block would you suggest using?What is this about missing another resistor and DC blocking capacitors? I'm not sure I follow?

author: rfclown
author_rank: Frequent Contributor
timestamp: November 17, 2024, 12:36:20 am
content: Since we are guessing, I'd guess that this board is from a 354, 330, or "Other models available". As you found out, you don't usually find gain above 20 in this type of package. I would guess that the models with higher gain have different PCBs. I looked at the Miniciruits site (they have several amps in this package) and didn't find an exact match. The ERA-3SM+ which goes to 3GHz (like the 354) is close, but P1dB is less than 30 mW. You could look at other manufactures and probably find something to match the 354 or 330, or just choose and part you want.What I find more unusual is that the supply is configured as a current source. The gain block amps in this package are usually Darlington pairs, and the manufacturer will specify the Rbias resistor for a given supply voltage. The voltage at the device will be fixed, and the Rbias resistor is chosen for a specified current. Although I guess its ok to use a current source, I've not seen it done or shown in a datasheet. I've been working with devices like this for many years. If you pick a device with different current requirement, just change the current source resistors to match... or make the LM317 a voltage source, and follow the datasheet recommendations of the part for the bias resistor and voltage.

author: petemate
author_rank: Regular Contributor
timestamp: November 17, 2024, 08:10:32 pm
content: Thanks for replying, rfclown!Whats the chance of the Sonoma instruments having some sort of custom gain block?And yes, I'm also confused about the current source thing.. I was hoping that it would bring more clarity about the possible MMIC components, but it sounds like its a really weird solution?

author: rfclown
author_rank: Frequent Contributor
timestamp: November 17, 2024, 10:02:33 pm
content: I would say the the chance of the gain block being custom is close to zero. There are too many off the shelf gain block devices with which one or two stages can't meet the requirements for that list of Sonoma amps.I said seeing the current source was unusual only in that I've not seen someone do that, or seen a datasheet that suggests it. But... this type of amp is wanting to be biased by a current source. It's just that since the voltage at the device output is fixed, using a voltage source with a resistor amounts to a current source. Normally the amp won't have its own dedicated supply, so one of the voltage supplies in the product will be used with the appropriate resistor to set the current.An app note that describes biasing of these things is here:https://www.skyworksinc.com/-/media/SkyWorks/Documents/Products/1-100/200942B.pdf

author: G0HZU
author_rank: Super Contributor
timestamp: November 18, 2024, 12:06:15 am
content: The classic passive bias method using a resistor works well although it is a bit inefficient if you need to operate over a wide temperature range. This is because it’s generally best to have at least 3V dropped across the resistor although >4V is better if you want to operate over -40degC to maybe 60degC. The classic alternative is to use active biasing. Probably the simplest active bias circuit uses a PNP transistor and a diode and a few resistors to make a stable current source. I’ve used this active bias method with MMICs since the early 1990s when good efficiency and good performance over temperature are both required.You can find this PNP based active bias circuit in various old HP application notes for their old MODAMP series of MMIC amplifiers.

author: G0HZU
author_rank: Super Contributor
timestamp: November 18, 2024, 09:07:50 pm
content: Looking through the list of Sonoma amplifiers, the 310 version (32dB gain) is only spec'd to 1GHz at -3dB so this could (just) be done with an old school 86 packaged MMIC. This kind of performance was available decades ago. However, 62mA seems to be very high for the P1dB spec of only +10dBm for the 310 amplifier. So this may not be what you have there.

author: G0HZU
author_rank: Super Contributor
timestamp: November 18, 2024, 10:00:07 pm
content: Quote from: petemate on November 16, 2024, 07:32:15 pmThe part that isn't fitted at the input of the MMIC is probably a dual PIN diode in a SOT-23 package that acts as a protection limiter. Something like an old HP/Avago HSMP-3822. It would need another blocking cap ahead of it to protect it from external DC and it looks like this cap has been bridged out on your board.Something like the old HP INA-02186 MMIC could act as the active device in the 310 amplifier although this amp is normally biased at 35-40mA. 62mA is way too much.However, the INA-02186 is notoriously difficult to tame for stability and it usually requires a special PCB design to minimise ground inductance. Your PCB supports fitting the 86 package MMIC on the PCB underside and this might help improve the grounding of the device such that it could be used with such a basic low cost PCB as you have there. This is because it grounds direct to the groundplane on the PCB underside.The HP INA-02186 MMIC really is a scary device in terms of the PCB design requirements. These MMICs were used at work decades ago. They look just like a regular 86 packaged MODAMP but they are derived from the MagIC series of MMICs containing a two stage amplifier. This gives lots of gain and it can maintain it up towards 1GHz. The downside is that the part requires very careful grounding. Most MMICs require careful grounding but the INA series of MMICs require careful grounding^3This MMIC ought to provide a low noise figure down to a few MHz or maybe less as it is a silicon based device.Modern MMICs tend to use InGaP technology to obtain high electron mobility. This ultimately provides lots of gain over a huge bandwidth. So no need to adopt the scary two stage innards of the old INA MMICs. The downside of InGaP is that the noise figure tends to creep upwards once you get below about 10MHz although this will vary across the various types.

author: G0HZU
author_rank: Super Contributor
timestamp: November 18, 2024, 10:24:04 pm
content: I've attached the datasheet for the INA-02186 MMIC below. Way back in the 1980s these amplifiers were made by Avantek (INA-02170) and they were housed in a sensible package. At some point someone (at HP?) decided to put it in an 86 package to create the INA-02186 which seemed a tad reckless. If you look at the datasheet, it states that the device is unconditionally stable. However, this only applies if you ground the device very well using lots of via holes (directly under the device connections) and you use a very thin PCB material. This gets the ground inductance well below 0.1nH.If this grounding performance isn't achieved, then the device loses its unconditional stability in a big way. The datasheet below doesn't warn about this although it might be mentioned on a missing page? It's up to the designer to work this out from the supplied s-parameters and an estimation of the grounding performance of their PCB.

author: KE5FX
author_rank: Super Contributor
timestamp: November 18, 2024, 11:19:14 pm
content: The INA-02186 sounds believable, as Sonoma Instruments was founded by an ex-HP guy (to pick up where the HP 8447A left off, IIRC.)A voltage regulator might be noisier -- notice how this is basically an RC filter in disguise, with a pole at 1/(6.28*33E-6*20.3) = 240 Hz before you even consider the dynamic resistance of the series pass element in the LM317, which would presumably lower it further. This configuration also offers 0V dropout voltage, since it reverts to a passive filter when the regulator goes offline.So in return for the wacky topology he's getting more supply-voltage operating range than he would otherwise, plus some extra noise filtering, plus a slow turn-on characteristic that won't dump a lot of charge through the output coupling capacitor at turn-on. These amps see a lot of use in EMC labs between the antenna and a six-figure spectrum analyzer, so job #1 is to not do that. (Notice the DC bleeder resistors at the ports...)

author: coppercone2
author_rank: Super Contributor
timestamp: November 18, 2024, 11:26:54 pm
content: Does anyone find it weird that its AC powered and the rectifier is on the same board as a RF amplifier

author: KE5FX
author_rank: Super Contributor
timestamp: November 18, 2024, 11:35:22 pm
content: Nah.

author: G0HZU
author_rank: Super Contributor
timestamp: November 18, 2024, 11:53:59 pm
content: Quote from: KE5FX on November 18, 2024, 11:19:14 pmThanks. I think another indication that the INA-02186 is used in the 310 amplifier is that the datasheet plot for the 310 amplifier shows a familiar (to me) peak in the gain response at about 700 MHz. In my experience, the amount of peaking here is a clue to how well grounded the 02186 MMIC is. If it is really well grounded the peak will be barely noticeable. It only takes a tiny amount of degradation in the grounding to produce a >1dB peak here. If the grounding is even worse, the device will oscillate. It looks like the 310 amplifier has about 0.5dB peaking at 700MHz. In reality, the peaking will be greater than this as some of it will be masked by the losses in the PCB and the blocking caps.To get the peaking this low implies the part is quite well grounded in the 310 amplifier and this is presumably because they fitted the part to the PCB underside. Normally, the PCB has to be really thin for the INA-02186 and the ground via holes have to be fitted inside the PCB pads under the ground legs of the 02186.Even if it turns out that the INA-02186 isn't used in the 310 amplifier, it would be a reasonably good substitute device as long as it was biased at about 40mA and you can keep it stable!The noise figure is right at just under 2dB and so is the input and output VSWR vs frequency. It will probably manage 32dB gain if biased a bit harder at 40-42mA and the gain droops by 3dB at 1GHz just like the 310 amplifier.The P1dB is also similar to the 310 amplifier at +10dBm.I'm not sure how easy it will be to buy a genuine INA-02186 MMIC in 2024. There are plenty for sale on Ebay, but they may be modern clones or fakes.