A look inside some Mini-Circuits modules - Page 1
Forum section: electronics / rf-microwave

author: D Straney
author_rank: Frequent Contributor
timestamp: March 22, 2023, 07:57:39 pm
content: Had the opportunity a while back to take the covers off a variety of Mini-Circuits modules in lab stock. It's pretty standard stuff as far as RF is concerned, so this'll be old hat to most people here, but I'm only tangentially involved in the high-frequency world so I'm still easily impressed by things like special winding patterns on wideband RF chokes.Many of the older modules contained just a PCB with the metal-can version of the equivalent Mini-Circuits part (which makes perfect sense from keeping design/stocking/production efficient), and one switch module would've been visible only if desoldering the connectors, but there's still some things to see in the rest of them:ZX60-6013E-S+ wideband ampWas surprised at the small number of components actually - just a single transistor and/or MMIC, DC filtering then a choke and series resistor to bias the output/collector/drain, and DC-blocking caps at the input and output.Also looks like there's a small stub or capacitive bit on the output for matching (the frequency range does extend up to 6 Ghz).ZFSC-2-1W-S+ splitterSeems to use the common "center-tapped transformer with resistor across it + 2:1 auto-transformer for input impedance matching" scheme (described in more detail here:https://www.minicircuits.com/app/AN10-006.pdf, but with a little bit extra in the form of some planar spiral inductors (which must have values in the single nH range or below) in series with that resistor across the outputs, and an extra capacitor to ground. If I remember correctly, I think that capacitor to ground partly compensates for the magnetizing inductance of one or both transformers, and the series inductors with the 100Ω resistor compensate for the output transformer's leakage inductance - but can't find a reference on that right now.U2C-1SP2T-63VH SPDT RF switchLooks like what's an SPDT switch at a high level actually uses 6 SPDT switch ICs inside. The switch directly on the COM port seems to have been added so that it can be disconnected from both NO & NC and terminated to 50Ω. One of the extra pairs of switches on the NO & NC ports is there to terminate the port to 50Ω when unused (you can see the termination resistors, larger on the ones closer to the port and smaller on the switches one level back from the port), and the second pair I think is to add extra isolation when unused.Kind of funny in a digitally-controlled model like this (it takes USB, I2C, and SPI inputs) where the microcontroller and power supply just for interpreting inputs has 100x more transistors than are in the RF "business end" of things.LZY-22+ RF PAYou can see the signal path clearly here, where from the left it goes through a crossed diode pair to protect against input over-voltages, and then 3 gain stages: first a small SOT-89 class-A stage, then a larger heatsink-mount class-A stage, and finally what looks like a class-AB dual-transistor stage to provide the full output power. The 100Ω resistors next to the output transistors seem to be involved in providing some local feedback for the output stage. There's a bunch of stuff around for biasing that I haven't mapped out, regulators etc. The big TO-247 transistor screwed to the case seems to be a voltage regulator for the incoming power, and the "Airpak" device turns out to be a thermal switch for over-temperature protection.ZFSWHA-1-20+ switchFinally, an amusing example of the "standard Mini-Circuits PCB-mount part within a module":I like the hack-ish shield hanging over the board, hand-soldered at the connectors and wire-bonded to the IC package. Looks like someone realized too late that there wasn't enough isolation in the design... (or the enclosure created some kind of resonant mode: would think a square of damping material would be a simpler solution there, but who knows about the details)

author: RoV
author_rank: Regular Contributor
timestamp: March 22, 2023, 10:40:27 pm
content: Nice photos, thanks!I add one that I found interesting: a ZFL-1000VH2, a so-called medium power amplifier (25 dBm out) 10-1000 MHz. It surely required a lot of hand work to be assembled!

author: D Straney
author_rank: Frequent Contributor
timestamp: March 23, 2023, 07:22:17 pm
content: Also, that sure is a lot of glue...

author: stephen_g
author_rank: Newbie
timestamp: March 24, 2023, 03:11:16 pm
content: Wow, nice! I use a lot of their surface mount parts in our designs at work, it figured that their modules would mostly look like this and use their own parts but I'd never had a chance to look inside any!Did you take a picture of the other side of the drop-in wall on the U2C-1SP2T-63VH? It looks from the PCB like it had a form-in-place RF gasket?

author: D Straney
author_rank: Frequent Contributor
timestamp: March 26, 2023, 03:41:48 pm
content: I didn't, I don't think that module had a back cover (looked like it was milled from a solid block), or if it did I didn't notice.Here's a bonus photo from one that I actually have in person, a ZX73-2500-S+ voltage-variable attenuator that I kept and opened it after it got fried with some over-voltage on the power supply pin. Inside is basically just a filter on the power input, and a Mini-Circuits RVA-2500 SMT module that I removed the cover from:This one's super simple, as far as I can tell it relies on using the small-signal equivalent resistance of a diode, which changes depending on the forward bias, as a variable resistor. Same principle as the diode ladder filters in old musical synthesizers.You can pretty much read the schematic right off the board, and the RVA-2500's datasheet (https://www.minicircuits.com/pdfs/RVA-2500.pdf) shows the arrangements of the diodes minus the extra passives. Only differences are:Control voltage goes to a shunt cap, and then through 332Ω in series, before going to the diodes. This resistance together with the 562Ω described below on each diode makes the control act more like constant-current bias than constant-voltage bias for the diodes; putting a "voltage source" across the diodes as in the datasheet's simplified schematic wouldn't make any sense. Setting the forward current instead keeps it from being wildly sensitive to individual diode characteristics and temperature."Matching" on the input and output looks more like DC blocking than anything else, as it consists of a series cap, and 562Ω to groundPower comes in through a single shared series resistor, and then branches off to a series 1.5KΩ resistor and shunt cap for each diode pair, for filteringIt's also possible, since that shared input-power-series resistor looks like a different family than the others, that it's actually a low-value thermistor to do some rough temperature compensation with the bias current and keep the gain a bit more stable over temperature.

author: SmVyZW15
author_rank: Newbie
timestamp: September 16, 2025, 03:31:19 am
content: Hello everyone,I’m new to the RF world and recently bought a Mini-Circuits splitter model ZFSC-2-2500-S+ for about $65 from an unofficial supplier on AliExpress:https://pt.aliexpress.com/item/1005001807736118.htmlThe seller claimed it was original, but I had some doubts, so I opened it up to check. I’m still unsure since I don’t have an authentic one to compare it with.I’ve attached some photos of the unit I received. Does anyone here own the same or a similar model to compare, or can tell whether this one looks genuine?Thanks in advance for your help!

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 16, 2025, 12:39:09 pm
content: I can't find any photos of that exact model, but it's safe to assume it's counterfeit. Comparing it to the genuine ZFSC-2-1W-S+, the circuit is a far simpler despite having a much wider specified bandwidth. The board having no markings is a big red flag as well.As to its performance, only measurements will tell the truth. I'm betting it won't meet the VSWR and isolation specs in the datasheet.

author: KE5FX
author_rank: Super Contributor
timestamp: September 17, 2025, 02:10:49 am
content: Honestly, this doesn't scream "counterfeit" to me. A ZFSC-2-2-S+ (10-1000 MHz) looks similar in overall construction. The board markings or lack thereof don't mean anything. However, the other ZFSC parts I'm familiar with use two transformers and a couple of other parts in addition to the resistor. This one looks like a standard textbook Wilkinson splitter, but I'd expect the resistor to be 100 ohms rather than 470.So that's weird.You'll need to test it to be 100% sure, or buy a known-good one and take it apart for comparison. $65 is a lot of money on Ali Express! Almost half the price at DigiKey or Mouser.

author: RadioNerd
author_rank: Regular Contributor
timestamp: September 17, 2025, 08:24:43 pm
content: The ZFSC-2-1W-S+ in the picture looks genuine to me. Counterfeit items would modt likely contain a PCB made of standard yellow/green FR4 material and use lower-quality SMA connectors.

author: RFDx
author_rank: Regular Contributor
timestamp: September 18, 2025, 08:36:42 am
content: The ZFSC-2-2500-S+ looks off to me. No 2:1 transformer at the input and a 470 Ohm isolation resistor with not one port matched to 50 Ohm? The color of the resistor is weird too, all the MCL modules i have seen typically use (sky)blue resistors.

author: KE5FX
author_rank: Super Contributor
timestamp: September 18, 2025, 05:28:36 pm
content: Yeah, I 'd really like to hear the rationale behind that resistor choice. There's always a new trick to learn in this business, but...

author: D Straney
author_rank: Frequent Contributor
timestamp: September 18, 2025, 05:43:33 pm
content: Yeah can't give adefinite"no" just from visuals, but it's at least highly suspicious.For what it's worth, those blue resistors are likely the KOA Speer RK73H series. I liked using them in the lab because they were visually distinctive for IDing things, had better than normal specs, and still cheap enough to still treat as jellybean parts.

author: G0HZU
author_rank: Super Contributor
timestamp: September 18, 2025, 08:12:32 pm
content: If it helps, I'm a career long user of Minicircuits' modules and that one does look a bit suspicious for a few reasons.The main aluminium block isn't machined/cast the same way as many other Mccts modules. It looks cheaper to produce and lacks the cutout shapes in the corners.The PCB material looks different to any Mcct module I've looked inside. The weave pattern looks very different and is much finer and the PCB edges are a bit ragged. Also the PCB material lacks a thin isolated ground border around the edges on the reverse side and there are no markings or circular pip shapes on the PCB top side.The black resistor is unusual and Mccts usually use blue resistors as has already been mentioned.There appears to be a ground connection near the sum port and this is unexpected. I've used the ZFSC-2-2500-S+ many times at work as a GPS antenna splitter and it allows the same DC bias to feed between all three ports. In other words, there should be close to zero ohms between all three ports and there should be high resistance (open circuit) to ground on all three ports when tested with a regular DVM set to the ohms range. But that device looks to have a ground connection as there are lots of ground vias. Of course, this could just be for mechanical strength but this type of splitter from Mccts should not have a path to ground. So I would not expect the PCB to have any ground vias.Over the years, Mccts may have changed the design of a splitter like this so it may not be conclusive to directly compare it with another one. I can borrow one from work and take it apart but it won't be until next week.One way to check would be to measure it on a 3 port VNA and compare it to the factory s-parameters. I can do this next week when I borrow one from work. There will be at least a dozen of these splitters at work of various ages and I'll try and find a shiny new one and also something that looks older.

author: Miek
author_rank: Regular Contributor
timestamp: September 18, 2025, 09:48:00 pm
content: The part used on that board looks a lot like a TCP-2-25+, which also suggests a ~470 ohm resistor in the datasheet. Of course the minimum frequency doesn't match up, so it might be a slightly different part if it is a genuine module.Also, here's a genuine ZFL-500 amp I took apart, the ragged PCB material looks pretty similar.

author: G0HZU
author_rank: Super Contributor
timestamp: September 18, 2025, 10:38:36 pm
content: Thanks! I still think the PCB weave looks subtly different to what I'm used to seeing. The Mccts weave is slightly different in the x and y directions but maybe I'm being too critical.I'll see if I can get hold of one tomorrow. I don't live far from where I work but I don't normally work on a Friday.I still think that Minicircuits may have changed the design over the years so it's going to be difficult to prove if it is real or not unless someone can find a genuine one that looks exactly the same inside. i.e. find a version that is of a similar vintage with the same internal design and construction method.

author: G0HZU
author_rank: Super Contributor
timestamp: September 18, 2025, 11:32:48 pm
content: Normally, the wideband versions of these things are designed using a tiny bifilar wound transformer as in the diagram below. The port 1 VSWR will be poor at low frequency at about 2:1 here because the port 1 impedance will be close to 25 ohms.Up at UHF, the VSWR of port 1 can normally be improved (to much lower than a 2:1 VSWR) with a shunt capacitance at port 1 as indicated by C1 in the diagram below. Maybe this is why there is a ground connection on the PCB as there may be some form of shunt capacitance at that node? Normally, this would be fabricated using a tiny SMD MLCC but there will be other ways to do it using a printed capacitor or maybe that transformer has an integral capacitance to the ground connection? This would explain why the ground vias are there.I can definitely state that these ZFSC-2-2500-S+ parts can be used to distribute a common DC bias to all three ports and that there should be an open circuit to ground at DC at all three ports. I suspect that the suspect version still meets this requirement because the ground connection is probably there to allow some sort of gimmick shunt capacitance. Without this capacitance, the port 1 (and port 2 and port 3?) VSWR up at UHF would be much worse.

author: G0HZU
author_rank: Super Contributor
timestamp: September 19, 2025, 05:35:33 pm
content: Found one today... see below for an image of the insides of a Minicircuits ZFSC-2-2500-S+. I can't guarantee that this is a genuine part but it was at work and the company usually buy direct from Minicircuits.The PCB has the weave pattern that I'm used to seeing. I also borrowed a couple of other splitter types ZFSC-2-1+ and ZFSC-2-11-S+ if there is any interest seeing inside these?

author: G0HZU
author_rank: Super Contributor
timestamp: September 19, 2025, 06:35:19 pm
content: To boost confidence that my ZFSC-2-2500-S+ is genuine, I put it on my 4 port VNA and compared it to the factory s-parameter data file from Minicircuits up to 3.5 GHz. See the plot below.The amplitude agreement is very good through to about 3.5 GHz. The phase agreement is within about 1 degree at 2.4 GHz.The port VSWR agrees very well too. So I think I've got a genuine ZFSC-2-2500-S+ here

author: Kean
author_rank: Unknown
timestamp: September 19, 2025, 08:29:42 pm
content: Quote from: G0HZU on September 19, 2025, 05:35:33 pmThat is the exact same internal construction of the older ZFSC-2-2500 in this video - internals can be seen about 3 minutes in.Quote from: G0HZU on September 19, 2025, 05:35:33 pmThe only ZFSC part I have here is an old ZFSC-2-2 (10-1000MHz, 1998 vintage?) and the PCB is apparently sealed in white potting compound or something.See attached photo. I wonder if your other ones will be like that, or mine is special. I'm happy to try remove the compound as this is not likely to get used again.

author: shabaz
author_rank: Frequent Contributor
timestamp: September 19, 2025, 08:50:41 pm
content: Here's a genuine ZFDC-20-5-S+ coupler (PDF datasheet) bought in 2018, direct from Minicircuits.I took quick photos with a mobile phone, but can take better ones later.

author: SmVyZW15
author_rank: Newbie
timestamp: September 23, 2025, 04:07:47 am
content: Hi folks,I got one more ZFSC-2-2500-S+. It claims to be an original used one.The overall construction looks closer to the original pictures fromG0HZUand the video fromKean.It has the blue resistor, PCB markings on the top side, and the corner cutouts in the main aluminum block.The PCB material also looks very similar to the "ZFSC-2-1W-S+ splitter" picture shared byD Straney.Now that I have a 2-port LibreVNA, I’m planning to run some tests. I’m not sure how reliable the results will be with a non-professional VNA, but I’ll give it a try. Any advice or suggestions would be very welcome.

author: Kean
author_rank: Unknown
timestamp: September 23, 2025, 05:36:17 am
content: Just for the heck of it before I put it away, I removed the silicone material covering the PCB in my ZFSC-2-2 part.Slightly different design (older, lower frequency).

author: KE5FX
author_rank: Super Contributor
timestamp: September 23, 2025, 06:03:08 pm
content: Seems downright surreal to see those through-hole resistors with long leads used in 2 GHz+ rated microwave components. But they know what they're doing.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 25, 2025, 12:58:27 pm
content: Quote from: KE5FX on September 23, 2025, 06:03:08 pmYeah, but the downside for them is that it's also easy to reverse engineer and copy such a design. The transformers are likely the only "secret sauce".

author: tszaboo
author_rank: Super Contributor
timestamp: September 25, 2025, 01:26:19 pm
content: Quote from: mtwieg on September 25, 2025, 12:58:27 pmTrue, but the trouble with microwave frequency circuits is mostly going to be mechanical engineering, not electronics.You can probably make a lot of money with a lathe if you can make RF stepped impedance filters. Cut the connector to the extreme precision, integrate it together with the internals of an N connector, make a bunch of different diameters on a piece of metal, and weld/braze it into a tube, ask a thousand dollars for it.

author: G0HZU
author_rank: Super Contributor
timestamp: September 25, 2025, 02:19:08 pm
content: Quote from: SmVyZW15 on September 23, 2025, 04:07:47 amI had another rummage at work and found an old box containing about twenty ZFSC-2-2500-S+ splitters. These were bought for use in a suite of test jigs but never got used in the end. They are probably quite a few years old but still look new.The first one I took apart looks identical to yours with the same colour PCB and the same PCB number 001-162. These would normally have been purchased direct from Minicircuits so I think it's safe to assume they are genuine. I could put it on a VNA and compare it to the other one I have here but it probably isn't necessary.

author: SmVyZW15
author_rank: Newbie
timestamp: September 26, 2025, 03:29:49 am
content: Quote from: G0HZU on September 25, 2025, 02:19:08 pmThanks for checking. I agree it looks original. I don’t think there’s any need to put yours on a VNA.The only thing we can’t really know is what conditions my unit was exposed to before it was taken out of service and ended up on AliExpress.I’ve been running some S11 and S21 tests and trying to match the data from the manufacturer, but I’m seeing a few differences.I’m probably messing something up in my setup, and of course this one is a used part and my VNA isn’t professional-grade either.I’m going to dig into The VNA Applications Handbook to get a better handle on things, and I’ll probably grab a brand-new unit at some point just to take that variable out of the picture.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 29, 2025, 11:42:35 am
content: Considering what RF eval boards routinely cost, I am always a bit surprised Mini-Circuits modules aren't more expensive. It very much looks like they're hand assembled, and I believe individually tested.Anyway here's an older ZFSC-2-1W 2-500 MHz splitter/combiner. The enclosure design seems to have come a long way!

author: tszaboo
author_rank: Super Contributor
timestamp: September 29, 2025, 12:03:14 pm
content: Quote from: ChristofferB on September 29, 2025, 11:42:35 amBecause Eval boards, you buy it once to select an IC and verify the performance in your system.The Mini-Circuits boards, you don't know typically what's inside, and you are supposed to build your system with those.

author: G0HZU
author_rank: Super Contributor
timestamp: September 29, 2025, 07:01:33 pm
content: Many years ago (early 1990s) the UK Minicircuits rep JP.N gave me some background about how the early Minicircuits modules were hand assembled. It was very different back then in terms of how the modules were put together. Hand tacked together (ugly build) methods were common but very effective. That's why I earlier suggested that you would have to obtain all build types over the years for a given Minicircuits module as they do change the way they make them. For example, I think we have so far found two genuine ZFSC-2-2500-S+ modules that are different in terms of the PCB material used and there are also subtle differences in the PCB artwork because of this.

author: mawyatt
author_rank: Super Contributor
timestamp: September 29, 2025, 11:52:50 pm
content: Quote from: G0HZU on September 29, 2025, 07:01:33 pmRecall the hand assembly as well, and recall they also used prison labor.Best

author: D Straney
author_rank: Frequent Contributor
timestamp: September 30, 2025, 03:16:46 am
content: Wellthattook a darker turn real quick. Before, I'd just been thinking about how much the questionably-wetted blob of solder on the BNC threads sketched me out.

author: SmVyZW15
author_rank: Newbie
timestamp: September 30, 2025, 11:54:48 pm
content: I found a teardown video on the IMSAI Guy YouTube channel showing the inside of ZB6PD-2-S+ Mini-Circuits splitter. I thought it might be interesting to share here:

author: soyouzka
author_rank: Newbie
timestamp: October 09, 2025, 05:04:46 am
content: The minicircuits lumped-element filters here.Top is with the RF shield, bottom is without.