• IEEE MTT-S and Microwaves101
    An association with IEEE will likely be stuffy and bureaucratic; you already mentioned committees and stamps of approval. I’d much rather have the politics and content of questionable taste. It also seems all the forum content was lost when IEEE re-wrote the site.

    Looking at EE enrollment over the last 30 years, MTT is definitely not going to grow. Not sure why they’d even try to monetize the content, or charge a subscription. This isn’t exactly a huge industry. If you had a YouTube channel with every IMS attendee subscribing, it’d be 10k subscribers, which is laughable small.
  • Reactive power combiners
    It probably depends on how a single amplifier fails. Say you had a 1/4 wave combiner (i.e. a Wilkinson without the resistor). If one amp fails with a short circuit at its output, that places an open circuit at the summing node, so the working amplifier just sees a 100 Ohm impedance mismatch. If the amp fails with a short circuit, the working amp sees an open circuit. If you added additional quarter wave lines at the amps, a short failure would present a short to the working amp. Perhaps a short is a better failure than an open since you have over-current (thermal problem) rather than breakdown from over-voltage.

    I’m sure there’s topologies that optimize degradation. I wonder if quadrature would be better.
  • wire and coax impedance
    Yes, though Z0=sqrt( (R+jwL)/(G+jwC) ) so a high R will boost the impedance; it’s not artificial. You may have to increase C to compensate. Though I would expect R to be very frequency dependent. Can you sweep it on a VNA, and see what happens at very low frequency such as 300 kHz? You may see a spike in Z0 at very low frequency, which will be exasperated by the R since you are getting sort of a divide by zero.
  • Standing waves observed using near-field radar measurements.
    Use a VNA in time domain mode to gate out the reflections.
  • Connectors suitable for a TRL calibration (microstrip)
    The connectors are mechanically compatible, but you should not use a 3.5 mm cal kit on a 2.92 mm connector as you will have an electrical discontinuity at the reference plane. Rather you use the adapter to get the proper interface.

    No, you should not use a dielectric filled 2.92 mm on a 2.92 mm cal kit for the same reason. Sounds like a bastardized 2.92 mm.

    Now SMA cal kits are rare (as they are an abomination) so they expect you to use 3.5 mm on SMA. Though never connect an SMA to a 3.5 mm cal kit, especially an HP. Use a 3.5 mm jack saver.
  • Phase noise from a splitter/combiner?
    I'm pretty sure thermal noise is half phase noise and half amplitude noise. In other words, -177 dBm/Hz from phase noise and -177 dBm/Hz from amplitude noise gets you -174 dBm/Hz thermal noise. So technically it's a low phase noise splitter just because it has low loss.

    Absolute lower limit of phase noise for a passive, room temp device is -177 dBc/Hz, and if that splitter is 0.6 dB excess loss, I'd think that would put the phase noise at -176.4 dBc/Hz, but this lists -175 dBc/Hz.

    That said, I'm sure piezo effect from substrate dielectric or capacitors will cause phase noise, so maybe they use low-glass substrates or certain caps.

    This is interesting. Maybe there is more to it:

    So the above paper describes how an isolation resistor in a power splitter introduces a differential thermal noise that messes up (underestimates) the DUT phase noise in a cross-correlation phase noise analyzer. They fix it by cooling the splitter to 4K but 77K may also work.

    Maybe the Holzworth splitter is a purely reactive type (though not with 38 dB isolation), specifically needed for their analyzers, though the paper mentions issues with reactive types too, though I don’t understand what they mean.

    It’s also neat to see the splitter isolation decrease near 4K, which makes sense as the resistor is changing.

    Note I’m using the term power splitter, but maybe a power divider would behave differently.
  • Wilkinson resistor test
    Maybe use a TDR. This guy has some lower cost pulse generators and probes. In this video he demonstrates probing some traces:

    Probe across the resistor to measure the impedance at the probe tips. Of course you could probably just use the probe with a VNA too in TD mode, and gate out the response just after the probe tips, but in essence you can isolate the measurement just around the resistor, limited by bandwidth or rise time.

    Even if you can’t probe directly across the resistors, if you can probe the splitter outputs differentially, you’d see the resistor further down the lines. This is assuming the resistor is closer to the probe than the stuff the splitter is feeding, as the pulse would travel in both directions.
  • Why we use Multi-Line TRL calibration?
    Some multi-line TRL compute a frequency weighted average, thus if you overlap the lines, you won’t get the discontinuity at the transition frequency. Yes though, it allows wider bandwidth than a single line.
  • Wilkinson resistor test
    I assume you are trying to do this in-situ without access to any of the ports?

    I was messing around several months ago with non-destructively measuring dielectric constants of curved plastic radomes. You can cut a piece of 1/2 wavelength copper tape and lay it on the plastic, then take two small coax loop probes and magnetically couple near the center of the dipole, looking at Q and resonant frequency. It easily tells the difference between ABS and anti-static plastics.

    Maybe you can couple to the output traces, as the isolation will degrade with the resistor open or shorted.
  • I am looking for a way to plot both the Time Domain and Frequency Domain at the same time
    I’d think you’d need to write something in Matlab or Python to get the 3D orthogonal view like that.

    Have a look a Baudline. It does sliding FFT analysis, and you can have the time domain displayed with the spectrogram. You can slide the window along in the time domain and see the spectrum change. You can also pipe it signals from GNU Radio via a FIFO to run in real-time.

    GNU radio would also work as you can plot time domain (with triggering), spectrum, and waterfall.

    AWR VSS will also run in real-time (continuous) and you can move tuners to change signal properties.

    There is a open source copy of Baudline called Inspectrum, but it is not as fancy.
  • Transmission line maximum impedance
    Is there a limit? Traveling wave antennas are tapered transmission lines. Maybe they have to be non-TEM lines? If you had a high impedance transmission line made of perfect conductor, with no dielectric loss, then there would be no loss. Those transmission line formulas do not account for radiation. So if a transmission line is always driven at a low enough frequency for TEM, I assume that means it can never radiate?
  • Transmission line maximum impedance
    I don't think so, but now I see you can buy 600 Ohm ladder line, so I don’t know. Skim through this, particularly section 5 where he looks at TM waves on two wires lines. The TM cutoff is lambda/2 wire separation, so it'd be lambda/4 for wire-over-ground if you treat it as an image. Really no different than microstrip.

    What's more interesting is the TE mode for really large wire diameters, but that's expected as it's in microstrip too.

    I'd just add a disclaimer saying it's for TEM only.

    I was also looking in the Ramo Fields and Waves book, and they had the reminder that TEM wave impedance can be derived by the static DC fields. So it reasons that the wire separation can go to infinity if the frequency goes to zero.

    This, and that other post about quasi-TEM causing nulls for long lines, really makes me want to redo my whole education; dumber everyday.
  • Is it worth using a choked WG flange for lab testing?
    The WR-15 worked well. For a 3” length, the insertion loss was 0.3 dB, compared to 0.2 dB for the 3” reference. The WR-10 did not work well.

    The surface roughness models in the 3D simulators do not work well; they overestimate the loss for the rather rough surface.

    I had previously stated the flange was polished. It was just machined flat. I suppose you could polish it, but the machining is probably adequate.

    I don’t know how high-Q structures would perform. I’d like to make a WR-15 filter or resonator; just no time.
  • Is it worth using a choked WG flange for lab testing?
    I had some WR-15 and WR-10 AM waveguides made on the Protolabs AlSi10Mg high-res process. I did not use a choke but I did have the flanges polished; they will do that.
  • Group delay in a receiver
    If it's a high sided LO, that mean the output frequency is being swept backwards too. So the GD (derivative of phase wrt. freq) still comes out positive.

    R&S has a two-tone GD measurement technique. I have never used it, but they say it can even be used to measure satellite links. In this they talk about using a reference mixer fed from the same LO as your mixer, bringing it back up to RF, and I assume that would take care of the frequency inversion as well.
  • How do you document your design work?
    I can tell you what not use use; MS OneDrive. It would work great IF they added a .gitignore type feature to prevent syncing of files or folders. Despite requests from thousands of users, they have not implemented it. One Drive works well for backing up work, but say you have a project folder with GB of 3D EM data cache, it will attempt to back that up. So what I'm having to do is keep my working projects outside of "My Documents", then remember to copy them in periodically for backup. That defeats the purpose of having a cloud backup.
  • Coupler Design Question
    Well I'll pay you $100M to design a 4 way splitter with under 5 dB insertion loss.
  • How do you document your design work?
    I use MS Word, just copy and paste stuff from Microwave Office, and paste in photos from a pocket camera. I use the Testwave software (within MWO) heavily in the lab. Each project has it's own document; nothing shared like a lab notebook. This is more for my own purposes than any patent stuff, because invariable I'll have to dig up something a year later, and I can't remember how I did it. It's easy to give the report to customers too.
  • Videos
    I figured out where that loop fixture is from. The oscillator class by Jeremy Everard and Simon Bale. It is shown in the recent MTTS webinar. I'll have to update the video.
  • Videos
    Video 7 details a coupled loop fixture designed to measure the quality factor (Q) of surface mount (SMT) resonators to 1 GHz.

    The fixture has adjustable loop spacing to null the magnetic coupling between the loops. The design and analysis is performed in Cadence AWR Microwave Office using the Axiem and Analyst electromagnetic simulators.

    The initial design has poor high frequency isolation due to common mode currents on the interconnects. The design is revised using EM simulation to show additional isolation may be achieved with the addition of ferrite loaded baluns.


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