high resistances at RF?

[coppercone2]

So I was wondering, since I repaired the impedance analyzer, that it can measure a fairly high resistance at high frequencies.

What are components that have resistance at high frequencies used for?

For instance a 1Kohm resistor at 1GHz. Maybe I can imagine a resistor thats somewhat big being used in a attenuator?

Is this mostly for evaluating ferrite beads to make sure they are correctly lossy at high frequencies? How about 10Kohm at 100MHz

I am wondering because normally I guess it makes more sense to test something like an attenuator with a VNA. I don't think I have ever seen a resistor much larger then 500 ohms even in 'only' a HF attenuator.

[Bud]

Even small capacitance with high resistance will kill the bandwidth.

[coppercone2]

 I guess, what I am asking is that (this seem to be confirmed in general when this question was brought up on the forum indirectly) that usually RF+ engineers never really want a component that has this high resistance range, other then in the case of a bead, right?

[PA0PBZ]

What are components that have resistance at high frequencies used for?

Typically to separate RF from DC, like when you have a supply voltage to an RF amplifier stage, mostly a coil, could be a ferrite bead or wire wound. Also used back in the days to supply DC voltage to the anode of a tube, you'd often see wire would on a high value resistor as support. Also see 'Bias Tee'.

[Wallace Gasiewicz]

PA0 has a good point
A Ferrite bead is an inductor. Not a resistor.
Higher reactance at higher freq, like a small coil.
Commonly used to quell "higher freq oscillations" in amplifiers.
Physically huge inductors are used in tube amps with a common heater / cathode. Lets you use line (60 Hz) for heating and keeps the RF from going thru the "wrong way"

There are a lot of designs today that use a lot of SMD inductors to separate DC from RF on PCBs

I just used a 5K resistor in an atttenuator to get -40 dB, works just fine at HF, not so well at higher than 1 GHz, I suppose it would be helpful to have an instrument that would tell me what the actual resistance and inductance or capacitance or total reactance would be at 1 GHz for my resistor.
The values of components change, sometimes drastically, when you change the freq.
Everything has resistance, inductance and capacitance.
A resistor can become more of a cap or inductor when you change the freq. The resistance value can also change. eg "skin effect"
Good RF engineers know which components do this, I do not know much at all.

[coppercone2]

I see the explanation commonly that the ferrite bead is resistive (eddy currents) at its frequency range of operation so it dissipates RF energy as heat. I figure a high value ohms might be good there, for a high voltage signal you want to get rid of. I see these beads specified as high as a few K at 100MHz commonly. I have a ferrite bead kit that specifies the impedance around 100R to 2000R iirc. Not sure if anyone would use that for a bias T since the bead would get saturated if current flow is high.. maybe a low power one.
at least that is what I was thinking about in the first post, in case that caused any confusion.

That one luckily has a graph that shows me each bead parameters, but I noticed alot of beads never have much specified other then 100MHz resistance.

[Wallace Gasiewicz]

Here is a result of a test at 1 MHz
I zeroed my HP 4332A LCR         Then:

Measured wire
R = .006 Ohm
L = .0222uH

Same wire with one bead
R = .16 Ohm
L =  .5 uH

So according to this the bead has both resistance and inductance that is considerable

I also used a longer bead that comes installed on a lead (not loose) about 3X as long as a bead.
it measured
1.2 uH
2.73 Ohm

On my 3456  (DC Measurement 4 wire) the tests show almost zero ohms with or without the beads

[coppercone2]

What bead are you using? Because I noticed that.. of the 16 beads I have in the all encompassing kit, with the curves drawl, all the interesting stuff happens at 50MHz+



So I think that you would have to find the correct operating regime for every bead

[Wallace Gasiewicz]

Using :
Philmore
No, 86-1570
Toroids     
Ferrite Beads

[coppercone2]

I can't find any information on that bead.

[Warpspeed]

There must be a million different sizes and types of beads in different materials.
They all have some very different characteristics.

They only work really well in low impedance circuits, and at higher frequencies.

[Kahooli]

We love being able to measure high Q components. It is as useful to an RF eng to measure High Q low impedance as high Q high impedance.
So for say, a PIN diode in shunt config - you want to measure in the off, reverse bias state a very high Rp, and a Very low Cp.
And for a shunt inductor, you may want 15 ohms reactance but still high Q of 500. low resistance.

[coppercone2]

the pin diode is a interesting thing to test. Makes sense since you have a bias voltage feature, so its not only for inductors.

[coppercone2]

Does it make sense to test other active components in the impedance analyzer by themselves rather then in a network analyzer? If you have both, I would love to know why use one over the other.

[jonpaul]

Bonjour à tous

Used ferrite beads for DM noise surpression

1/Measured with network /impedance Analyzers since 1980s. HP4194A with impedance test set 41952 characterized 100 khz..500 MHz with TH and SMD adapter.
4191A and others exist.  HPIB plots in various formats.

2/ Coilcraft, and other manufacturers have a huge selection of wound beads, with very fine spec and plotting of Zo, L, R vs frequency.

3/ We used single and multi hole slug beads from TDK, Fair-Rite, Ferroxcube with SMPS and electronic ballasts. The 6 hole slugs were best but hard to build.

4/ The ferrite material choice affects the performance, eg effective frequency for transition from an inductor to a lossy resistance.

5/ See Fair-Rite, Coilcraft application notes on beads and EMI surpression.

6/ most effective filters use a series bead and shunt capacitors to form an LC, T or pi 2 pôle filter.

Just the ramblings of an old retired EE

Bon week-end

Jon

[joeqsmith]

What are components that have resistance at high frequencies used for?

If you have both, I would love to know why use one over the other.

There are a lots of examples for a VNA.  Measuring dielectrics for example:



I was recently using the TDR to look at some open ended conductors. 

[coppercone2]

looks like you might be able to find a obsolete freon substitute for those old HV probes in a 'scientific manner' using this.

When installing water purification equipment I made a probe for a LCR meter that used the theoretical model for the impedance range, using poland spring distilled water datasheet as a calibration constant it got within 10%, with non ideal chamber geometry (there was a necking at the top, because I used a glass chocolate milk bottle instead of a tall beaker that was not available).

[joeqsmith]

I think Tektronix solved their Freon problem three decades ago when they started using silicone.    When I attempted to design my own wide band HV probe,  I used ceramics for an insulator.   

[coppercone2]

I read some people tried it and they could not get the trimmers to adjust right, so its somewhat tricky to find the correct substance, if you wanna retrofit a old probe.

[joeqsmith]

I wonder if Tektronix had changed the compensation network along with the change to silicone.  Should be easy enough to check. 

[joeqsmith]

A few more thoughts on where a VNA may be a better fit than an impedance analyzer.

Some time ago I was attempting to use one of the low cost VNAs to make PDN measurements.  Difficult problem and required a decent transformer to break the ground path.  I made a demo video showing the end results
***
Also, the wideband transformer that I used to make these measurements was also home made.   One of our members posted S parameters for an off the shelf product that I then used to give me an idea on the length and type of coax used as well as compare it against my own design.   This was also measured on my VNA.
***       

I built an X-Y controller that I can use with the VNA to look at radiation patterns in 3-D.  For a demonstration,  I posted some data off a couple of pork and bean cans I soldered together to form a waveguide.   

Along the same lines, we had our friend from France(?) posting about his vitals measurement experiment using the VNA.  Like my bean cans, he was bouncing the radio waves off his chest and decoding the data.  One of the more interesting experiments I have seen posted about with these low cost VNAs.

Q was already mentioned.  I too tried to use the low cost VNA to make some measurements of some ATC jelly bean caps.   This took us to how the firmware worked which led us back to the PDN measurements.     
https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg4126171/#msg4126171   

With the more recent firmware and low cost VNAs,  I attempted to use it to listen to a local radio station.   It seems I was able to get enough signal to detect audio but it would rank as possibly the worse radio ever.