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.