Mini-Circuits KARN-50-18+ as the Reference Termination for a VSWR-Bridge

[TimYTTT]

Recently I was looking for a 50 Ohm Reference Termination for my VSWR bridge. Someone on the EEV-Blog forum recommended Mini-Circuits KARN-50-18+ termination so I ordered from Mouser Electronics 3 pieces, just to be sure to get proper stuff.
After order arrival I checked DC resistances of the terminations with my 5 1/2 digit multimeter and found out to my greatest surprise that  their resistances are 51.90 Ω, 51.78 Ω and 51.86 Ω. I was expecting the values very close to 50 Ohm.
The Mini-Circuits data sheet specifies impedance of 50 Ohm, but provides also information that the minimal return loss for KARN-50-18+ is 33 dB for the range DC-0.5GHz . The 33dB return loss corresponds approximately to VSWR=1.0458. Calculating VSWR for value of Termination with 51.90 Ω ( comparing to hypothetical 50 Ω reference) results in the VSWR = 1.0379. This complies also with the data in Mini-Circuits Data-sheet. However I do have understanding problems and therefore following Questions:

-Is it so difficult to produce the exact 50 Ohm termination?
-Why Mini Circuits specifies 50 Ohm termination and delivers 51.9 Ohm Termination?
-What happens if I use this termination as the reference termination in my VSWR bridge?

Thanks for all inputs
Regards, Tim

[mag_therm]

If Mini Circuits are following common convention (and ISO 5725) in the data sheet:
The margin of error is one half of the last significant place.

50 Ohm as shown the data sheet with no decimal places, can legitimately be between 49.5 and 50.5 Ohm

To check for systematic errors (bias) in you measurements (of around 51 Ohm), you could use
Mini Circuits Type N calibration (short and open and load) Part number SOL-63-NM+
The parameters with their decimal places are shown in the datasheet.

You may be able to find a more accurate type N termination, I did not look.

[RoV]

-Is it so difficult to produce the exact 50 Ohm termination?
-Why Mini Circuits specifies 50 Ohm termination and delivers 51.9 Ohm Termination?

I would be tempted to say that they sacrified a bit the response at low frequency (although remaining within specs) to get a better overall response up to 18 GHz, where a bit of internal capacitive loading, combined with the pin electrical delay, will tend to lower impedance seen from outside.
However, this interpretation conflicts with the "typical" response table, where they claim >50 dB RL below 0.5 GHz.

[TimYTTT]

To check for systematic errors (bias) in you measurements (of around 51 Ohm), you could use
Mini Circuits Type N calibration (short and open and load) Part number SOL-63-NM+
The parameters with their decimal places are shown in the datasheet.

Well, that is what I wanted to avoid. Such calibration kits are quite expensive. Thanks for your
valuable comments.

[LM21]

How much  does that MiniCircuits SOL-63-NM+ cost. I can't  find a  price for it.
Is it the same as in SDR-kits page, which has other calibration  kits, too
https://www.sdr-kits.net/index.php?route=product/category&path=66_68_69

[TimYTTT]

How much  does that MiniCircuits SOL-63-NM+ cost. I can't  find a  price for it.
Is it the same as in SDR-kits page, which has other calibration  kits, too
https://www.sdr-kits.net/index.php?route=product/category&path=66_68_69

- I paid CHF 19.76 pro termination. This is the price in Switzerland.
- I do not actually know if this termination is also included in the SDR-Kits offer. Certainly you can use it as a 50 Ohm calibration standard, but you have to calculate/measure yourself other parameters relevant for this use (delay, L, C).

[Ringmodulator]

Hi Tim,

at first, you need to get an accurate measurement of the resistance. For such a low resistance, a 4-wire measurement with kelvin probes is highly recommended.
If you use a 2 wire measurement, you will likely err on the high side.

Second: It is not a problem to build a high precision 50 ohm resistor for dc al low cost.
But the issue is, to create a tremination, that is 50 ohm over the specified frequency range. The higher the frequency, the more the parasitic capacitance and inductive components come into play.

The KARN50-18 is specified up to 18 GHz. This is quite something.

The sdr-kits calkits have each 50 ohm load specified with the resistance measured individually with the kelvin measurement.
The value is entered in the vnwa3 sotware and therefore taken into account for each masurement after calibration.

Regards
Chris

[shabaz]

I've noticed that, too. My Mini-Circuits 50-ohm terminators have a DC resistance approx 51.3 ohms, whereas the Huber & Suhner ones that I have are approx 50.3 ohms. However it's not a large sample (I only have two Mini-Circuits ones, ballpark $20 value, and two H+S, one was approx $30, the other is a 15W load, $$$).

[TimYTTT]

at first, you need to get an accurate measurement of the resistance. For such a low resistance, a 4-wire measurement with kelvin probes is highly recommended.
If you use a 2 wire measurement, you will likely err on the high side.

Hi Chris,
Thanks for your comments. I do not believe that 4-wire measurement will so dramatically improve the values I have measured. Having said that, my measured values of 51.90 Ω, 51.78 Ω, 51.86 Ω can differ by 4-wire measurements only in the places beyond the decimal point. That means that I still do not have answer why Mini-Circuits are selling this product with the value higher than 1.5x Ohm from  the nominal value. I think that they have a good reason for that, but this is still hidden to me. Maybe I have to ask them directly.
Regards,
Tim

[TimYTTT]

I've noticed that, too. My Mini-Circuits 50-ohm terminators have a DC resistance approx 51.3 ohms, whereas the Huber & Suhner ones that I have are approx 50.3 ohms. However it's not a large sample (I only have two Mini-Circuits ones, ballpark $20 value, and two H+S, one was approx $30, the other is a 15W load, $$$).

Hi Sahbaz,

Thanks, I am obviously not alone! I noticed this discrepancy too, comparing the Mini-Circuits KARN-50-18+ values with other 50 Ohm terminations I have (Rosenberger, Huber Suhner and even Mini-Circuits (BTRM-50+->50.27 ohm) terminations ).

Regards,
Tim

[joeqsmith]

Recently I was looking for a 50 Ohm Reference Termination for my VSWR bridge. Someone on the EEV-Blog forum recommended Mini-Circuits KARN-50-18+ termination so I ordered from Mouser Electronics 3 pieces, just to be sure to get proper stuff.
After order arrival I checked DC resistances of the terminations with my 5 1/2 digit multimeter and found out to my greatest surprise that  their resistances are 51.90 Ω, 51.78 Ω and 51.86 Ω. I was expecting the values very close to 50 Ohm.
The Mini-Circuits data sheet specifies impedance of 50 Ohm, but provides also information that the minimal return loss for KARN-50-18+ is 33 dB for the range DC-0.5GHz . The 33dB return loss corresponds approximately to VSWR=1.0458. Calculating VSWR for value of Termination with 51.90 Ω ( comparing to hypothetical 50 Ω reference) results in the VSWR = 1.0379. This complies also with the data in Mini-Circuits Data-sheet. However I do have understanding problems and therefore following Questions:

Thanks for all inputs
Regards, Tim

I'm a bit confused by your post.   

-Is it so difficult to produce the exact 50 Ohm termination?

I would suggest exact is impossible to achieve but I don't know why you would need that for measuring VSWR.

-Why Mini Circuits specifies 50 Ohm termination and delivers 51.9 Ohm Termination?

They provide a tolerance.   Your comment of their specified 33dB return loss corresponds approximately to VSWR=1.0458 is correct.  Assuming no errors in your measurements, your worse case of 51.9 falls within the specified 52.3 & 47.8.  I don't understand why you would ever "expect" them to be closer.   

-What happens if I use this termination as the reference termination in my VSWR bridge?

You calculated the error correctly.  Guessing you read the datasheet prior to purchase, knew the errors and found them acceptable for your needs.  You measured it and found it in spec, so I am not sure what you are asking.   

To check for systematic errors (bias) in you measurements (of around 51 Ohm), you could use
Mini Circuits Type N calibration (short and open and load) Part number SOL-63-NM+
The parameters with their decimal places are shown in the datasheet.

Well, that is what I wanted to avoid. Such calibration kits are quite expensive. Thanks for your
valuable comments.

Assuming you are are cheap ass like myself and really want (from your posts, it seems you don't actually need it) that higher accuracy, you can always order several parts and sort them. 

***
Of course, sorting them means having a way to measure them, meaning you need standards.  In my case, I borrowed a couple of sets.   You may be able to rent a set. 

[mag_therm]

Assuming you are are cheap ass like myself and really want (from your posts, it seems you don't actually need it) that higher accuracy, you can always order several parts and sort them. 

***
Of course, sorting them means having a way to measure them, meaning you need standards.  In my case, I borrowed a couple of sets.   You may be able to rent a set.

In eng school 55 years ago in Australia we had a Very British supervisor "Dr Chris" for first year physics.
If a report was submitted that did not have  correct number of places to match the precision of the measuring instruments, and the experiment calculations, (including slide rule and sometimes log table precision) he would roar expletive at the poor student and sometimes throw the report on the floor.

[joeqsmith]

In eng school 55 years ago in Australia we had a Very British supervisor "Dr Chris" for first year physics.
If a report was submitted that did not have  correct number of places to match the precision of the measuring instruments, and the experiment calculations, (including slide rule and sometimes log table precision) he would roar expletive at the poor student and sometimes throw the report on the floor.

Funny, but I don't see the point you are trying to make, if any.     

If Mini Circuits are following common convention (and ISO 5725) in the data sheet: The margin of error is one half of the last significant place.  50 Ohm as shown the data sheet with no decimal places, can legitimately be between 49.5 and 50.5 Ohm

There is no need to guess.  The manufacture's datasheet provides the return loss down to DC. From there we can calculate the acceptable range of DC resistance.

[TimYTTT]

In eng school 55 years ago in Australia we had a Very British supervisor "Dr Chris" for first year physics.
If a report was submitted that did not have  correct number of places to match the precision of the measuring instruments, and the experiment calculations, (including slide rule and sometimes log table precision) he would roar expletive at the poor student and sometimes throw the report on the floor.

Hi mag_therm,
Thank you, I perfectly understand what you wanted to say. A long time ago I had also at Uni a math professor who was looking just for our faults and didn't bother very much if we understood his lectures. That was not quit good experience for all of us, but at the end we all survived.
Kind regards,
Tim

[mag_therm]

Hi, TimYTTT Yes, I did not keep fully up to date with electrical metrology, but was always aware of the basics of uncertainty of measurement after that experience!
Reading again your o/p, how did you zero and then measure the DC R? . You did not have a type N short (post #3)?
Did you screw the 50 Ohm, fully compressing the gasket, into (for example) a bulkhead jack Amphenol 082-97-RFX ?
If the DC measurements (51.78 Ohm etc)  were with a calibrated and zero-ed DC Ohm meter, it might be appropriate to raise a "Tech Support and Satisfaction" ticket with Mini Circuits.

[TimYTTT]

Hi, TimYTTT Yes, I did not keep fully up to date with electrical metrology, but was always aware of the basics of uncertainty of measurement after that experience!
Reading again your o/p, how did you zero and then measure the DC R? . You did not have a type N short (post #3)?
Did you screw the 50 Ohm, fully compressing the gasket, into (for example) a bulkhead jack Amphenol 082-97-RFX ?
If the DC measurements (51.78 Ohm etc)  were with a calibrated and zero-ed DC Ohm meter, it might be appropriate to raise a "Tech Support and Satisfaction" ticket with Mini Circuits.

Hi mag_therm, thanks for feedback! Here are my comments:

• For the measurements I used a DIY fixture (Photo) with N-,BNC- and SMA-connectors. The N-female connector on the fixture (bulkhead jack) comes from Huber+Suhner.
  
• The first measurement is with "N-short" termination in order to find out what is the "zero-level " resistance of the fixture + N-Short termination (000.10 Ohm).
• Than I press "zero" button on the multimeter. Now I have reference zero for the KARN-5-18+ termination measurement (000.00 Ohm)
• I replace the N-short connector with the KARN-5-18+ termination . The multimeter displays now the KARN-5-18+ termination resistance (51.90 Ohm).

I do not think that there is a substantial measurement error in my measurement procedure. Most probably joeqsmith is right telling that Mini-Circuit specification shall be interpreted using their Return Loss specification. However I measured various terminations from other known and unknown vendors (10+ samples) and all of them had quite close values to 50 Ohm. I am going to ask Micro-Circuits for their opinion on this.

Kind regards,
Tim

[tszaboo]

In eng school 55 years ago in Australia we had a Very British supervisor "Dr Chris" for first year physics.
If a report was submitted that did not have  correct number of places to match the precision of the measuring instruments, and the experiment calculations, (including slide rule and sometimes log table precision) he would roar expletive at the poor student and sometimes throw the report on the floor.

Well, he wasn't an engineer.
We had other things written down (especially for telecommunications class) for example:

sqrt(2) =1
sin(x) = x
g = pi²
1/2 = 1

Because we were engineers, and it's usually good enough, especially when you have to calculate stuff in decibels.

[joeqsmith]

Check my math but Mini-Circuit's specified worse case 33dB return loss at DC = +/-0.0224 +0j = 52.29/47.81 ohms or a VSWR of 1.046.   Expecting it to be tighter than what is specified makes zero sense to me. 

As was mentioned, we don't normally measure VSWR at DC.     

A similar discussion about the tolerance:     
https://www.eevblog.com/forum/projects/20db-rf-attenuator-seeking-feedback-to-improve/msg2978858/#msg2978858

I do not think that there is a substantial measurement error in my measurement procedure.

I agree.  Assuming the meter is in working order, what you show should be fine.

Shown is my meter with various adapters, no null, with a decent fixed resistor along with a Narda and HP terminator.   

[joeqsmith]

Sure the 4W and null is going to remove some of those errors but I doubt anyone measuring the VSWR of their antenna would care.   

[TimYTTT]

Hello all,

I contacted Mini-Circuits sales and asked them about the DC resistances of KARN-50-18+  terminations. Here is their response:

"Our KARN-50-18+ and BTRM-50+ terminations designed to use for commercial – testing application, they are cost effective options, and they are not precise terminations. Due to internally built structure for KARN-50-18+, as this contains more resistors due to higher power handling, so more variations occur."

After that I asked them to suggest me a suitable precise termination from their portfolio. Again, here is their response:

"Unfortunately, we do not have any precise terminations available in our standard catalogue."

By the way, does someone can provide me description or specification for the "Precision RF Connectors and Terminations"?  Are these specifications for mechanical or electrical or deployment characteristics of RF connectors/terminations?

Thanks, Tim

[Bud]

Nobody measures VSWR to 4 decimal points, it makes no sense for any practical purpose. What is your use case?

[shabaz]

If you need 50 ohm for DC or near-DC applications, you'd be better off making a DIY one. Sometimes it can be useful, for instance, I sometimes use 50 ohm components to deliver a low frequency signal to a 'scope for convenience, rather than 'scope probes. But usually I don't mind a few percent error.

Otherwise, for $20-$30 ballpark the Huber & Suhner ones appear to be a lot more close to 50 ohm at DC than the MiniCircuits parts, but that's just an observation based on a limited sample, and would be easy to improve on with a DIY load, at DC. If you search for 'DIY OSL' or similar, you'll see lots of examples of how those can be assembled. I have a few DIY ones, and personally I measure a dozen or two resistors (2-wire, I don't bother with 4-wire for this use-case), to pick the ones closest to 50 ohm (when combined; usually I'll use two 100 ohm resistors in parallel), simply because it's easy to do.

Even calibration kits (3- or 4-digit $) might not come with any measurement at DC. My fairly decent (but not metrology grade) OSL 'T' cal tool doesn't come with any measurement at all for the 50 ohm load below 50 MHz.

[joeqsmith]

Hello all,

I contacted Mini-Circuits sales and asked them about the DC resistances of KARN-50-18+  terminations. Here is their response:

"Our KARN-50-18+ and BTRM-50+ terminations designed to use for commercial – testing application, they are cost effective options, and they are not precise terminations. Due to internally built structure for KARN-50-18+, as this contains more resistors due to higher power handling, so more variations occur."

Were you expecting something else? Keysight has a similar note for some of their terminators, for example:

The model 11593A is not intended to be used as a precise termination.

After that I asked them to suggest me a suitable precise termination from their portfolio. Again, here is their response:

"Unfortunately, we do not have any precise terminations available in our standard catalogue."

By the way, does someone can provide me description or specification for the "Precision RF Connectors and Terminations"?  Are these specifications for mechanical or electrical or deployment characteristics of RF connectors/terminations?

Thanks, Tim

I'm sure it came as no surprise that Mini-circuits does not produce standards.  If you want some data for actual standard, the best thing would be for you to look up something that meets your requirements (something I am still not sure of).   

***
If your looking for something low cost:
https://www.ebay.com/itm/266817014023

LOAD
DC-resistance: 50 Ω ± 0.5 Ω
Return loss, min.: 42 dB @ DC to 6 GHz
Power rating, max.: 0.5 W