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precision dummyload?

B

bob85

Supporting Member
Mar 30, 2005
3,480
1,463
173
england
i am looking for an accurate load for testing/calibrating my analyser,
commercial loads are way too expensive to warrant buying one for a mickeymouse analyser so i thought i would make one that has got to be better than the many cheap 50 ohm bnc terminators i have,
im still on the hunt for a none inductive 0.1% 50r resistor to carefully solder into a bnc plug,
heres a possible candidate that i may try if its suitable for hf,
http://www.vishay.com/docs/31019/ptf.pdf

seems to me that if i keep the leads as short as possible and the resistor central in the plug or one of the larger terminator plugs i have it would do the job good enough for a cheap analyser,

what do you guys think?
any tips tricks i need to know?

thanks.
 

What freq range do you expect to get?

Also, look on the N2PK site for their suggestions on building calibration standards. They use several SMT metal film, high precision resistors oriented in a spoked pattern. The also employ all three standards (Open and Short) for a good reason, something to do with the "calibration plane" Depends on how accurate you want it. And don't forget that you are using BNC connectors, which have their own limitations...

Your biggest problem is going to be getting rid of capacitive and inductive reactances. I had a real hard time getting something that I was going to be satisfied with that worked over 500 MHz. I was shooting for 3 GHz. LOL, what a joke it was.

Anyway, I got lucky on an Anritsu precision load (~$500US) and also bought a used Narda precision load off of eBay for $35US. It actually turned out really good too.

I'll post more in a bit.
 
I found one of my old loads that I built inside of a pl259 using SMT film resistors (4) in a spoked pattern. If you look into the cable end of the connector, you can see the tiny resistors. I soldered them to the end of a solid copper wire first (#14 I think), then slid them down into the connector, tack soldered the copper wire to the connector and snipped off the excess (like you would normally solder the center conductor), finally I soldered each resistor through the hole in the outer connector.

OK, for BNC, just chop the connector at the back side of the insulator and file smooth to make the calibration plane. Make 3 the same. Solder about 6, precision SMT film resistors (small ones), about 300 ohm each (or 4 if you like, 200 ohm each) to one of the connectors for the "Load." Solder a copper round for the "Short" to another. Leave the last alone for the "Open."

Here are the plots from my initial trial. After that, I went to rigging up shunt capacitors, inductors, trims, and the like to try and get it better. I got just a little bit better, but my approach was all wrong.


You can see in the lower Smith plot (starts right to left) that my load goes from 50 ohms resistive to about 40 ohms resistive and swings to about 2 ohms capacitive and ends up about 5 ohms inductive by the time I hit 1 GHz.

HomeBrewLoadVSWR.png


HomeBrewLoadSmith.png

Note to freecell, that is a normalized smith plot with a characteristic impedance of 50 ohms...
 
c2 i only need it to be good enough for cal/testing the minivna, absolute accuracy is not needed for a cheap unit like this providing it is not reactive, 30mhz is good enough, vhf would be nice but not essential as i don't use vhf uhf much,
if it works ok i will make a few so i can play with coaxial transformers and such,

note to c2 smith charts baffle me, i think there is software for the vna that outputs smith charts, one day i will learn how to read one.
 
Smith charts are real easy to read and truly a wonderful thing.

Your characteristic impedance is at the center point (1 in my plot).

The ~concentric circles represent constant resistance. For example, the 0.2 to the left of center is 10 ohms. Anywhere on that circle is 10 ohms.

The horizontal line through the center is zero reactance.

The left outer edge is a short. The right outer edge is an open.

The entire upper half of the chart is inductive reactance.

The entire bottom half of the chart is capacitive reactance.

The arcs are constant reactances.

Now, you can throw a dart at that chart and tell instantly what is R and X.
 
yup 3, 7, and double top to finish ;)
i will figure it out

that load like youres is nice i would buy one if it was bnc at that price and specs,

i have plenty of loads but the only one that measures 50ohm dc exact is an old philco 60w all the others are fractions of an ohm off.
 
C2 said:
Note to freecell, that is a normalized smith plot with a characteristic impedance of 50 ohms...
Click to expand...

:LOL:

bob85 said:
i have plenty of loads but the only one that measures 50ohm dc exact is an old philco 60w all the others are fractions of an ohm off.
Click to expand...

bob85, do you really think the load needs to be that precise? How much error do you think will occur if the load is 5% off? The reason I ask is because you are generating a DAC test signal and recovering/converting an ADC signal when testing, so there is going to be step error and probably some nonlinearity in gain as well. Plus, a small change in R probably means a small change in X too.

I would try a few very similarly sized loads (48, 50, 52, etc...) and test your system with minor load variations and see how it responds. If the results stay pretty consistant, I would call a close enough.

:)
 
ken i know where your comming from and totally agree,
unfortunately im a guy with caesium standards in a timex wallet,
good enough is never good enough for my own use i just dont have hp/anritsu finances,
i realise it is a joke when compared to a real vna but after reading several articles on the cheaper analysers very popular with hams/cbers i come to the conclusion that the manufactureres think that within a few ohms is good enough at 50ohms and away from that i recon i could stick my thumb in a guess more accurately,
this unit shows my better loads within a few tenths of an ohm at 50 and within an ohm or so at 75 before i software calibrate it,
if it cannot be calibrated to consistently read 50ohms XS=0 or very close then i will sell it and look around for something better,
i would like a small cheap load that is far more accurate than it ever needs to be for an instrument like this,
its just a mental thing that wont effect my onair performance in the slightest but its just how i am :LOL:
 
Bob,

The Narda load on ebay is very good and will last you a lifetime for anything you will likely buy in the future. You would just need a precision N(f) to BNC(m) adapter.

For the miniVNA, I think you will be golden up through VHF if you make a BNC load the way I said.

I looked around for the instructions (pictures really) on how to build the calibration standards on the N2PK site, but I could not find them.
 
bob85 said:
ken i know where your comming from and totally agree,
unfortunately im a guy with caesium standards in a timex wallet,
good enough is never good enough for my own use i just dont have hp/anritsu finances,
i realise it is a joke when compared to a real vna but after reading several articles on the cheaper analysers very popular with hams/cbers i come to the conclusion that the manufactureres think that within a few ohms is good enough at 50ohms and away from that i recon i could stick my thumb in a guess more accurately,
this unit shows my better loads within a few tenths of an ohm at 50 and within an ohm or so at 75 before i software calibrate it,
if it cannot be calibrated to consistently read 50ohms XS=0 or very close then i will sell it and look around for something better,
i would like a small cheap load that is far more accurate than it ever needs to be for an instrument like this,
its just a mental thing that wont effect my onair performance in the slightest but its just how i am :LOL:
Click to expand...

I know exactly where you are coming from too. Have fun...

:D
 
hey C2 while looking for a precision load i picked up a very clean one of these from ebay,
http://www.bird-electronic.com/products/product.aspx?id=150

looking at the manual for it the dc-1ghz vswr is below 1.1:1 not 2.2 as it says on the frontpage there,
the guy selling it wont be happy i got it for £16 plus shipping,
i hope he ships it :D

i think i may have a thing for buying loads at a bargain price, cant help myself :oops:

still keeping my eyes open for a bnc calibration load ;)
 
Hey, thats a good price for about $32 buckanages!

It will be interesting to see your characterization when it comes in!
 

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