[volt-nuts] Impedance bridge information

[Charles P. Steinmetz]

Fred wrote:

>I am looking for a bridge to measure things like small caps, 
>dielectric constants ad other "weard" parameters. I have several 
>VNA's and RLC bridges but they used to make bridges that you can see 
>as a manual VNA. It gave R and jX, sometimes with build in detector 
>and oscillator, sometimes using these external.
>*     *     *
>Can someone give me a hint what to search for. For instance the 
>right name of such a bridge ( i think Z/Y or admittance cq impedance 
>bridge) and what is important so I can determ if they are for what I want
>
>I want to measure the more extreme things like ESR, fF, delectric 
>proporties/losses ect but for most I love old precision measurement 
>instruments which make you think about what you do and have lots of 
>knobs instead of menus.

Instruments of that type are known as "impedance bridges," 
"admittance bridges," or "immittance bridges."  If they are designed 
to be used at RF frequencies, they may be called "RF bridges."  So, 
the first thing to figure out is what frequencies you want/need to 
use (for example, if you want to measure the driving point impedance 
of an AM radio antenna array for a station on 680 kHz, you need to 
measure at 680 kHz +/-).  To measure the S-parameters of a 440 MHz 
matching network, you need to measure at 440 MHz +/-.  Some 
general-purpose bridges work with (or are calibrated only for) 
excitation at audio frequencies.

The most often encountered quality general-purpose impedance bridges 
are the GR 650, 1608, and 1650-A.  The 916-A was GR's RF impedance 
bridge.  The Boonton 250-A has a built-in oscillator and detector, 
and covers from 0.5 to 250 MHz.  Here are some references to get you started:

http://www.jamminpower.com/main/GR%20Bridge.html

http://www.radiomuseum.org/r/gener_raco_impedance_bridge_1650_a16.html

http://www.seed-solutions.com/gregordy/Amateur%20Radio/Experimentation/RFBridges.htm

These bridges will generally measure the loss (real part) of a 
reactance (ESR, dissipation, Q).  They will not measure the 
dielectric absorption of capacitors.

Best regards,

Charles