Measurement Receiver Inductors

K5ESS
 

Wonder if others have found that the inductors called for in the Measurement Receiver need tweaking?  I found that the inductors in the impedance matching networks at each end of the crystal filter needed one less turn (11 vs 12) and needed for the turns to be all scrunched up to arrive at a value close to 8.05µH. Value was determined using the Crystal Test Fixture and measured known capacitors.(This is another good use for the Crystal Test Fixture).  Still,  I can't see any difference when tuning the variable caps associated with the matching networks at each end of the filter.  I modeled the matching networks in LTSpice with a 600 ohm 6dB pad between them and the response is pretty broad.  So are the variable caps even needed?  I'd be interested in others experience. 

Also, the inductors in the Bridge Tee Diplexer needed three fewer turns to arrive close to the specified value of 2.43 µH. 

In both cases using the AL value given by Amidon, the number of turns stated in the schematic give the specified inductance.  Somewhere i saw that Amidon stated the accuracy of AL as ± 10 to 20% so there is a wide margin on the inductance you'd get for a given number of turns.  Apparently the cores I bought vary considerably from these values.  I bought my cores from Kits & Parts.  Don't know if they were manufactured by Amidon or not.

Anyway, I'd like the hear any comments on these questions and how best to determine proper inductance values.

I did see in one of K1RF's recent posts that the inductors in the 70MHz LPFs needed reduced turns to achieve the proper cutoff frequency.


Mike K5ESS


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