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Measurement Receiver Inductors

John Kolb
 

I was a manufacturing engineer for Southcom International back in the dark ages. We had the coils wound by a subcontractor (our production ladies working at home) and then adjusted them tighter or loser using Boonton Q meters. I was going crazy writing engineering change notices on the coil drawings with each new batch of cores till I was allowed to specify the winding instructions as + or - a number of turns. It was probably +/- 4 turns for a coil of 120 turns.

So yes, for critical applications, toroids have to be tweaked by squeezing or expending the winding around the core or even changing the number of turns.

We once received a coil from a commercial coil manufacturer where their wire spool had run out in the middle of the winding and another wire was joined with nice square knot on the insulated wire. Couldn't figure out how a single toroid could not have continuity from one end to the other :)

John
KK6IL

On 2/19/2016 9:23 PM, k5ess.nothdurft@... [PHSNA] wrote:
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.

vasilyivanenko@...
 

Hi Mike


Toroid turns calculators and tables give approximate L values with tolerances that may vary with toroid AL variations, wire spacing and to some extent frequency in a functional sense ( we usually put them in circuits with stray L and C )  Wire gauge may effect turns spacing and also perhaps how the wires orient and stay put on the toroid core. Even the target L can influence these as more windings might mean thinner gauge wire and wire turns can bunch up and not stay in place compared to a small uH coil wound with thicker wire etc.


With nH coils @ VHF, the lead length may affect precision. I don’t use turns calculators and just measure them with a LC meter and scrunch or spread the windings to get the desire L.  When I did use turns calculators, I always wound 1 less turn than calculated and then applied the scrunch/spread method to get the desired L.


In most L-networks, a tuning cap proves critical. Most of mine apply 2 caps [ L-C-C] network because 2 caps gives you 3 tweeks. The other tweek is to again scrunch or spread the inductor while peaking the trim cap(s) for the best impedance match. SPICE might not account for stray L and C and the reactive components at 1 or both ports.


If you experiment with a return loss bridge and a simple L or L-C-C network to match a network into 50 ohms --- while measuring RL on the 50 ohm port, you will learn just how “touchy” the tuning can get.

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