Inductor Guide

Anecdote of inductor development [No. 7] Days worrying about high-frequency high-speed measurement technology

This story is from approximately 30 years ago, when high-frequency chip inductors were developed. Mobile communications at that time referred to car telephones, which were so large that storage required the entire center console of a luxury car. I suggest removing the "nowadays" phrase because the paragraph begins 30 yrs ago...and next sentence gor to present day...and next phrase goes to 30 yrs ago. So the structure sentence is a little confusing. 

One day we received a request from a particular customer, "Do you have any high-frequency chip inductors with inductance up to 1 GHz, a high Q and a small inductance tolerance?" To be honest, my first thought was "What!? G- Gigahertz?"

At the time, our company had only one PC per section, with an 8-inch floppy-disk drive whirring and clicking as an external memory device. This was the era when documents were hand-written, copies were blue print, and ashtrays could still be seen on desks. The biggest obstacle was "How can we guarantee inductance values with narrow deviations for all units at high frequencies?"

High-frequency coils have been around for a long time. Spring coils were also used in mechanical tuners (the dials turned while clicking to change the channels of old TVs), and work by soldering the terminals and then deforming the coil with a spatula to tune the frequency. However, the specifications of those spring coils simply defined the physical dimensions such as the wire diameter, number of windings, and coil length, and did not guarantee the inductance value (nH ±x%) or other characteristics. This was right after Murata had started mass production of wire wound-type chip inductors, when we had the ability to make high-frequency inductors, but we worried about how to quickly and automatically measure the inductance and Q at high frequencies. Impedance analyzers (measuring instruments) capable of measuring up to 1 GHz existed, but at the time these instruments were not suitable speed-wise or mechanically for high-speed automated equipment.

We made some prototypes and the performance satisfied the customer's specifications, so adoption was decided and we started receiving orders. At first we relied on sheer force of numbers and measured each product by hand, but we were worried about how to cope as orders increased in the future.

While measuring and worrying in the shipping inspection room, an old Q meter (*1) off to the side caught my eye. The sight brought to mind nostalgic memories of my student days, but the Q meter would be of no use as it measured only up to 25.2 MHz, not to mention using analog tuning operation (turning a dial).
At that moment,  a second thought struck me, "A Q meter is a reactance resonance bridge (*2), isn't it? That's right! We don't need to measure the inductance itself." And once I had that thought, the rest was easy. I went and got a high-stability capacitor (after all, Murata is a capacitor manufacturer) and made a Colpitts oscillation circuit (*3). Next, I connected a power supply and a frequency counter, and conducted some tests. As expected, the oscillation frequency and the inductance correlated beautifully. I then made some modifications so that oscillation stopped when the Q value was low and finally connected the frequency counter output to a comparator (*4) to complete a high-speed automatic measurement and taping machine for high-frequency chip inductors. This provided prospects for mass production, enabling supply of sufficient quantities.

Mass production initially started up with two machines based on this measurement system. Some time thereafter, high-speed high-frequency impedance analyzers became available, which enabled direct high-speed measurement of inductance and Q from the third machine onward. Our first high-frequency chip inductor (LQN2A type: 3225 [mm]) was favored by many customers for many years. Since then, mobile communications have evolved at a tremendous pace, and the lineup has expanded accordingly and grown into the LQW series of wire wound-type high-frequency chip inductors.

High-frequency chip inductor series
<High-frequency chip inductor series>

[Simple explanation of terms]

(For more detailed information, please refer to a technical manual.)
*1 Q meter:
 An old analog measuring instrument used to measure the electrical characteristics of coils and capacitors. Wireless technicians with many years of experience may have nostalgic memories of using Q meters.

*2 Reactance resonance bridge:
 The principle of electrical balance for high frequencies. This system enables calculation of the value for an unknown coil when there is a resonator for which the value is already known and a balance can be achieved between the two.

*3 Colpitts oscillation circuit:
 A typical high-frequency oscillation circuit. Incidentally, the reason why I chose this oscillation circuit is that I was new on the job and this was the only oscillation circuit I knew at the time.

*4 Comparator:
 A device that sorts measured values into pass and fail and sends these results to machinery. A flag raiser.

 

Written by: T.M., Ceramic Production Dept. 4, EMI Division,

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