HP 4274A & HP 4275A LCR Meters

While working in the microwave semiconductor business I used many of the H-P Kobe Instrument Division (KID) products. These are the component test boxes like these Inductance Capactance Resistance (LCR) meters.

The HP 4274A covers the frequency range of 100 Hz to 100 k Hz
The HP 4275A covers the frequency range of 10 k Hz to 10.0 MHz

Measurement Methods

There are a large number of methods of measuring L, C or R, but the 4-port method used in the HP 4274A, HP 4275A and many other LCR meters is the most precise. For more on impedance measuring methods see the HP/Agilent Impedance Measurement Handbook (5950-3000) written by Makoto Honda and my web page on Impedance.

4-Terminal Pair

The electrical node where all four shields are connected together is "Guard".

DC Resistance in Ohms to Ground
	Lc	Lp	Hp	Hc
Shield	10	0	0.8	>200k
Center	100	>30k	open	>200k

a) shields connected together at DUT = guard
b) currents flowing on Hc & Lc shields
c) With proper connections the measurement range is from 1 milliohm to 10 Megohm (or better).
d) WRONG connection, like Kelvin Clips below.

4-Terminal Pair LCR Meter Functional diagram from The Impedance Measurement Handbook.
4-Terminal Pair LCR Meter Functional diagram

Fixtures

There are four BNC-male connectors at the lower left of the front panel for connection of a fixture or cable adapter. Mechanically they seem to be on 22.0 mm pitch. If you know the actual pitch, Let me know.

HP 16047C LCR Test Fixture	This fixture has an advantage over the 16047D when the leads are either very small in diameter (i.e. arn't stiff enough to push into the spring loaded sockets) or when they are very long. The bottom of the 16047C looks like the top, i.e. it's open so you can have test leads sticking out the bottom.
HP 16047D Test Fixture	This is the test fixture that shipped with all the HP LCR meters. I'm looking for one and both the axial and radial inserts as well as the notched metal plate for doing the zero cal. Let me know if you have one to sell.
Reproduction HP 16034E SMD Test Fixture Labeled TH26008A
4-Terminal Pair Tweezers
Kelvin Clips	These test leads will NOT work correctly because they don't tie the Hc and Lc shields toghther, see fig 3-5d above.
Home Brew Test Leads	I will modify this cable to add another lead connected to the shield common point, i.e. guard. Then make up a demonstration of guarding.

Guarding

When the shields of all four test ports are electrically connected, that connection point becomes a "guard" terminal. By connecting "guard" in a knowledgable way you can eliminate capactiors from the measurement. These can be real caps or parasitic caps, but either way you can make better measurements. If you guard something that you don't want to guard you may be making a measurement that's different from what you think you're measuring.

Three test items were made using six each 22 uF caps.

	# of 22 uF Caps in Parallel	Meas by itself
C1	1	21.75 uF
C2	2	42.03 uF
C3	3	63.3 uF

Now when all three are connected in a delta arrangement and the leads of C1 are placed into the test fixture the capactance reads 47.01 uF.
C2 and C3 in series: 1/C23 = 1/C2 + 1/C3 or 25.25 uF
The C2-C3 series connection in parallel with C1 = C1 + 25.25 uF or 47.008 uF (i.e. what's displayed above)

Now when the common point between C2 and C3 is connected to any of the coax shields the reading changes to 21.75 uF, i.e just C1 all by itself.

1909842 Method and means for measuring direct capacity, Albert M Skellett, Bell Labs, 1933-05-16, - It's an old idea. "According to this invention in measurement of a component capacity in a system of conducting elements between which capacities exist, the shunting effects of other capacities in the system are eliminated by maintaining all elements not directly in the measurement at the same potential as one of the elements associated with the component capacity being measured." i.e. "guarding"

HP 1980 Catalog Component Measurement Equipment

Model	Description	Remote
4260A	Universal Bridge	na
4261A	LCR Meter	BCD not HP-IB
4262A	LCR Meter	BCD or HP-IB
4265B	Universal Bridge (GR, Boonton?)	na
4271B	1 MHz LCR Meter	HP-IB
4272A	1 MHz Preset C Meter	BCD
4273A	1 kHz Preset C Meter	BCD
4274A	Multi-frequency LCR Meter ($7930)	HP-IB
4275A	Multi-frequency LCR Meter ($8720)	HP-IB
4282A	Digital High Cap Meter	HP-IB
4328A	Milliohm Meter	na
4329A	High Resistance Meter	na
4332A	LCR Meter	na
4342A	Q Meter	na
4191A	RF Impedance Analyzer (new 1980)	HP-IB
4140A	pA Meter/DC Voltage Source (new 1980) Semi: I-V and Quasi-static C-V	HP-IB
4800A	Vector/Z/Meter	na
4815A	RF Vector/Z/Meter	na
5045A	Digital IC Tester	HP-IB

HP 4274A & HP 4275A LCR Meters

Background

Measurement Methods

4-Terminal Pair

Fixtures

Guarding

HP 1980 Catalog Component Measurement Equipment

Related

Links