CALIBRATION TECH BRIEF: BK Precision 4010A – VCG Out of Tolerance


The Out of Tolerance Solution

We had two BK4010A generators in for calibration in the JM Test Alexandria Lab recently, and all parameters except one were well in tolerance.  The out of tolerance condition was the Voltage Controlled Generator(VCG) frequency section of the verification.

Briefly, the verification consists of the following:
Set the Frequency Range to 200 kHz, and the dial to 2 (setting the frequency to 200 kHz).  Apply a DC voltage to the VCG input, about 6-8VDC initially.  Increase the DC voltage until the Frequency output drops by a factor of 100 (to 2 kHz).  Voltage should be between 9.0 and 11.0 VDC.

The generators were falling in at 8.3 VDC and 8.8 VDC.  Searching our libraries of materials, I could find no information about adjusting this function.  I wrote BK Precision, and they wrote back to insert a 500 ohm to 1000 ohm resistor in series with the VCG input, and this will put the frequency division voltage within the specification tolerance.

See picture below for the BEFORE condition of the 4010A

out of tolerance




The VCG input is the leftmost BNC connector from this view.  I reasoned that a with a DC control, adding resistance to this lead would not have an effect on the balance of the parameters of this unit. (A reasoning since verified through testing).  I also reasoned that if BK specified a 500Ω through 1000Ω resistance, then an adjustable resistor was in order.  I selected a 2kΩ potentiometer, shorted one of the legs and the wiper connection together and mounted as seen in photos 2 & 3 below:

Photo 2: After modification for adjustment

out of tolerance




Photo 3: Side view of modification:

out of tolerance




I inserted the single turn potentiometer (variable resistance) in series with the VCG input, and set the pot to approximately ½ turn (mid range).  With this adjustment in place, I set up for VCG verification,  and applied the DC voltage.  By adjusting the potentiometer, I was able to bring the voltage for proper function from approximately 8.3 or 8.8 VDC to right at 10.0 VDC.

On one of the above units, I found that the 2 K variable resistor was insufficient resistance, and substituted a 5 K variable resistor  (mounted in the same fashion as the 2000 ohm variable resistor).

It has been seen here in the lab that most of the 4010A units fall very close to the lower level (9.0 VDC) for proper operation of the VCG function.  This modification allows for adjustment to make the voltage for operation fall right at the mid range of the specification.

This is defined as an ADJUSTMENT, not a repair.  (So, a unit with an original setting of say 8.97 VDC, which rounds to 9.0 VDC – can be adjusted to the center of the range).

Article written by Mark Price,
Sr. RF Technician
JM Test Systems, Inc.

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