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12. MEASURING IMPULSE NOISE
12.1 General – During our wideband noise example we noticed some level changes indicating possible
shot noise from some IC’s, a common problem with many inexpensive consumer solid state IC chips. In
this chapter we will use the impulse measuring mode to test the shot noise from these chips and illustrate
the use of this measuring mode. Unfortunately, the HP3586C doesn’t have this SSB channel mode and it
can’t be simulated by any other mode so we can’t do this measurement with a C version instrument.
In this mode the instrument will output the number of pulses during a preset time that exceed a preset
amplitude level.
12.2 Setup – Since the frequency display will be used for counting pluses in the impulse mode, enter the
filter’s center frequency before entering the impulse measurement mode. Press [FREQ] then enter 5.000
on the keyboard followed by pressing [KHZ]. Press [AUTO] on the measure/entry sub panel.
Press [SHIFT] on the measurement mode sub panel to select the impulse mode button. Press [IMPULSE]
to place the instrument into the impulse measurement mode. Now press [OFFSET] on the
measurement/entry sub panel to turn off the offset calculation if its button’s light is on.
We’ll arbitrarily decide to measure any noise pulse above –35dbm. Press [USB] or [LSB] to activate either
one of the SSB channels, for this test either will work fine. Be sure the widest filter without WTD is
selected and set its center frequency at 5KHz midway in our amplifier’s audio pass band. Next set an
interval measuring time of 5 minutes and 30 seconds and a minimum pulse threshold of -35dbm. On the
entry keyboard press [TIME] and enter 5.30 on the keyboard followed by pressing [MIN]. Next press
[THSHLD] followed by 35 on the keyboard, then press [-db] and [MEAS CONT].
12.3 Taking Measurements – Now install one of the IC’s we want to test into our audio amplifier.
Start the test by pressing [START] on the measurement mode sub panel. After our 5 minute 30 second
time interval we should see the number of shot noise pulses that exceeded our maximum noise level of –
35dbm on the RH side of the frequency display on the frequency/entry sub panel.
We can now compare the shot noise performance of our amplifier by the number of pulses over the time
interval using different chips. This example is not especially the best way to measure shot noise in IC’s but
chosen only to illustrate how the HP3586’s impulse measurement mode works and could be used.