Specifications

Con nect the fre quency coun ter to the os cil la tor, the os cil la tor to the fil ter `+’ in put, and the AC me ter to the fil ter in put.
Set the fil ter to butterworth with the `TYPE’ key, lowpass with the ‘MODE’ key, 0dB in put gain, 0dB out put gain and
5kHz with the `FREQ’ key . Set the os cil la tor to 5kHz, 1VRMS. Set the me ter to read 0dB (dB and REL mode on the
Fluke 8920). Con nect the me ter to the fil ter out put; ad just the os cil la tor fre quency to get –3.01dB. The fre quency on the
coun ter should be 5kHz ±3%. Change the fil ter to highpass, ad just the os cil la tor fre quency for –3.01dB; the coun ter
should read 5kHz ±3%. Change the fil ter to lowpass mode, bessel type. Ad just the os cil la tor fre quency for –12.6dB; the
fre quency on the coun ter should be 5kHz ±3%. Change the fil ter to highpass, ad just the os cil la tor fre quency for
–12.6dB; the coun ter should read 5kHz ±3%.
Re peat the en tire above pro ce dure for 50Hz, 500Hz, 50kHz and 200kHz. It is im por tant to re-ref er ence the volt me ter at
each new fre quency since the am pli tude of the gen er a tor may change with changes in fre quency.
3.4 STOPBAND AT TEN U A TION
Ac cu rate stopband at ten u a tion mea sure ments re quire some sim ple pre cau tions be cause of low level sig nals. The fil ter should
be shielded with the top and bot tom cov ers in place. BNC ca ble only should be used be tween os c il la tor, fil ter and volt me ter,
and no other in stru ments should be con nected.
Set the os cil la tor to 7Vrms, 20kHz and con nect it to the `+’ IN PUT with the fil ter set to a cut off fre quency of 1kHz and 0dB
of In put and Out put gain. Con nect the OUT PUT through a 6kHz pas sive high-pass fil ter, as shown in Fig ure 3.1, to the
ac volt me ter. (Fluke 8920 should have `low range en able’ pressed in).
Set the fil ter to low-pass mode. The fil ter OUT PUT should be <700µVrms (–80dB).
3.5 PRE-FIL TER AND POST-FIL TER GAIN AC CU RACY
With a BNC tee, con nect both the os cil la tor and the AC me ter to the fil ter `+’ In put. Set the fil ter to 10kHz, low-pass.
With the me ter in volts mode, set the os cil la tor to 1kHz, and about 700mVrms. Set the me ter to read 0dB (dB and REL
mode on the Fluke 8920). Con nect the me ter to the fil ter out put. Set the in put gain to 10dB, the me ter should read 9.8 to
10.2dB. Set the in put gain to 20dB, the me ter should read 19.8 to 20.2dB. Set the in put gain to 0dB, set the out put gain to
1dB by press ing `1’ and ei ther of the out put gain ar rows. The me ter should read 0.8 to 1.2dB. Set the out put gain to 2dB
to 20dB in 1dB steps, check ing the me ter for ±0.2dB ac cu racy. Set the out put gain to 0dB.
Con nect the me ter to the fil ter `+’ In put and set it to volts mode. Set the os cil la tor to about 70mVrms. Set the me ter to
read 0dB (dB and REL mode on the Fluke 8920). Con nect the me ter to the fil ter out put. Set the in put gain to 30dB, the
me ter should read 29.8 to 30.2dB. Set the in put gain to 40dB, the me ter should read 39.8 to 40.2dB.
Con nect the me ter to the fil ter in put and set it to volts mode. Set the os cil la tor to about 25mVrms. Set the me ter to read
0dB (dB and REL mode on the Fluke 8920). Con nect the me ter to the fil ter out put. Set the in put gain to 50dB, the me ter
should read 49.8 to 50.2dB. It is im por tant that the gen er a tor be low noise; pas sive BNC attenuators pro duce the best re -
sults, placed at the fil ter in put.
Section 3 - Incoming Acceptance Model 3380
3-2
Figure 3.1 Passive 6kHz High-Pass Filter