Bird Technologies 3170A Series - Manual (Page 15)

Chap ter 2
The ory of Op er a tion
In put Sig nal
Di rect cur rent sig nals rep re sent ing op er at ing power lev els are brought into the RF mon i tor from
the sens ing el e ments through the rear panel jacks J1 and J2. These cur rents are shunted to ground
through 1400 ohm re sis tors. The volt age de vel oped across these re sis tors is am pli fied by a pair of
op er a tional am pli fi ers to ap prox i mately one volt (me ter full scale).
The drive cur rent to the main in di cat ing me ters is taken from the am pli fier out puts through se ries
re sis tors of 33.2k ohm each. The drive volt age for re mote me ters also co mes from the am pli fier
out puts and is buf fered by a pair of op-amps con fig ured for unity gain. This pro tects the
3170A/71A cir cuitry against ac ci den tal short ing of the re mote me ter line.
Ad just ment
The cal i brate/op er ate switch and the two ref er ence po ten ti om e ter al low the user to sub sti tute an
ad just able cur rent in place of the sig nals from the sens ing el e ments to fa cil i tate ad just ment of the
set-points.
The three set-point po ten ti om e ter (for ward, re flected, and con firm) pro vide dc volt ages which are
com pared to the am pli fier out puts by three com para tors.
The LED to the left of each po ten ti om e ter in di cates whether the in com ing sig nal is higher (LED
ON) or lower (LED OFF) than the set-point of the po ten ti om e ter.
2-1