BRUEL & KJAER 1621 - Frequency Tuning Circuit, Manual; Overload Circuit

I
1
Technical Description 1621.1
~3
~"Frequency Tuning"
OmV to -60 mV
P120
> .... -----------------1.to Integrator circuit
+ 6V
1
L ____________ J
(HV)
Q3
-300 mV
beLow 300 mV p
(above ,,300 mVp)
Input from V 12
" 3G ot Y!?_-_
Ref. tor
Overloan
-mV
-300 mV
tamp On La mp On
Feedback l
Input
t-L.}---:-----<~--{) I
Vl
V4
I 1----------
3%~ 1V~
23'100 O)VO
01 Nl
FiLter
Max voltage
1-2
(V 9, 10 "A")
"A"
I
45 mV to 450 mV
> ..... ;-----J~to Integrator circuit
(V9, 10 "8")
Vl0 I
Q4
I
1
bandwith
seleetor
(V4)
cLosed during overLoad
( 3'10 position)
Frequency Tuning Circuit, Manual
The filter centre frequency is controlled by FREOUENCY TUNING potentio-
meter P120 on the front panel. The voltage to the potentiometer is
taken from the voltage divider R20, R21 and P3. The output of V17 can
be variated between O mV and -60 mV DC.
This voltage is fed to the transistor "A" in the tuning circuit (and to "A"
in the two integrator circuits). Transistor "A" is basicly used to controi
the current to transistor "B" which acts as a variable resistor tuning the
integrators within one decade, but "A" is also used for temperature and
distortion compensation.
The output of V 15 will be approximately + 45 mV to + 450 mV DC de-
pending of the P120 position.
Sync.
from
o---
Overload Circuit
The input to the overload circuit is taken from the Feedback I. The over-
load is activated when the feedback is more negative than -300 mV
peak.
For
an input voltage to the overload circuit more positive than -300 mV
peak the voltages will be as follows:
V14 pin 6 (output) will be -9,5 V DC. and the "Overioad" lamp is inac-
tive. At pin 3 there will be approximately -300 mV DC due to the resistors
R58, R59 and R63. The two diodes 03, 04 will not be conducting
When the input voltage at pin 2 goes more negative than -300 mV
peak, the output voltage (pin 6) changes to approximately -r 8,5 V. The
"Overioad" lamp is now activated, and the voltage at pin 3 changes
value from -300 mV DC to + 4 DC due to the diode 04. C22 starts
charging against + 8,5 Vand the input at pin 2 increase as weil due to
the diode 03.
When the voltage at pin 2 reaches + 4 V DC the circuit switches back to
the previous position, -9,5 V DC at pin 6 which switches off the lamp,
and -300 mV DC to pin 3.
Now C22 discharges against -9,5 V DC. When the voltage across C22
is decreased to -300 mV DC the "Overioad" is switched on again and a
new cycle takes place.
This function will be continued as long as the voltage from Feedback I is
more negative than -300 mV peak.
The drawing shows the voltage across C22 during overioad.
The signal from the output of V14 pin 6 to the bandwidth seleetor is
used to switch automatically the 3% bandwidth to 23% during overioad.
This is done in order to get a quicker reaction of the filter circuit espe-
cially at very low frequencies.
9.77
9.77
470 k n..
R 62
- 9,5V
(+8, SV)
to