BRUEL & KJAER 2006 - Input; Main Unit of Type 2006

Main Unit of Type 2006.
A
block diagram of Type 2006 is shown in Fig. 1.2. The input stage of the
Heterodyne Voltmeter consists of an untuned mixer stage containing a PNP-
transistor in " ground ed emitter" confi g uration. The sig nal from a variable
oscillator is supplied to the emitter while the collector circuit is tuned to
the IF-frequency, which has been chosen to be 11.3 MHz.
By careful amplitude stabilization of the variable oscillator the conversion
transconductance of the mixer is kept independent of frequency. To further-
more ensure frequency stability the oscillator is temperature compensated
and different oscillator components are used for different frequency ranges.
Following the mixer stage is a crystal filter with a bandwidth of 2.5 kHz.
This filter is shorted when the BANDWIDTH switch is set to "Wide".
From the filter the IF -signd is fed into a 3-stage IF -amplifier via an emitter
follower and a stepped attenuator stage. The IF -amplifier itself is stabilized
by means of current feedback. The first stage of IF -amplification contains a
special diodc-attenuator which is op£rated from the sensitivity controls
marked CARRIER ADJ. and LS AND MOD. ADJ., on the instrument front
panel, respectively. The diode voltage divider avoids different damping levels
influencing the tuned stage. Actually the potentiometers marked CARRIER
ADJ.
and LS AND MOD. AD.J. only determine the working points of the
diodes. Having passed the first two stages in the IF -a mplifier the signal is
-200
-100
I.
+100
+200 kHz
/~ I ~,
~0---- ------ ~ ---- -,~ ~----~~ N----- --~~~-- -- ~ ---------- --~
/ I "" "Wide"
-20
-------- -----1
// I ~~
-30 ------ --~ -+--------~~---- -1~-- -----+--------,_ ~ --------~
/ -5 11 .3 v +S ""
-
40
/
10
• "Narrow" "'-
-30 "'-
;7 ~: ---- ~ -------+------~-------- ~"'.-4
-60 ----------~--------~------ -1~ ------~--------~-- ---------- ~
-~ ---- ------ ~ ---------r-- -----+~ ------~---------r------------ ~
-80 - --+- -+--- ~/\ ~t----+-------1
dB
4654-33
Fig. 1.3. Bandwidth of the Heterodyne Voltmeter in BANDWIDTH switcl1
position "Wide" and "Narrow".
5