L - VIR History

ELECTRONIC CIRCUITS NAVSHIPS
the previous checks foil to locate the trouble, the circuit
supplying the plate voltage is probably at foult.
Lor Output. A lo,^ output -~o,uld lormally be caused
by a defectj,,e or weak VI, or o low filament or plote volt-
oqe, or lf the i-f ond r-f tsnk circlits ore no! tuned to the
proper freq~encies. A ;:eck local oscillator voltoge cln
cause a low output. Check the flloment and plate voltages
wlth a VnM. If they art. not riornnl, refer to the procedure
in the previous paragraph. If they ore normal, check the
amplitude of the loco1 oscillator signol. If it is low,
check C3 with an incircuit copocitor checker. If the out-
put is still low, the trouble is probably in the local oscil-
lator circuit.
Distorted Output. A distorted output can be caused
by a defect in nearly any component in thecircuit. With an
oscillascope, check far an r-f siqnd on Cqe secondary
winding to TI. If the r-f sign01 is not present, check the
windings of T1 with an ohmmeter for continuity. Should
the windings not be defective, the trouble lies in the pre-
ceeding stages and the mixer is probably not defective.
If the signol is present on the secondary winding of T1,
check for the presence of the local oscillator signal on
the high side of C3. If it is not present, the loco1 oscillo-
tor is at fault. If it is present, both the I-f and local
oscillator signals should be present on the control grid of
V1. If the local oscillator signal is not present, C3 is de-
fective. If the r-f signal is not present, C2 is defective.
If bath the r-f and local oscillotor signals are present on
the control qrid of V1, Tube V1 is probably defective. If
the output is still distorted, check the plote and sneen
voltages with a VTVM. If both voltages are low, check
the outplt of the plate voltoge supply. If it is low, the
trouble lies in the plate supply. If it is normal, and the
screen voltage is low, check R2 with an ohmmeter, and
C4 and C5 with an in-circuit capacitor checker. If the plate
voltage is low, check C6 with an in-circuit capacitor checker
and the primary winding of T2 with an ohmmeter. If the out-
put is still not present, check the secondary windinq of T2
with on ohmmeter.
PENTAGRID MIXER
APPLICATION.
The pentaqrid mixer IS used in modern superheterodyne
receivers 0s o frequency converter. Incoming r-f s~gnals
are combined wlth signals iron! .I local oscillotor to prm
duce an intermediate frequer:~; (i-f).
CHARACTERISTICS.
Offers qood selectivity.
Serves both os a frequenq converter md o hizh qain
amplifier.
Siqnal-to-noise rotio is poor.
Requires o separate local oscillator to supply the iet
erodyning voltage.
Uses two input control q ds to provide electron mu-
plinc.
CHANGE 2
0967-000-0120 FREQUENCY CONVERTERS
Operates with either cnthode-self, flned, or ovc bias
voltage.
CIRCUIT ANALYSIS.
General. Tne function01 operation of the pentaqrid
mixer is very similar to !hot of other mixer circuits discussed
previously in this hmdbook. R-f and oscillator
voltoges ore injected into the tube a d added olgebrclcolly.
The fundomertal ireqljen-~s, along vrith their swm and
difference frequencies, appear ocros: the outout circuit.
The output circuit is a parallel resonant tank, tuned to the
i-f. The desired I-f sional is transformer coupled into the
next stage.
The prlmary difference between the pentamid mlxer circuit
and other mixer circuits is the input arrangement. In
the diode, triode, md pentode nlxer the r-f md oscillator
wltoges are inserted on the same tube element, allowinq
for greater ~nteraction between input signals. In the pentogrid
mixer, r-f and oscillotor signals ore inserted on seporate
control grids, isoloted from ear5 other and the plote by
screen grids. Coz;eq'~endy, the frequnq pulling effects
md signal interocticn, cormon to other mixer circuits, is
virtually eliminated.
Circuit Operotion. Before discussinq ooeration of the
pentogrid mixer it ..will be helpful to review :he operation of
the pentagrid tube.
The pentagrid tube consists of a plote, cothode, filaments
and five gr~ds, hence the name pentogrid. Two of
the grids ore used as control grids (GI and G3), two are used
as screen grids (G2 ond G4), and the fifth is used as o
suppressor grid. For all procticol purposes the gain of
the pentagr~d tube is compcroble to that of the pentode,
however the inuoductian of an extro screen grid increases
the partition noise, and consequently, the circsit noise.
The screen qrids are operated at a positive voltaqe and
serve as the occeleratinq anodes for electrons leovinq the
cathode. However, the electrons strike the plate of the
tube wlth such force that they tnunci off (semndory emission)
and form a spcce chorqe around the positive sneen
qrid (G3).
The space charge greatly limits the plate voltoqe swinq,
so a negative grid (G5) is placed between screen and plate,
md its negative charge diverts electrons jock to :he plote.
By following the above discassion it con be seen that
the pentagrid tube plote current is made independent of
plate voltage. In fact, tile plote voltage may smng as low
as, or lower thm, tk,~ screen voltoge ..xithout serious loss
of amplifier qoir capobil~ties. In rnixer clrcuits jnin (qr)
is referred to os "con,,ersicn transmnductance" ond rep
resents the quotient of i-f octput current divided by r-f
input voltage; a:, conversion trunsconductance = Iit/Eri.
In pentagrid tubes con.Jerslon trarscond~ctance may run
as high as 500 micromhos.
In the mixer circuits previously discussed, such as the
triode ond pentode mixer, the r-f and oscillotor siqnols ore
injected on th.e some control grid. Tnus the r-f input circuit
is "seen" by both inp~ts cnd stray mupllnq induces
oscillator detuning, or !req'~ency pulllnc. In pertaqrid