1
Introduction
The Marconi F.M./A.M. Signal Generator
TF
995A/5 covers the frequency range
of
1·5
to
220 Mc/s in
five
bands. Its output can be un-
modulated, frequency modulated,
or
amplitude
modulated;
if
required,
both
frequency and ampli-
tude modulation can be applied simultaneously.
The modulation frequency
is
obtained from either
an
internai 3-frequency oscillator,
or
an
externai
source.
The open-circuit output voltage
is
variable by
means
of
resistive step attenuators from 1
Il.
V to
100
mV
at
52
ohms
and
1
Il.V
to 200
mV
at
75
ohms. A plug-in 20-dB Attenuator Pad, available
as
an
optional accessory, extends the range down
to a nominal
0·1
Il.
V
at
both
impedances.
The high-discrimination tuning required for
testing narrow-band systems
is
facilitated by the
inclusion
of
a
fine
tuning controI.
In
addition, small
known changes in carrier frequency can be made
by means
of
two incremental-frequency controis;
one
of
these controis gives a stepped adjust-
ment while the other allows continuous inter-
polation between steps. On the two highest bands,
the stepped controi provides shifts
of
20
and
40
kc/s
in
either direction and the calibration marks
on
the continuous controi are only 1kc/s apart;
on
the lower bands, the total shift
is
determined by
a simple division
of
the reading
on
both
diais.
A high degree
of
frequency stability has been
achieved by use
of
temperature-compensating com-
ponents;
af
ter warm-up, drift
is
less
than
400 c/s per
minute
at
a carrier frequency
of
200 Mc/s.
The inclusion
of
a Carrier on/off switch makes
it possible for the Generator output to be tem-
porarily interrupted without affecting the output
impedance; this facilitates a number
of
two-signal
receiver tests such as intermodulation and blocking
which involves the simultaneous use
of
two signal
generators. Spurious f.m. due to hum
is
less than
25
c/s deviation, and the low level
of
noise modu-
lation makes the
TF
995A/5 fully suitable for appli-
cations such as adjacent-channel testing
on
receiver
systems using a channel separation
of
25
kc/s
or
less.
OM 995A/5 7
1-1/61
