B+K precision 2165A, Instruction Manual, Page: 9 / 28

9
INTRODUCTION
General
The spectrum analyzer permits the detection of spectrum
components of electrical signals in the frequency range of 0.15
to 1050 MHz for Models 2625 and 2630 and 0.15 to 500 MHz
for Models 2615 and 2620. The detected signal and its content
have to be repetitive. In contrast to an oscilloscope operated in
Yt mode, where the amplitude is displayed on the time domain,
the spectrum analyzer displays amplitude on the frequency
domain (Yf). The individual spectrum components of “a signal”
become visible on a spectrum analyzer. The oscilloscope would
display the same signal as one resulting waveform.
Models 2625 and 2630
The spectrum analyzer works according to the triple superhet
receiver principle. The signal to be measured (f
in
= 0.15 MHz to
1050 MHz) is applied to the 1st mixer where it is mixed with
the signal of a variable voltage controlled oscillator (f
LO
1350
MHz – 2350 MHz). This oscillator is called the lst LO (local
oscillator). The difference between the oscillator and the input
frequency (f
LO
– f
in
= 1st IF) is the first intermediate frequency,
which passes through a waveband filter tuned to a center fre-
quency of 1350 MHz. It then enters an amplifier, and this is
followed by two additional mixing stages, oscillators and
amplifiers. The second IF is 29.875 MHz and the third is 2.75
MHz. In the third IF stage, the signal can be selectively
transferred through a filter with 400 kHz or 20 kHz bandwidth
before arriving at an AM demodulator. The logarithmic output
(video signal) is transferred directly, or via a low pass filter to
another amplifier. This amplifier output is connected to the Y
deflection plates of the CRT.
Models 2615A and 2620A
The spectrum analyzer works according to the triple superhet
receiver principle. The signal to be measured (f
in
= 0.5 MHz to
500 MHz) is applied to the 1st mixer where it is mixed with the
signal of a variable voltage controlled oscillator (f
LO
610 MHz –
1110 MHz). This oscillator is called the 1st LO (local
oscillator). The difference between the oscillator and the input
frequency (f
LO
– f
in
= 1st IF) is the first intermediate frequency,
which passes through a waveband filter tuned to a center
frequency of 609.5 MHz. It then enters an amplifier, and this is
followed by two additional mixing stages, oscillators and
amplifiers. The second IF is 29.5 MHz and the third is 2.9 MHz.
In the third IF stage, the signal can be selectively transferred
through a filter with 250 kHz or 20 kHz bandwidth before
arriving at an AM demodulator. The logarithmic output (video
signal) is transferred directly, or via a low pass filter to another
amplifier. This amplifier output is connected to the Y deflection
plates of the CRT.
The X deflection is performed with a ramp generator voltage.
This voltage can also be superimposed on a dc voltage which
allows for the control of 1st LO. The spectrum analyzer scans a
frequency range depending on the ramp height. This span is
determined by the scanwidth setting. In ZERO SCAN mode
only the direct voltage controls the 1st LO.
Models 2620A and 2630 each include a tracking generator.
This generator provides sine wave voltages within the
frequency range of 0.1 to 1050 MHz for Model 2630 and 0.1
to 500 MHz for Model 2620A. The tracking generator
frequency is determined by the first oscillator (1st LO) of the
spectrum analyzer section. Spectrum analyzer and tracking
generator are frequency synchronized.
Operating Considerations
It is very important to read
Safety in the GENERAL
INFORMATION Section including the instructions prior to
operating the spectrum analyzer. No special knowledge is
necessary for the operation of the spectrum analyzer. The
straightforward front panel layout and the limitation to basic
functions guarantee efficient operation immediately. To
ensure optimum operation of the instrument, some basic
instructions need to be followed.
CAUTION
The most sensitive component of
the spectrum analyzer is the input
section. It consists of the signal
attenuator and the first mixer.
Without input attenuation, the
voltage at the input must not
10 dBm (0.7Vrms) AC or
±25 volt DC. With a maximum input
attenuation of 40 dB the AC voltage
must not 20 dBm.
Exceeding these limits will
damage the input attenuator and/or
the first mixer.
Prior to examining unidentified signals, the presence of
unacceptable high voltages has to be checked. It is also
recommended to start measurements with the highest
possible attenuation and a maximum frequency range. The
user should also consider the possibility of excessively high
signal amplitudes outside the covered frequency range,
although not displayed (e.g. 1200 MHz). The frequency
range of 0 Hz to 150 kHz is not specified. Spectral lines
within this range would be displayed with incorrect
amplitude.
A particularly high intensity setting shall be avoided. The
way signals are displayed on the spectrum analyzer typically
allows for any signal to be recognized easily, even with low
intensity.
Due to the frequency conversion principle, a spectral line is
visible at 0 Hz. It is called IF-feedthrough. The line appears
when the 1st LO frequency passes the IF amplifiers and
filters. The level of this spectral line is different in each
instrument. A deviation from the full screen does not indicate
a malfunctioning instrument.