SECTION III
THEORY OF OPERATION
3-1 BLOCK DIAGRAM
The Model 540A Transfer Oscillator consists of the basic sections
shown in the block diagram in Figure 15. The operation of the
circuits is as follows:
The fundamental frequency in the 540A is generated by
an extremely stable, push-pull oscillator and is brought
out of the front panel at the OSCILLATOR OUTPUT jack.
The signal is then normally coupled through an attached
jumper to the MIXER INPUT A jack.
A signal to be measured is connected to the ATTENUATOR
INPUT jack for large signals, such as high amplitude pulses,
or, directly to the MIXER INPUT B jack for small signals,
below 1 volt. The A and B MIXER INPUT jacks are tied
together inside the mixer housing and connect directly to
the base (cathode) electrode of crystal diode CR1.
Mixer crystal CR1 serves both as the mixer and as the
harmonic generator for the fundamental frequency gen-
erated by the 540A oscillator, producing useful harmonics
up to approximately the 25th harmonic. When an input
signal is applied, mixing action occurs with all harmonics
generated. If the difference between the input signal and
one of the harmonics is less than the bandwidth of the
following amplifier a response will be seen on the oscil-
10s cope.
The Video Amplifier which is separate from the oscil-
loscope amplifier, amplifies the output of the mixer and
feeds both the VIDEO OUTPUT jack on the front panel
and the vertical oscilloscope amplifier. The maximum
bandwidth of the Video Amplifier is adjustable at both
the high and low-frequency ends to obtain the most easily
read beat-frequency pictures for certain types of input
signals.
Of all the frequencies applied to the Video Amplifier only
frequencies below approximately 200 kilocycles (such as
an input frequency approaching zero-beat with some
harmonic) will be pictured on the self-contained oscillo-
scope. Consequently, when measuring CW signals, any
active indication at all on the 'scope indicates that a zero-
beat is being approached. With amplitude -modulated '
carriers , vertical deflection appears be tween ze ro-beats
because the modulation frequencies are also pictured on
the 'scope.
3-1 |
ACEC
HEWLETT-PACKARD
