Frequency Marks - Philips PM5324 Instruction Manual

2.3.5. FREQUENCY MARKS

Marks, wh ich i nd icate a certa i n frequency on the band-pass curve, often are i m portant. T h i s can be met if
additional equ i pment i s ava i lable.
F ig. 2 1 represents a schematic survey of th is p r i nciple.
signal
a.f.
PM
5321.
f 2
Marker
oscillator
Fig. 2 1. Generating frequency markers
The signals of both the generators PM 5324 ( f l ) and the marker generator (f2) are appl ied to a m i x i ng c ircu i t
via separator stages I and I I . By superposition of the signa l s at the input of the m ix i ng c i rcuit sum- and d ifference
frequencies (fl + f2) and ( f l - f2) occur at the curved characteristic of the m i x ing d iode due to mod u l ation.
O n l y the difference frequency (fl - f2) is ampl i fied by a smal l -band LF ampl ifier. This d i fference signal is
appl ied to a l ow-pass fi lter. As soon as both frequencies (fl a nd f2) approximate sufficiently, the l ow d ifference
frequencies pass through the low-pass fi lter. At the moment both frequencies are equa l , the d ifference is zero;
provided that they have the same phase and ampl itude. If the frequencies deviate aga i n , another I.f . component
is formed.
The resu lting output pulse is represented in F ig. 24. The m a r k i ng pulse obta i ned i s added to the output voltage
of the test object and is fed to the V-i nput of the osci l loscope.
The ampl itude of the frequency marks with respect to the band-pass curve can be influenced by changing the
ampl itude of the marker frequency f2 or by adjusting the gain of the I.f. amplifier. The frequency marks on
the band-pass curve of a test object, obtained in th is way can be sh ifted by vary ing the frt;!quency of the m a rker
generator.
Fig. 22. Mixing circuit
- - - -
1
Object
I
under
test
1
1
1
I
I
1
mixing
1
circuit
1
I
II
L
- - -
---f 5 6 0 0 P F
�
Fig. 23. L ow-pass filter
-
- - - - - -
100
k
[>-100X
%
- - -
- - - -
MA8216
25
y
- I
- -
- - -
I
1
1
1
I
I
1
x
1
I
- - - - - -
MA 8214
MA8217
Fig. 24. Freq uency mark