1250 Frequency Response Analyzer The Display Menu and Mini Status
CHFR/1250/2 7-3
7.2 SOURCES
With two Analyzers in use, they may be employed as two separate measuring systems,
called Channel 1 and Channel 2. Establish the required Channel [Ch 1] or [Ch 2] as
source, by using the SELECT and ENTER keys in the usual way.
These two Channels can also be used together for point-to-point measurements, by
selecting [Ch 2/Ch 1]. In this case, Channel 1 would be connected to monitor the input
to the system under test, and Channel 2 to the output.
If more than two Analyzers are available, the other Channels are established by
selecting [Ch ( )] and entering the required Channel number in the round brackets. To
avoid confusion, the user should not enter a 1 or a 2 here. If the 1250 sees a 1 or a 2 in
the round brackets, it automatically selects Channel 1 or Channel 2 respectively, and the
STATUS display will show Ch 01 or Ch 02 as the chosen Source.
However, if [Ch ( )] is selected again, the round brackets will be empty, whereas if, for
example, Channel 11 had been established, the Display would show [Ch (11)].
The remaining mode is [Ch ( )/Ch m]. Using this, the ratio of any two Channels can be
displayed, for example by setting Channel 1/Channel 2, the reciprocal of the point-to-
point measurement described above can be calculated.
To set this up, type 1 on the key pad and enter it. TheDisplay will now show [Chl/Ch( )].
Now enter 2; the Display will revert to [DISPLAY] MENU but if the SOURCE soft key is
pressed again, the Display will show [Ch (1)/Ch 2]. STATUS will show Ch 01/02.
To avoid confusion, the user is advised not to enter the specially-allocated Ch 2/Ch 1
mode here. If this is done, the 1250 will correctly establish the required sources, and
STATUS will show Ch 02/01. However, the originally selected source title will be un-
numbered, i.e. show [Ch ( )/Ch m].
Either of the dual-channel modes can be used for measuring the forward gain of a
closed loop system, which might become unstable if the feedback loop were broken. It
is a straightforward matter to measure the error signal at the same time as the output.
7.3 CO-ORDINATES
In addition to Cartesian, the other co-ordinate presentations available are:
1. Polar, which is displayed as r, θ
where r = √ (a
2
+ b
2
) and θ = arctan b/a.
2. log Polar, which is displayed as log r, θ.
Where θ = arctan b/a as before, and log r is defined as follows.
a. For Single Channel measurements, log r represents:
20 log
10 √
(a
2
+ b
2
) in dB relative to 1V.
By using an appropriate scaling factor, the units can be changed to dB relative to any
reference voltage.
b. For Dual Channel measurements, log r represents the ratio of the two readings in dB,
e.g. for Ch 2/Ch 1:
log r represents 20 log
10
√ (a
2
2
+ b
2
2
)
dB
√ (a
1
2
+ b
1
2
)
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