Frequency setting possibility with resolution of 1 Hz - without changing band in the whole operation frequency domain - is a basic requirement. It is realized on the principle of the so called heterodyne generator. Its essence, that frequency of the outgoing signal of the transmitter is the mathematical difference of the frequency of two oscillators, (1) and (2). The advantage of this process is that the frequency of the generator can be changed continuously in a broad band.
Generation of difference is executed with the (3) modulator, and the (4) low-pass filter. In measuring transmitters with lower demand the (1) tuneable oscillator is a freely running oscillator, and the frequency setting is supported by a built-in frequency measuring tool. In the more up-to-date measuring transmitters a frequency synthesizer operates instead of the tuneable oscillator.
Level setting:
Setting of the level of the generated measuring signal is
executed in relative units (decibels) in the output stage of the measuring transmitter (6).
The set value of level refers to the terminated output.
Outgoing level of the measuring signal must be set with a minimal resolution of 1 dB
(0.1 dB is desirable).
If the reference level is an absolute performance level, then the output voltage of the
terminated measuring transmitter depends also on the output impedance.
Value of the correction factors to the standard impedances:
When setting the output impedance the measuring transmitter will adjust the output voltage with the values mentioned above.
Level forbidding:
Level of the output signal has to be forbidden by changing frequency. It is necessary in case of measurement executed on operating FDM equipments, not to feed a signal into the operating channels of the equipment, when tuning the measuring transmitter.
Output impedance:
The possible values of the output impedance are the usual /75, 150, 600 ohms/. Some of the measuring transmitters have an output of 0 ohm.
Output connector: