Frequency and wavelength Measurement
Microwave frequency is meaured using a commercially available frequency counter and cavity wavemeter. The frequency also can be computed from mesured guide wavelength in a standing wave pattern aliong a short circuited line by using a slotted line .
Wavemeter Method :-
A typical wavemeter is a cylindrical cavity with a variable short circuit termination which changes the resonance frequency of the cavity by changing the cavity length. TE101 mode is most suitable for wave meter because of its higher Q and absence of axial current. Since this is higher order mode, possibility of generation of lower order modes exits. Hence lor practical purposes dominant TM010 mode is used in wavemeter applications. Wavemeter axis is placed perpendicular to the broad wait of the waveguide and coupled by means of a hole in the narrow wall as show n in Fig. 1. This excites TM010 mode in the cavity due to the magnetic Held coupling. A block of absorbing ma- terial (Polytron) placed at the back of the tuning plunger prevents oscillation on lop of it. Thus the cavity resonates at different frequencies for different plunger posi- tions. The tuning can be calibrated in terms of frequency by known frequency input signals and observing the dip in the display unit (power melcrt connected at the output side of waveguide. The accuracy of such a wavemeter is in the range of 1% to 0.005% for available Q of 1000-50,000 respectively. Since the power is absorbed in the wavemeter at resonance this is called absorption type wavemeter.
Figure 1 : Wavemeter method of frequency measurements
Slotted Line Method :-
Since the distance dmin between two successive minima voltage standing wave pattern in a short circuited line if half wavelength λg/2 frequency can be determine from the relations
and measuring dmin by the slotted line probe carriage.
Down Conversion method :-
An accurate measurement of microwave frequency can be done by means of a heterodyne converter which is shown in the following figure
Figure : Down conversion method