of the drift regions of course in an extremely long device with appropriately shallow drift regions the harmonics could be close enough together to have multiple stable oscillation frequencies if it were desirable to configure a millimeter wave comb filter it could be done in this fashion in addition the resonant frequency is somewhat sensitive to the thickness and doping of the contact layers 20 and 26 these contact layers load the transmission line and therefore can lower the phase velocity of propagation in the transmission line and lowering the resonant frequency depending on the dimpatt characteristic impedance the side coupling microstrip characteristic impedance may need to be small in this case it is desirable to transition from standard 505l transmission line at the input to low impedance 23 in several lambda 4 transformers lambda corresponds to the wavelength in the microstripe at the center frequency of frequency band multi section transformers give broader band impedance matching a similar x 4 transformer sections increase characteristic impedance level at the output terminal figure 16 shows this conjunction preferably the impedances increase geometrically for specific applications it may be desirable to change dimpatt width in other words use wider devices closer to the output end to ensure that each device experiences similar power densities in this case it may be desirable to couple devices closer to input prior at a different impedance level than those closer to output as shown in fig1 alternatively similar size diodes can be used but with the coupling changed towards the output this can be achieved as shown in fig1 by again changing the characteristic impedance of the microstrip near the input.