Microwave mixing circuit and a VHF-UHF tuner having the mixing circuit

摘要

In a double superheterodyne tuner for receiving TV channel signals, a frequency in a range defined between 2520 and 2700 MHz is selected as the first intermediate frequency, while the first intermediate frequency is obtained by subtracting a desired channel frequency from a first local oscillator frequency. The tuner comprises first and second mixing circuits respectively used for effecting frequency conversion, where each of these mixing circuits has parallel coupled lines so that each mixing circuit operates as a balance-to- unbalance transformer. The second mixing circuit further comprises a short-circuit line placed between the parallel coupled lines. The tuner also comprises a band pass filter having a band width less than 5 MHz, which selectively transmits the first intermediate frequency signal from the first mixing circuit.