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一台标准的超外差式半导体收音机只要对其线路稍加改变,增加一个不太复杂的辅助线路,就可以改制成一台效果良好的超声多普勒胎心听诊仪,有兴趣的读者不妨一试。一、接收线路的改制去掉图1虚线框中的收音机的磁性天线和双连调谐电容,并把变频线路改接成图2所示的不调谐输入线路。收音机原来的振荡频率(中波)上限约为1.6MH_Z,由于去掉双连电容,其谐振电容改为10P以下,因而调整振荡线圈的磁芯可使其本振频率提高。一般多普勒超声探头的工作频率为2.5MH_Z,故在此本振频率应调为:2.5MH_Z+0.465MH_Z=2.965MH_Z≈3MH_Z
A standard super-heterodyne semiconductor radio as long as the line a little change, add a less complex auxiliary line, you can transform into a good ultrasound Doppler fetal heart rate auscultation, interested readers may wish to one test. First, the restructuring of the receiving line Remove the magnetic antenna of the radio in Figure 1 dotted line box and dual-connected tuning capacitor, and the conversion of the conversion line into the shown in Figure 2 does not tune the input line. The original oscillation frequency of the radio (the upper limit of the wave) is about 1.6MH_Z. Since the double capacitor is removed, the resonance capacitance of the radio is changed to below 10P, so adjusting the magnetic core of the oscillation coil can increase the local oscillation frequency. General Doppler ultrasound probe operating frequency of 2.5MH_Z, so the local oscillator frequency should be adjusted to: 2.5MH_Z + 0.465MH_Z = 2.965MH_Z ≈ 3MH_Z