The 70MHz L/C band-pass filter used in the receiver front end and the transmitter driver output.
The graph above shows the simulated response of the BPF. The actual prototype fell short of 70MHz it would only center ~ 68MHz. To make the filter center on 70MHz L1 and L2 were changed to 860nH. Coilcraft 7M2 861 coils were used. The simulated insertion loss is ~6.5dB with a bandwidth of ~4MHz.
To measure the filters response I used the input filter on the front-end amplifier prototype.
The measured response of the filter is shown below.
Insertion Loss 4.5dB
Return loss 10.5dB
Bandwidth 3.2MHz
Frequency Response
Transmitter Output Filter 9th Order Chebychev LPF
Click on the diagram to download a PDF version
The filter design was done using an on-line filter designer the one I used is Chebychev LPF Designer which give ideal values. The inductors used are Coilcraft Maxi range which are rated at 3A. The 0805 capacitors are rated at 100V. The circuit also shows the power monitor circuit which consists of a 9:1 current transformer which is terminated into 50ohms. Measuring the voltage across the 50ohm load enables the power to be calculated. I tried the circuit with the reverse power transformer fitted. The high Z side of the transformer needs to be connected on the power line and ground. This created a problem the filter match S11 was totally destroyed. I have now resolved the problem by re-optimizing the filter. To achieve a good return loss over the band C11 was added an C7 changed to 8.2pf. This now gives the radio the capability to measure SWR.
The measured response is shown in the graphs below.
Insertion Loss @ 70.25MHz ~0.6dB
Return Loss @ 70.25MHz ~ 24dB
Attenuation @ 140MHz ~ 51.5dB
Close In Frequency Response
Milled Prototype PCB
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