HiAll,
It was Matt, M0LMK who de-potted the PCB by boiling it, and then sent it to me to try and measure the performance.
Today, I managed to repair the amplifier and get it fully running.
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As I suspected, the badly mismatched 30MHz input low pass filter messes up the frequency response. The designer has really slipped up here by using a filter designed for 50 Ohm termination impedance, but with a low impedance loop as the source and an amplifier having an input impedance ranging from 1.5K Ohm to a few hundred Ohms, as the load the response curve is all over the place.
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The following graph was taken with a low value of source impedance in order to better simulate a loop.
I tried adding various values of terminating resistors between the amplifier chip Pin 1 and the PCB ground and Pin 8 and the PCB ground to try and improve the match, and it looks like 75 or 100 Ohms is about the optimum (50 Ohms rolls the gain off too rapidly at 30MHz) however it does reduce the overall gain, but this can be increased in order to compensate.
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The big problem is that under certain conditions the unmodified amplifier can have a 20dB higher level of gain at around 7MHz, which is bad news, especially in Europe, as this is where all the very strong broadcast stations appear at night.
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The next stage is to draw out the bias tee circuit and then try the loop outside to see how it actually performs at my location.
Regards,
Martin
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