Rana and I spent some time looking at the IMC demod board earlier today. I will post the details shortly, but there was a label on the front panel which said that the nominal LO level to the input should be -8dBm. The new 29.5MHz routing scheme meant that the LO board was actually being driven at 0dBm (that too when the input to the RF distribution box was attenuated by 5dB).
An elog search revealed this thread, where Koji made some changes to the demod board input attenuators. Rana commented that it isn't a good idea to have the LO input be below 0dBm, so after consulting with Koji, we decided that we will
- Remove the 5dB attenuator to the input of the distribution box such that the LO is driven at ~5dBm
- Remove the input 10dB attenuator, first ERA-5SM amplifier, and the mini circuits power splitter from the demod board (schematic to follow).
After implementing these changes, and testing the board with a Marconi on the workbench, I found that the measured power levels (measured with an active FET probe) behave as expected, up till the ERA-5SM immediately prior to the LO (U4 and U6 on the schematic). However, the power after this amplifier (i.e. the input to the on-circuit LO, Minicircuits JMS-1H, which we want to be +17dBm), is only +16dBm. The input to these ERA-5SMs, which are only ~2years old, is -2dBm, so with the typical gain of +20dB, I should have 18dBm at their output. Moreover, increasing the input power to the board from the Marconi doesn't linearly increase the output from the ERA-5SM. Just in case, I replaced one of the ERA-5SMs, but observed the same behaviour, even though the amplifier shouldn't be near saturation (the power upstream of the ERA-5SM does scale linearly).
This needs to be investigated further, so I am leaving the demod board pulled out for now... |