HMC849ALP4CE SPDT not working

Dear all,

I'm using the hmc849alp4ce switch to select which channel to be transferred to the rest of the chain. That is, there are two different RF signals on each of RF Port 1 and RF Port 2 that I wish to pass on to RFC Port.

I'm working around 5.9GHz and placed a 1.6 pF DC block caps on each port.

The control bit appears well and the enable pin is tied to GND.

The problem is simple - the switch doesn't apear to be working. No matter what the state of the control pin, I still observe -25dB loss when RF2 should be connected to RFC and similarly RF1 to RFC.

When I change between states there is a slight improvement towards -23dB. either way, it's not working.

Vdd is fine and according to specs.

My questions are:

1. The switch is bidirectional - Is that correct? I assume it is but just to make sure.

2. Do you think that 1.6pF caps should be replaced to different vals? The datasheet suggests 100pF but this values look too much for me considering 5.9GHz.

Thank you,

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  • Hi,

    The HMC849A conducts the signal in bot direction, from RFC to RFx or from RFx to RFC.

    The dc blocking capacitor should behave like a short at operating frequency. 1.6 pF could introduce some loss due to its reactance at 5.9 GHz so I would recommend you to use larger dc blocking capacitor (i.e.10 pF).  

    I think the problem you experience could be board or test related.  Could you provide me with the followings?

    • Board Schematic and layout in pdf showing vias clearly around the transmission lines and under the package.
    • PCB layer stack-up
    • RF transmission line design variables (i.e width, ground gap)
    • Test setup diagram and some measurement screens.


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