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DC Coupling HMC194A

Question asked by Aeroplane_man on Mar 11, 2016
Latest reply on Mar 29, 2016 by Aeroplane_man

Hi,

I am trying to use a pair of HMC194A devices dc coupled together. Internally these seem to be pre-biased to the supply rail, in this case +5V, and I understand from other enquiries that it is possible to do this. The problem I have is that in trying to use the device from low frequency to HF which means that the coupling capacitors need to be quite large. This does not tie up well with trying to get good transmission line impedance matching at high frequency. Furthermore, the constraints of the circuit imply that I need several of these capacitors which causes awkward low frequency roll-off problems. Experimentally I am wondering if it is possible to bias the logic controls at ground potential and the GND terminals of the device to a decoupled -5V line. This would allow me to remove most of these capacitors.

 

A further problem I have found when using the devices at low frequency (i.e. less than 5 MHz) is that the 1 dB compression point seems to drop down significantly and results in clipping of the top part of the waveform. Experimentally I found that it is possible to put in a small amount of pull down bleed current to the RF inputs to bias them at, say, 1V below the logic high voltage. This seems to improve the compression problem but does sacrifice some of the low frequency OFF isolation. In order to acheive this and with dc-coupling one would need a suitably biased dual supply system. Am I working the device correctly? This solution is quite messy has anyone a better method?

The CMOS devices such as ADG919 seem to have too much RF loss for my application.

Thanks.RF and Microwave       

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