CAV-NB Application Assistance

The cavity preamp functions by using a high Q
cavity as the preamp input circuit. Out of band signals are rejected, but because the cavity is part of the preamp input circuit, losses can be reduced.
If you are using an older, low bias L-C preamp
and are having problems, a cavity input preamp
may solve your problems. The combination of the cavity input, and the high dynamic range PHEMT device, running at a high bias level is a good combination. We can help determine the best course of action to improve your receive capability. We can’t however, work magic! Some locations are just not good places for weak signal receiving.
It is important to remember that each CAV-NB
unit is custom designed for your application.
If you know that the primary out of band problem is a pager at 150 Mhz, we can tailor the response of the pass band to provide high rejection at that frequency.As the spectrum fills with more and more signals, it is not unusual to have a reduction in receive capability due to out of band signals.

Improving your system performace by just a few db can make all the difference !

If your location is near an FM Broadcast Station, Paging System Tower, Weather Radio Transmitter, or Public Service (police / fire / ambulance) Transmitter, you may notice that your weak signal receive performance degrades when the out of band transmitter is transmitting. In many cases, the pass band of older L-C preamps extends well above and below the band they are intended to be utilized for. One out of band signal alone may not be a problem, but when you add up two, or more strong signals, the preamp might be “overloaded”.

When at the compression point, the receiver may seem to go dead, or the weak signals may fade into the noise. If the out of band signals are even stronger, you may hear clunks, noise, or the messages being transmitted. At that point (Ip3) the preamp is now useless. The cavity input preamp is designed to roll off gain rapidly, outside of the band it is designed for. Since the out of band signals are not “using up” amplfier power, the preamp will continue to function.

Some preamps have filters on the output, to
limit out of band signals from reaching the receiver front end. This will help prevent front end overload in the receiver, but does nothing to protect the amplifier stage in the preamp!In some applications a Helical filter or cavity filter can be placed in front of the preamp to
limit out of band signals. The down side is you have now placed loss in front of the first gain stage, adding to the system noise figure. In the most severe cases this may be the only solution.

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