Saturday, July 11, 2009


Regarding the 147.360 squelch... It has been re-calibrated. But here is a little nugget for those curious:

Most FM squelch circuits incorporate a feature called hysteresis, which is a deliberate change in sensitivity between squelch-closed state and squelch-open state. If you design the circuit so that it is more sensitive after opening than before it functions much better in the real world. Without hysteresis, the squelch may drop out while someone is talking. If there is too much hysteresis, squelch threshold becomes hard to adjust properly.

Motorola coined this dynamic relation as "smart squelch", which I think sticks with people. The gist of it is as you get noisy due to mobile flutter or what have you, the squelch loosens, to help cut down on picket fencing.

A good example of a radio with no hysteresis is the Icom 706. If you have ever use this for FM mobile you know how annoying a lack of hysteresis is.

Hysteresis actually helps the weak signal performance, because it decreases threshold once the squelch pops open. Imagine how choppy every mobile users would sound if a repeater had no hysteresis. You want the carrier to reach a point that the noise level is driven below the critical point and hold the repeater COS open.

Steve, KB9MWR

No comments:

Post a Comment