‰ NOW 25 WPM ‰ TEXT IS FROM JULY 2012 QST PAGE 38‰ BEEN A COMMON IF AND IS STILL FREQUENTLY ENCOUNTERED AS THE SECOND IF FREQUENCY IN MANY HF RADIOS. MOST HF RADIOS ALSO PROVIDE ONE OR MORE OPTION SLOTS FOR MULTI POLE CRYSTAL FILTERS FOR SSB AND CW. IN THE RECEIVE FLOW, THE SIGNAL IS FILTERED TO SELECT THE USB OR LSB PORTION OF THE IF SPECTRUM BEFORE IT IS DEMODULATED. WHILE THE RADIO IS IN TRANSMIT MODE, THE SSB IS FILTERED INTO USB OR LSB USING THE STEEP SLOPE OF TYPICALLY THE SAME FILTER TO REMOVE THE UNWANTED SIDEBAND. THIS HAS TO BE TAKEN INTO ACCOUNT WHEN THE IF IS SENT FROM THE OPTIONAL FILTER PORT TO THE LIF CONVERTER, WHICH ONLY WORKS FOR RECEIVE SIGNALS. A LOW QUALITY CRYSTAL FILTER IS PLACED INTO THE SIGNAL PATH BETWEEN THE OUTPUT AND INPUT OF THE OPTION FILTER JUST TO KEEP THE AGC WORKING. NOTE THAT TRANSMITTING IS ONLY POSSIBLE IF THE TRANSMIT CHAIN HAS ITS OWN FILTER. IT MIGHT SEEM COUNTERPRODUCTIVE TO DISABLE THE TRANSMIT, BUT IN DOING SO, THE USER GAINS ‰ END OF 25 WPM TEXT ‰ QST DE W1AW ƒ