LPF switching

Mike KU4QO suggests that how the relays select the relevant output low pass filter is done in a very clever way.

Control switching of LPFs

Three relays control four different filters.

  • When KT1, KT2 and KT3 are not energized, the 21-30MHz filter is selected.
  • With KT1 energized it bypasses the 21-30MHz filter and selects the 14-21MHz filter.
  • With KT1 and KT2 energized it bypasses both of those filters and selects the 7-10MHz filter.
  • With KT1, KT2 and KT3 energized it bypasses all filters except for the 3.5-5MHz filter.

The relays are only energized when transmitting (and only when needed).  It is an unusual configuration, saving the expense of a 4th relay.

Mike had a problem where he had no power out on 30m or 40m. Receive was fine. It ended up that one of the legs on relay KT2 was not soldered. A quick soldering of the pin and he was back in business.

Raj VU2ZAP responded, “Farhan has his mysterious ways of being thrifty. The real reason is as I think is to save one data out control line from Raduino.”

“The relays are known to fail, so I cut the TX line to filter ralays and linked it to +12V . The relays remain on during RX and  when you tune across bands you can hear them clicking! This affects power consumption but I can live with that.”

RF Path analysis

Jerry Gaffke KE7ER, provided an analysis of the RF path through the relays and suggested:

“With no relays energized as shown in the uBitx schematic,  RF power from the finals goes through KT1-14 to the 30m lowpass filter then through KT1-3 out to the antenna.   That’s as short as it can get.

“With KT1 energized and KT2 not, RF power goes through KT1-16 and KT2-14 to go through the 20m lowpass filter.  Still pretty short.

“With KT1 and KT2 both energized, we get the 40m lowpass filter through KT3-14.

“With KT1,KT2,KT3 all energized, we get the 80m lowpass filter through KT3-16.

“That’s getting to be a pretty long path for that RF, going through all three relays.  But at 3.5 – 4mhz it simply doesn’t matter.”