EMRFD Message Archive 8333

Message Date From Subject
8333 2013-03-12 11:27:54 Romi RF power switching circuits question
In some modern and enough expensive HF transceivers like FTDX5000, IC9100, IC7410 etc. the manufacturers used for antenna switching between the RX input or TX power amplifier output (>100W) a mixed circuit, made with classic electromagnetically coil actuated relays in combination with a diode, high power switching circuit. In RX mode, a selected antenna is connected to the RX input only using switching relay contacts, while on transmitting the output of the final amplifier, after passing the LPF is switched to the antenna through a regular 3A rectifier diode (type U15J or U05G, trr=3us, not PIN diodes), which is saturated in DC using a simple 3 transistor DC driver scheme. In the RX function, this DC 3 transistor circuit applies a reverse blocking DC voltage to this diode (13...25VDC) in order to reduce the RF leakage from the antenna to the inactive TX circuitry, which is also passed to ground by a relay contact. While transmitting, this 3 transistor DC circuit applies through the diode a forward current of about 300mA DC to saturate it, which conducts the 100W or more RF power to the antenna (about 70.7Vef & 1.41 Amps). It seems to me normal that they should use for connecting the output of the power amp. to the antenna also a relay contact, which anyway has a lower resistance than a saturated diode. I am shure that not the lowest switching time between RX and TX was the reason why they used a diode circuit to connect only the transmitter output to the antenna, while the RX is connected through switching relays, because anyway they lower the commutation speed, due to their electromechanically delays about few milliseconds; must be another reason, that I would like to find out. And why used they regular rectifier diodes instead special PID diodes especially developed for RF power switching ? Any useful answer or idea will be appreciated.
Regards, Romi YO8CAN