[G0HZU_JMR]

If I get a chance this week I'll compare a few diodes in a 50 ohm RF switch for loss and distortion. It's also worth checking the datasheet for the capacitance of each diode type. The 1SS83 has about 1pF capacitance for example.

A few years ago I did similar tests and I made up a test jig using a 1MHz LPF + 1.9MHz notch filter and also a 1.9MHz narrowband diplexer to allow fairly serious testing for IP2 and 2HI distortion. The idea was to combine signals in the MW band to generate a distortion term at 1.9MHz in the RF switch. The various filters ensure that the weak link for distortion is the diode and not the test gear. I think I posted up some of my results on QRZ.

I'll warn in advance that I recall that the 1N4148 diode performs really badly in this 50 ohm test. The winner will be the £20 Macom PIN diode but I do also have some nice PIN diodes from Microsemi that are cheaper and also other classic diodes like the BA282, 1SS53, BA479 and 1N4007 to try.

Testing with big signals down in the MW band really is quite harsh but it will show up the differences in these diodes. I don't have the 5082-3081 PIN diode that Ulrich Rohde used but looking at the datasheet I think this diode would have the highest insertion loss. The RF resistance of this diode at 10mA bias is 10 ohms. This would be OK in some radios but if the whole signal path and band switching is done at 50 ohms this would become quite lossy with several of these diodes in the signal path.


I recall some of the older Yaesu HF radios used low voltage Schottky diodes for the band switching and these were prone to failure. A Schottky diode seems an odd choice of diode for this application and I think Yaesu stopped using them in newer radios.

Looking at the datasheet for the 1SS83 silicon switching diode it looks to be a fairly rugged diode. I'm not sure why these would prove unreliable in this application. I'd expect this diode to be more robust than the earlier Schottky diodes.