Saturday, March 16, 2013

Standard diodes for RF switching

This idea of pressing ordinary diodes for RF work is not new, many LED's are working as varicap's, some diodes are protecting antenna front ends, other are working as zener's for transistor biasing and the list can go on...can you imagine that some diodes are even working as rectifiers?!!!...

 Now, this idea come because I have some (8) band modules to build for the Speaky and each module uses 4 PIN diodes, so, that's 32 diodes more than the ones I have... or for the mater, the local electronic shop.

(PIN diode is BA479 on the schematic)

Using ordinary diodes for RF switching is nothing new, many circuits use them.... but are they good? Or just reasonable? I had to test for myself!

Built a small circuit: the diode being tested in series with an 82pF capacitor and connected to an C meter. By placing in series I avoid any possible influence on the meter measuring method by DC blocking with the series cap, anyhow I think the measuring method of the multimeter is by impedance, injecting an alternate signal, and not by capacitor charge.

Here's the measured and computed results from the series association:

Light blue/grey is capacitor impedance above 500 Ohm, less than that and gets to close to the usual 50 Ohm influencing circuit performance. That's the importance of having a very low diode capacitance for RF switching. If signal is of high value it's also needed a diode with high forward current to keep things linear.

So, from this simple test results, and if you go bellow 28 Mhz, the 1n4148 is probably a safe alternative for RF switching. Interesting is also the yellow LED result by would not use it in circuit due to voltage drop.
I don't know what was the issue with the Germanium diodes, didn't investigated further since I was more interested in Si type.

 In the 14Mhz band module I built for the Speaky I used 1n4007 and 1n4148 without noticeable issues, let's see if it doesn't mess things with more modules in parallel.

 Just a side note:
BA479 diodes have an capacity of 0.5pF at 100Mhz (from datasheet), 1n4148 have 4pF at 1Mhz (also from datasheet), so, if you go the VHF side I think the only way is to use real PIN diodes for RF switching. I have to find alternative varicap's and ferrite cores...

Have a nice weekend!

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