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.
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.
...now I have to find alternative varicap's and ferrite cores...
Have a nice weekend!
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