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jdirga
Post subject: High Frequency diode Posted: Wed Apr 15, 2009 1:27 pm

Captain

Joined: Wed Apr 15, 2009 12:58 pm
Posts: 5
I am looking for a diode which still functions as a diode at 100MHz. All diodes that I know have junction capacitance which varies with voltage. This capacitance makes the diode pass current in both direction so the diode does not function as a diode any longer. I tried to cancel out this cap by putting inductor in parallel with the diode but since the cap varies with the voltage while the inductance value is fixed, I can't cancel it. Any suggestion is greatly appreciated.


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biff44
Post subject: Re: High Frequency diodePosted: Wed Apr 15, 2009 5:24 pm

Colonel

Joined: Wed Apr 08, 2009 11:07 am
Posts: 33
Well, just about any diode on this page will work like "a diode" at 100 MHz, i.e. it will rectify an RF signal.

http://www.avagotech.com/pages/en/rf_ic ... ace_mount/

If you want something that has very little capacitance modulation as the RF voltage passes by it, there are "PIN Diodes" that have a very thick middle "I" region, that will not conduct current readily at 100 Mhz, and will have almost no capacitance modulation.

hsms2802 is an industry standard.

_________________
Rich
Maguffin Microwave Consulting
www.MaguffinMicrowave.com

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jdirga
Post subject: Re: High Frequency diodePosted: Wed Apr 15, 2009 9:10 pm

Captain

Joined: Wed Apr 15, 2009 12:58 pm
Posts: 5
The simple schematic that I am simulating can be seen below

Please ignore the name "1N4148", I've modified the diode model so that the Cjo is only 1p
.model 1N4148 D(Is=2.52n Rs=.568 N=1.752 Cjo=1p M=.4 tt=20n Iave=200m Vpk=75 mfg=Motorola type=silicon)
With Cjo=1p, however, the diode D1 still passes current in both direction.
Below is the diode D1 current vs. time



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biff44
Post subject: Re: High Frequency diodePosted: Thu Apr 16, 2009 8:10 am

Colonel

Joined: Wed Apr 08, 2009 11:07 am
Posts: 33
Did a quick look, and most diode voltge doublers have a DC blocking cap at the input, as in:
http://www.play-hookey.com/ac_theory/ps ... liers.html

In any event, if you want a diode to operate like it would in real life, you would need to simulate it like a real-world circuit. What that means is to have some sort of load impedance, even if only a megohm or so. You need the capacitors and resistive load to allow the diodes to attain the proper quiescent DC operating point.

Also, since diodes are pretty much voltage controlled devices (Vd>0.6 volts = conducting, Vc<0.3 volts = not conducting for a junction diode), I would simulate it all with a 4 volt pk-pk voltage source, instead of a current source.

_________________
Rich
Maguffin Microwave Consulting
www.MaguffinMicrowave.com









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