Diode Help-

[solderfumed]

I'm gathering parts for a field strength meter- it calls for a diode which has a "wide bandwidth, 5 GHz range", with a "lowe than- normal forward voltage compared to a 1N34A or 1N60." . (I'd like up to 8 GHz or so)"
I am going to try a 1N23.
Any suggestions?
thnx

 

[duffy]

That low forward voltage spec is hard to beat, a 1N34A is a germanium. 5ghz is a tough thing to design for, diode capacitance gets to be a bigger factor than anything, and board layout is critical even for a field strength meter. A schottky RF diode like an MMDL301T1G might do it, otherwise I would say use a PIN diode.

 

[solderfumed]

Thanks for the reply-

That low forward voltage spec is hard to beat, a 1N34A is a germanium. 5ghz is a tough thing to design for, diode capacitance gets to be a bigger factor than anything, and board layout is critical even for a field strength meter. A schottky RF diode like an MMDL301T1G might do it, otherwise I would say use a PIN diode.

Thanks- I'm going to try several different ones- starting with a 1N23, and it'll be a good learning experience.I am going to compare it with some store- bought 'bug detectors" for different frequencies with a micro- ammeter for display. The instructions were vague on the diode specs.

 

[duffy]

VERY vague on the diode specs if they are asking for lower forward voltage than a germanium and don't give you any example part numbers. Where did you get the schematic from? There are some schottkys with, like, 220mv Vf, but they are mostly power rectifiers with hundreds of picofarads of capacitance - 5ghz or anything close will waltz through it in both directions.

 

[audioguru]

An untuned "bug detector" might pickup mains hum and every AM, FM and TV station in town. Taxi-cab radios too.

 

[duffy]

He's trying for 5ghz - you forgot cell phones, microwave ovens, bluetooth, computer noise, wifi, cordless phones, wireless temperature transmitters, RC transmitters, remote car starters, garage door openers, RF remote controls...

 

[solderfumed]

Yep, all of the above- which is why broadband receivers are like snake oil; I use an AVCOM PSA-65C spectrum analyzer. But I want to experiment w/ how to find, d/f, locate, if not demodulate, but to find the source- of nearfield emissions and want to build something myself.
The field is full of P.T. Barnums who sell magic buzz- boxes.
I have an Optoelectronics Digital Scout as well.
Want to use an oscilloscope to examine AC for carrier current signals. I talked to the designer of the 'magic buzz box' I bought- he's actually their IT guy, didn't understand that you could send signals along carrier current lines, so I'm selling said magic buzz box.
The guy who designed the schematic did say that he'd substituted a spectrum analyzer as well-
I'd like to make a silent- alerting device if possible so this one uses a meter or optional r.s. mini audio amp
plus, he has a switch to isolate a 100uH choke for carrier current detection.
I would also like to figure out how to vary the sensitivity w/ a three- position switch.

 

[duffy]

I like the wall outlet carrier current detector gizmo. Have you thought about detecting modulated IR from optical transmitters or from the mythical window laser-bounce eavesdropping devices? Also microwaves bouncing off the infamous Soviet "thing"?

 

[Roff]

PIN diodes act as current-controlled resistors at high frequencies - not as rectifiers.
**broken link removed**

 

[solderfumed]

wall outlets- can be lamps, lots of things. laser- they cost ~ $50K so it'd be someone you're not 'sposed to interfere with. Modulated IR; you can see it w/ night vision like a beacon.
hard to find decent, affordable training

 

[duffy]

PIN diodes are the workhorse of the solid-state microwave switch industry.
**broken link removed**

 

[Roff]

PIN diodes are the workhorse of the solid-state microwave switch industry.
**broken link removed**

Yes, they are - but not as rectifiers, which is what he needs for a field strength meter. Here's a quote from your reference (and mine):

[FONT=Tahoma, Trebuchet MS]A PIN diode only acts like a rectifier at low frequencies. At microwave frequencies, the IV curve undergoes a change, so that it behaves like a resistor, whose resistance value is determined by the level of DC current that is present in the I-region. Thus a PIN diode is essentially a DC-controlled high-frequency resistor. Just as important, if no DC current is present, the diode behaves like an open circuit.[/FONT]

 

[duffy]

Thank you. I have misunderstood why these things were used for many years now.

 

[Roff]

Thank you. I have misunderstood why these things were used for many years now.

I only used one once, and it was in a commercial product (video routing switcher). I didn't use it as a current controlled resistor, but as a low-capacitance diode switch for composite video. It worked great, but I still only know about them from what I've read.

 

[duffy]

I apologise for not checking into this more carefully before doubting you. Years ago we started using PIN photodiodes in optical data recievers because they were faster. Having seen them used in microwave equipment, I assumed it was because they were just incredibly fast. Never occurred to me they had a second property that made them useful as a variable attenuator.