RadioBanter - View Single Post

July 13th 03, 09:32 PM

In article , says...

"Active8" wrote in message
...
Hi:

i've found out from my reading that pin diodes are only used as attns
and switches down to about 10MHz.

Where'd you get that idea?

as i said, from my reading (i'll let the author know when i come across
the article again). i know better than to believe everything i read.
that's why i'm getting a second (or better) opinion.

Back in 1975 I was using PIN diode T/R
switching and a bridged T PIN diode voltage variable attenuator in a
military HF transceiver (100W TX 2-30 MHx frequency coverage).

Both the T/R switching and the RX attenuator (in front of the 1st mixer
at RF) worked fine. In fact, the RX would take 2 tones 3 kHz apart
at 300,000 uV each and the 3rd order IM products were down 55dB
or better.

The key to using PIN diodes at HF is to select PIN diodes with a long
(say 2usec) "minority carrier lifetime" (necessary to minimize distortion
on lower freq signals.

that would have been my guess from what i read in the HP app note #922.
that note was dealing with a diode where Fc was 10MHz, below which, it
acted as a regular diode. elsewhere they give Fc=1/2pi*t and mention
that t typically ranges from 3uS to .005uS so 3uS corresponds to
something like 35kHz. O'Tay, that's range i can easily live with. once
i've found some good specs i can do a cost/performance study.

At the time "Unitrode" made the diodes I was
using ... but they may have been acquired by someone else, I can't
recall for sure because I haven't been in the market for such diodes
for some time.

i'm wondering if i can use another
type of diode as an attn or switch at lower freqs. i don't recall ever
hearing that.

i think i've seen common diodes (maybe gen purpose signal diodes) as RF
switches and the passive DBM can be used as an attn, though i'm not sure
about the freq range for the DBM attn.

PIN diodes are the best for RF switching ... other diodes will switch, but
don't have the same distortion characteristics.

BTW .. the common 1N4007 rectifier diode has a PIN structure and
has been used by many with some success in switching HF signals, but
I don't know offhand what maximum RF power level that particular
part will handle. (though I suspect it should be fairly respectable, since
the part is rated as a 1 Amp rectifier, IIRC.) Also note that AFAIK,
the 1N4007 is the ONLY part in the 1N400x series that has a PIN
structure.

i've noticed they're used for something other than rectifiers. i think
i've seen some of the ones like 1N4148 used also, thought i know not of
their structure. i'll keep my eyes open. i think they were used as
varactors, though.

BTW i've got the HP app notes on pin diodes, but there's no info there
pertaining to my question.

HP makes some very nice parts for lower powers and also for higher
frequency use, but I'm not sure that they make a part that's capable
of handling something like 100W of RF at 2-30 MHz.

One thing to keep in mind ... the more switching (forward bias)
current used to make the PIN diode conduct, and the higher the
reverse voltage used to reverse bias it to turn it off HARD, the
lower the distortion created (within the current and reverse voltage
ratings of the diode, of course).

just like the DBMs ring diodes need an LO drive that switches them fully
on and off. they (Mini-Circuits) even say a pulse (which i take also
includes a square wave ) used for the LO drive can lower distortion.

IIRC, in 1975 I was using something like 10mA of forward bias for
the diodes in the RX leg and about 500mA for the diodes in the TX
leg of the T/R switch and the reverse bias was the full supply voltage
of 12 or 28 volts, depending on which primary voltage the particular
model of the radio was intended for ...

I hope this information is of some help.

damn right it is, thanks.

mike

73,
Carl - wk3c