Re: Rf switch using pin diodes

Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:
> Hello Jamie,
>
>> Thanks guys for the links!
>>
>> I found an alternative to using pin diodes for the RF switch that seemed
>> better in all ways so I made a new board using it.
>>
>> It uses the upg2009tb SPDT RF switch from NEC:
>> "www.cel.com/pdf/datasheets/upg2009tb.pdf"
>>
>> It is easier to drive than pin diodes, and is only $0.99US at
>> mouser.com. The circuit is simpler too.
>>
>
> Yabbut.... where's the sportsmanship? That is almost like buying a can
> to cook a meal. Or a frozen pizza instead of rolling your own dough (we
> always do pizza from scratch and make burgers from raw meat ...).
>
> Also, PIN diodes can be stacked, T-ed, Pi-ed and what not to yield a
> stunning off-isolation. Try that with those chips in a can for the same
> money.

One of the problems with the topology of PIN switch chosen by Jamie is
that it requires bias current in both transmit and received mode - i.e.
one or other of the diodes needs to be biased in order to transmit or
receive. If you're designing battery-operated kit, this is not good.
A better design would be the quarter-wave switch, which only consumes
(PIN diode) bias current in transmit mode.

It's also notoriously tricky to achieve good off-state isolation through
a PIN-diode switch unless you've got a lot of reverse-bias available,
which may or may not be the case. These snazzy CMOS RF switches have
become almost ubiquitous in the last 5-10 years because of the
advantages they offer over PIN solutions.

>
> Ok, rant mode off now. If it works for you, why not? After all, nobody
> expects us to re-invent the wheel ;-)
>
> Regards, Joerg
>
> http://www.analogconsultants.com
.

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