1. Home
  2. Electronics Design
  3. Rf switch using pin diodes
  4. Page 3

Rf switch using pin diodes

I was going to take two of those SPDT switches and make a DPDT switch, for an RX/TX switch with a single antenna but is it ok to just use an SPDT switch at the antenna or at the transceiver? If the SPDT switch is at the transceiver, then when in receive mode the output of the poweramp would be driving the antenna but there is no power into the poweramp so is this ok? In transmit mode the LNA input would be hooked to the poweramp output, so it may be overloaded? The single SPDT switch could also be put at the antenna, but this requires a higher power rated switch for handling the poweramp. Also the LNA would be driving the input of the poweramp! DPDT is the only way I guess, or maybe a SPDT and a SPST could work and avoid switching the poweramp output?

Here is a possible DPDT switch to use:

formatting link

cheers, Jamie

Reply to
Jamie Morken
Loading thread data ...

Hello John,

But the worst is that afterwards many of the will say "See, told ya so".

Regards, Joerg

formatting link

Reply to
Joerg

Hello Jamie,

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.

Ok, rant mode off now. If it works for you, why not? After all, nobody expects us to re-invent the wheel ;-)

Regards, Joerg

formatting link

Reply to
Joerg

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.

Reply to
Rick

my $0.02:

if you are doing this as an educational experience, go with the PIN diodes. the layout issues dont change, but you get to learn how to drive PIN diodes. if you use the fancy chip, you get to learn how to solder it to a pcb....

Cheers Terry

Reply to
Terry Given

Hello Rick,

True, but it's not a whole lot of current.

That is where the engineering mind is being honed via doing projects. You do something and then, oh drat, there is no negative supply. So it needs to be generated which isn't a big deal here as there is no current.

For a T/R switch often you don't need a lot of off isolation. Just enough to guarantee nothing gets fried. For superb off isolation like in filter switching a T configuration works nicely.

They are nice although the off isolation often isn't much to write home about either.

Cost is often a big issue. I mean, you can buy a PIN diode like the BA592 for around 6 cents. Try that with a snazzy RF switch chip.

Regards, Joerg

formatting link

Reply to
Joerg

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.