[Premium-Rx] Relay vs. PIN switching on filters

[Tisha Hayes]

Charles said;
I agree with that. We have been through that several times with various R&D
designs.
Depending on the frequencies, filter loop imedances and other variables,
solid state switching can come with unwanted results. I suppose there are
some newer diodes that have lower capacitance and other more desirable
characteristics, but for the most part we have avoided them.
A recent project with headend amps and?bandpass filter switching would
never have passed spec without the use of relays. Admittedly, the
requirements were kind of tight, but then, that's a good thing...especially
when working around the front end of a receiver where compromises are not
good..
?
If anyone has any modern data on newer s/s devices or techniques to
accomplish this kind of switching - I'm am all ears, and would love to
learn about it.
?
Bob
?
<snip>
I'll take relays any day over PIN switching.? I think of PIN
switching as something used by manufacturers who are more interested
in cost than quality, and the use of regular signal diodes for RF
switching as something used by manufacturers who just don't care how
their gear works.? Sometimes, other considerations dictate the need
for solid state switching (vibration, magnetic shielding, power
limits, contract spec, etc., etc.), but IMO it is a last resort.? I
have almost always managed to find a way to avoid it.
---------------------------------------------------------------------------------------
As you mentioned about wiping contacts and oxide layers. To me, that was
one of Harris's biggest failures was in choosing that particular relay that
was more intended for switching a 5 amp load than for low signal work. They
lacked any sort of case shielding and the relays were stacked up, one right
next to the other, in banks. After much digging I found a very old BellCore
paper on relay current and the reliability of crossbar switches in
telephone exchanges and the need for a slight amount of wetting current to
keep the contacts from being insulating with an oxide layer.
On a mechanical filter like a Collins you are working with super fine wire
gauges for both sides of the mechanical relay with it's magnetic little
needle to make these metal disks all ring like a bell. Just plopping a 1
kohm resistor off of 12 VDC to provide leakage current to keep the relay
contacts clean seems so inelegant. That has been a technique used by some
folks to get around the oxide layer on relay contacts in exposure to
atmosphere.
Clare used to make a relay, similar to the HGSM 5328 FD1052EK that I used
in PTT circuits and for audo switching on some 4 wire telephone to radio
interface cards (PTT keys on tone, 20 mSec later after transmitter has
stabilized, send data,drop PTT when tone goes away). as a metal shielded
relay (just the top case, not a real RF "sealed" relay they worked great.
Now with mercury being banned in just about everything I cannot even find
the tiny little glass reed relay contacts that had a dot of mercury on the
contact points. As of 2007 it is nearly a death penalty offense to even own
a mercury wetted device in some states.
As Charles and Bob both said, diodes have capacitance, Another reason why
when they are used in surge protective systems for RF  they are wired in
series, to lower that capacitance way down. They definitely do not have the
isolation characteristics of an air gap, or on a SPDT relay, where you open
up the bus and switch both sides of the unselected filter to ground for the
ultimate in isolation. The only thing I like about PIN's are their
switching speed and near infinite cycle rate. Some of my gear is used in
ALE where the receiver is bopping around across five or six bands,
listening for beacon stations to do a BER and establish an ALE link. Relays
get pretty loud and it is not uncommon for the radio to throw a BITE code
out because it saw a filter as infinity open with no signal detected.
Usually when I would see an RF-350K (I know, a horrible radio) that is
marked down to $200 because it is generating a BITE code I go to my Harris
manuals and look it up, most cases it shows as a filter failure when in
reality it is a relay problem.

-- 
Ms. Tisha Hayes/ AA4HA
- Invisible airwaves crackle with life, bright antennae bristle with the
energy.
Emotional feedback on a timeless wavelength.
Bearing a gift beyond price, almost free.
Rush, Permanent Waves

"Whenever governments mean to invade the rights and liberties of the
people, they always attempt to destroy the militia, in order to raise an
army upon their ruins." --Elbridge Gerry