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'[OT] Solid State switch for antenna'
1998\02\25@165114 by ndie Ohtsji [4555]

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Hello,

Sorry to bother this list, but I know this is the only place
I can think of which has some RF gurus listening.  (I, on the
other hand, am very weak with RF design)

I'm trying to share one antenna between two receivers. Is there any
way to use a  solid state switch which will only connect up one
receiver at a time to the antenna?  At first I was just thinking
of using a relay as the switch, but I am concerned about the
reliability of the relay over time and would like to remove any
mechanical components which could fail.

I am going to use some RF modules in either the 433MHz or 900MHz range.

The antenna switching will be controlled by a PIC!  :)

-Randie
spam_OUTrohtsjiTakeThisOuTspamglenayre.com

1998\02\25@175446 by Dan Larson

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On Wed, 25 Feb 1998 13:40:39 -0800, Randie Ohtsji [4555] wrote:

>Hello,
>
>Sorry to bother this list, but I know this is the only place
>I can think of which has some RF gurus listening.  (I, on the
>other hand, am very weak with RF design)
>
>I'm trying to share one antenna between two receivers. Is there any
>way to use a  solid state switch which will only connect up one
>receiver at a time to the antenna?  At first I was just thinking
>of using a relay as the switch, but I am concerned about the
>reliability of the relay over time and would like to remove any
>mechanical components which could fail.
>
>I am going to use some RF modules in either the 433MHz or 900MHz range.
>
>The antenna switching will be controlled by a PIC!  :)
>

On the same topic, I myself, am also interested in a way to switch RF signals
electronically.

My project idea is for a PIC-based IR remote controlled 75ohm RF switch box
for my TV.

Dan

1998\02\25@195342 by Clyde Smith-Stubbs

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On Wed, Feb 25, 1998 at 01:40:39PM -0800, Randie Ohtsji [4555] wrote:

> I'm trying to share one antenna between two receivers. Is there any
> way to use a  solid state switch which will only connect up one

Yup, sure is. No, I'm no RF guru, but basically all you need is a couple of
diodes. The idea is that a reverse biased diode is a high impedance, a
forward biased (saturated) diode is a very low impedance. You need to couple
the RF through the diodes via capacitors, and the DC via resistors.

The RF is very low level compared to the DC, so the diode is linear as
far as the RF is concerned when switched on.

There are diodes manufactured specially for this application, mostly
PIN diodes (there is an intrinsic layer between the P and N layers) -
some data on HP diodes is available at
http://hpcc923.external.hp.com/HP-COMP/rf/1n5719.html


--
Clyde Smith-Stubbs               |            HI-TECH Software
Email: .....clydeKILLspamspam@spam@htsoft.com          |          Phone            Fax
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---------------------------------------------------------------------------
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1998\02\25@200022 by Sean Breheny

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At 04:22 PM 2/25/98 -0600, you wrote:
{Quote hidden}

Hi guys,

There is a very easy and common way to do this quite cheaply. It involves
four 1N914 or 1N4148 diodes (regular silicon switching diodes).
I'll attempt some ASCII art! ;)

                       ANTENNA
      C                   |                  C
RX #1-)|--+----|<--+-->|---+---|<---+--->|--+-)|--RX #2
         |        |       |        |       |
        1K        |      1K        |       1K
         |       RFC      |       RFC      |
        GND       |    C  |    C   |      GND
                  +---|(--+----)|--+
                 PA0      |       PA1
                         GND
RX #1 is receiver(or even low powered transmitter, if necessary) #1
RX #2 is receiver #2
RFC's are inductors with a reactance that is high(at least 10X the
impedance of the RX's) at the frequency you are using.
C's are capacitors with reactance that is at least 10X lower than the
impedance of the RX's at the frequency you are using.
PA0 and PA1 are any normal PIC output pins. When you want RX#1 connected to
the ANT,put PA0 high and PA1 low. When you want RX#2 hooked up, put PA1
high and PA0 low. If you want both hooked up, put both high.
IF you want to isolate both, put both low.
You should increase the values of the resistors if you are dealing with
high impedance loads. Just remember, the resistors must be no higher than
about 10K or so, to allow a small current to flow through the diodes.
As you can probably tell, the circuit works by not allowing any current to
flow between the antenna and an RX in either direction unless there is a
small DC bias flowing thru the diodes from the PIC. The resistors are to
provide a path to ground for the DC. The caps block DC from the RX's and
the RFCs and the caps near the PIC keep RF out of the PIC. Note that the
antenna will have DC on it. IF this is a problem, just stick a cap in line
with it. Also, you can use this for transmitters, too, you just must make
sure that the current which the transmitter produces in the diodes is less
than the DC bias current and that it is not too high for the diode to handle.

Good luck,

Sean


+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
Fight injustice, please look at
http://homepages.enterprise.net/toolan/joanandrews/

Personal page: http://www.people.cornell.edu/pages/shb7
.....shb7KILLspamspam.....cornell.edu
Phone(USA): (607) 253-0315

1998\02\25@200140 by Regulus Berdin

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Try pin diodes. You need two of these plus chokes and some caps
to form a SPDT switch.
These are used in switching RX/TX in almost all tranceivers.
Putting a forward bias on the diode turns it on.

Reggie.

----------
{Quote hidden}

1998\02\25@213234 by Clyde Smith-Stubbs

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On Wed, Feb 25, 1998 at 07:50:52PM -0500, Sean Breheny wrote:

> with it. Also, you can use this for transmitters, too, you just must make
> sure that the current which the transmitter produces in the diodes is less
> than the DC bias current

This is where PIN diodes come in - they can handle RF currents *greater* than
the DC bias current, while remaining low impedance. It's something to do with
the holes and electrons not immediately recombining in the intrinsic layer,
so that in a certain current/frequency region the AC current does not result
in a depletion layer forming.

I've seen specs for diodes handling 10A of RF with only 1A of DC bias, so
it's quite a significant effect.

The original question was about 2 receivers, though (though I'm not 100% sure
why 2 receivers even *need* the RF to be switched, unless the 3dB loss from
splitting the signal is important).

A 1N914 could be used to switch a receiver, but it has a capacitance of
around 4pF, which is an order of magnitude higher than a UHF PIN diode. This
might cause some problems at UHF.

--
Clyde Smith-Stubbs               |            HI-TECH Software
Email: KILLspamclydeKILLspamspamhtsoft.com          |          Phone            Fax
WWW:   http://www.htsoft.com/    | USA: (408) 490 2885  (408) 490 2885
PGP:   finger RemoveMEclydeTakeThisOuTspamhtsoft.com   | AUS: +61 7 3354 2411 +61 7 3354 2422
---------------------------------------------------------------------------
ANSI C for the PIC! Now shipping! See http://www.htsoft.com for more info.

1998\02\26@100349 by Wynn Rostek

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At 07:50 PM 2/25/98 -0500, you wrote:

>There is a very easy and common way to do this quite cheaply. It involves
>four 1N914 or 1N4148 diodes (regular silicon switching diodes).

>Sean


Not at 900 MHz.  I've been able to hit 400 MHz with some point contact
diodes, but they are on the way down by then.  I've had real trouble
getting above 300 MHz with 1N914 or 1N4148 (or any others such as the
1N4001-1N4007 which people say act like PIN diodes) diodes.  Maybe I'm
doing something wrong, but the on/off ratios are always poor at the higher
frequencies.

How about using one of the diode RF switches that are designed for this
use?  Mini-Circuits has tons of them.  A lot of other people make them as
well.  Has anyone with the test equipment to find out tried any of the more
common diodes at 900 MHz?  I know a lot more of us are going to be working
there soon. :-)

Wynn Rostek

WB4ZUY

1998\02\26@211435 by Sean Breheny

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At 09:51 AM 2/26/98 -0500, you wrote:
{Quote hidden}

Wynn and others,

Yes, I must admit, you are right. I was forgetting about UHF TV freqs. I
think the biggest problem would probably be the fact that even when reverse
biased, these diodes will present about 4pF each of cap so they won't
provide much isolation. As you suggest, there may be some other effect due
to the diode's response at high frequencies. I suppose that the PINs might
be better, but I don't know much about them. Would they be pretty much a
drop-in replacement in the circuit diagram I drew? Perhaps the switches
made by mini-circuits would be best.

Thanks,

Sean
+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
Fight injustice, please look at
http://homepages.enterprise.net/toolan/joanandrews/

Personal page: http://www.people.cornell.edu/pages/shb7
spamBeGoneshb7spamBeGonespamcornell.edu
Phone(USA): (607) 253-0315

1998\02\27@044616 by paulb

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Clyde Smith-Stubbs wrote:

> The original question was about 2 receivers, though (though I'm not
> 100% sure why 2 receivers even *need* the RF to be switched, unless
> the 3dB loss from splitting the signal is important).

 Good ol' Clive!  Not only does he know his PIN diodes (slightly better
than I do, so it seems), but he alone picked the trick question.  You
*don't* switch receivers, you leave them all running and connected!

 If the two receivers differ significantly in frequency, you connect
each with a series trap tuned to its frequency.  If they are almost the
same frequency, either use a splitter and "wear" the insertion loss,
(probable little more than the diode switches proposed) or precede it
with a pre-amplifier.

 Pre-amplifiers are readily available for these frequencies as TV
boosters and dead simple but extremely effective ones can be kit-built
using single devices such as the "MAR" (MAR-6 comes to mind) series
chips (these are small four-lead chips, risky to handle over a dark
carpet!).

 The only thing you have to add to the standard design using these, is
a tuned circuit or "trap" parallel resonant to your operating frequency.

 Cheers,
       Paul B.

1998\02\27@104910 by Scott Horton

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Attachment converted: wonderland:Re- (TEXT/CSOm) (000137DC)

1998\02\27@122042 by Dan Larson

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On Fri, 27 Feb 1998 08:47:52 -0600, Scott Horton wrote:

>
>RFM has a transistor in their 916.5Mhz reference design - BFS17 that they use
>for antenna switching.  HTH
>
>Scott
>

Whom / What is RFM?

Do you have a URL to an application note or shcematic for the antenna switch?

Thanks,


Dan

1998\02\27@122054 by ndie Ohtsji [4555]

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Hi Scott,

I have the RFM Product Data Book (Low-Power Components, Virtual
Wire Radio Systems, Frequency Control Modules Filters) and have
taken a brief look at their web-site (http://www.rfm.com), but I can't
seem to be able to find this transistor (BFS17) or the antenna
switching circuit you mention.  Do I have the correct data book
or do they have another one?  Is this info on their web-site?

Thanks in advance!

-Randie

TakeThisOuTrohtsjiEraseMEspamspam_OUTglenayre.com


{Quote hidden}

1998\02\27@124634 by ndie Ohtsji [4555]

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> From RemoveMEpaulbspam_OUTspamKILLspamMIDCOAST.COM.AU Fri Feb 27 01:47:21 1998
>
> Clyde Smith-Stubbs wrote:
>
> > The original question was about 2 receivers, though (though I'm not
> > 100% sure why 2 receivers even *need* the RF to be switched, unless
> > the 3dB loss from splitting the signal is important).
>
>   Good ol' Clive!  Not only does he know his PIN diodes (slightly better
> than I do, so it seems), but he alone picked the trick question.  You
> *don't* switch receivers, you leave them all running and connected!
>

Hi Paul,

Actually, I wanted to have the capability to switch:

1) One antenna between two receivers
2) Two antennas between one receiver
3) One antenna between one transmitter and one receiver

I'll take a look into using PIN diodes.  I took a look at the
HP URL (http://hpcc923.external.hp.com/HP-COMP/rf/1n5719.html)
which Clyde had suggested, but these appear to be only data sheets.

Does anyone know of a site which may have more info with application
notes.  Does anyone other than HP make these things?

Thanks to all the people who have responded.....that's why I like
this PICLIST so much......just a wealth of information PLUS we also
get to learn about the different cultures and animals (re: the number
4 translates to death, goats and lawnmowers, and the latest OT on
our bovine friends).  Oh, good PIC-tricks too!

Best Regards,

-Randie
RemoveMErohtsjiTakeThisOuTspamspamglenayre.com

1998\02\27@195417 by Mike Keitz

picon face
On Thu, 26 Feb 1998 20:39:43 -0500 Sean Breheny <EraseMEshb7spamspamspamBeGoneCORNELL.EDU>
writes:

>>>There is a very easy and common way to do this quite cheaply. It
>involves
>>>four 1N914 or 1N4148 diodes (regular silicon switching diodes).

For good RF switching, you need one or both of 2 conditions: a lot more
DC than RF current through the diode (only practical for receivers and
small signals), and/or slow diodes, diodes that won't attempt to rectify
the RF signal.  All diodes turn on quickly, but some, especially PIN,
take a long time to turn off (thus the common suggestion to use 60Hz
rectifier diodes).  Slow diodes can pass a much larger RF current than
the DC current that is holding them on. They won't be able to switch off
when the instantaneous current through the diode reverses, so they look
pretty much like small resistors to RF.

I'm familiar with 2 widespread applications of diode RF switches in
consumer equipment.  The first is a two-way radio.  A transmitter and
receiver alternately use the same antenna.  Low loss in the forward paths
is desirable.  The isolation only has to be good enough to keep a
damaging level of transmitter power from reaching the receiver.   The
generally standard design uses two diodes, both of which are not biased
when receiving and both are biased on when transmitting.  The first one
connects the transmitter to the antenna.  This diode needs to be rather
large since all the RF current from the transmitter passes through it.
The other diode shorts out the receiver input.  To prevent it from
shorting out the antenna as well, a 1/4 wave transmission line or
equivalent L-C circuit are inbetween the antenna node and the receiver
with it's shunting diode.  This circuit transforms the shorted diode to
an open at the antenna side, so the transmitter is not affected.  It has
negligble effect on the antenna signal reaching the receiver in the
receive mode.  The 1/4 wave line of course works only over a limited
frequency range.  This is not a problem since the radio is also built for
a limited frequency range.  The two diodes are usually connected in
series for DC, so the same current passes through them to switch them on.
In some designs, the diodes aren't biased with DC, they just rectify
some transmitter energy.  Of course this doesn't work as well.  In either
case, in receive mode, no drive current to the diodes is required.  This
is an advantage.

>think the biggest problem would probably be the fact that even when
>reverse
>biased, these diodes will present about 4pF each of cap so they won't
>provide much isolation.

The other application is to select from multiple inputs/outputs for a TV,
VCR, cable box, etc.  Here the signals are small, the bandwidth and
isolation requirements are large, and the insertion loss is not quite as
important.  To get high isolation, several (usually 3) switches are
arranged in series from each input to the switch output (tuner input).
Each switch consists of a diode in series with the signal path that is
biased on when the leg is to be on, and a diode that shunts the signal
path to ground when the leg is supposed to be off.  The shunt diodes are
of course omitted from the input and output nodes of the overall switch
and just used on the center nodes, which would be floating if the series
diodes are ideally off.  The diodes in these are not I believe anything
special, just small-signal diodes.  They must be rock-cheap in any case.
The diode does not need to be able to rectify 900 MHz signals, in fact,
it would be bad if it did.

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1998\02\28@074318 by paulb

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Randie Ohtsji [4555] wrote:

> Hi Paul,
>
> Actually, I wanted to have the capability to switch:
>
> 1) One antenna between two receivers
> 2) Two antennas between one receiver
> 3) One antenna between one transmitter and one receiver

> I took a look at the HP URL
> (http://hpcc923.external.hp.com/HP-COMP/rf/1n5719.html)
> which Clyde had suggested, but these appear to be only data sheets.

 Of course.  You're supposed to *know* the circuits!

 OK, I'll broadcast in part the reply I made to Dan Larson
<RemoveMEdlarsonKILLspamspamcitilink.com>.  Does this give you enough ideas?

> Hello Dan.

 <rending/ tearing noise>  (Can't find the snippers!  Never can when
you want them)

{Quote hidden}

 Cheers,
       Paul B.

1998\02\28@115006 by peter

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I have a JIL SX400  scanning receiver that goes from 26Mhz to 520Mhz
(guess what that cost 18 years ago)
Sorry no PIC's in it, the micro is NEC UPD 7508ac-035

There are 12 seperate RF + mixer sections each covering part of the band
(26-34, 34-50,...and 460-520Mhz)

Each of the 12 have diode switches to the antenna
they use a single diode for the top 4 sections and the signal passes
through
3 diodes in series for the other eight, all the diodes are 1S2222

It's a simple circuit, the incoming signal is connected to ground via a
1k in series with an RFC (1uH) and to the diodes,
the other side of the diodes are connected via a 1k to the transistor
switch connected to 8V

The local osc and the IF out are also diode switched also using 1S2222's


Peter Cousens
email: peterSTOPspamspamspam_OUTcousens.her.forthnet.gr  phone: + 3081 324450, 380534
snailmail:  Folia, Agia Fotini, Karteros, Heraklion  Crete, Greece.

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