VHF TX/RX switch w/ pin diodes
 

I am trying to design a solid state TX/RX switch for VHF (2-meter).

DC (when TX)
|
| pin diode
TX-------|---------- Antenna
|
delay line
|
|-------- RX
V (pin diode)
GND

(I am not good at ASCII drwaings !)

My problem is the design of the 1/4 wave line that allows the RX input to be
grounded while showing an open circuit at the antenna. One possible
implementation would be a length of coax but at VHF frequencies that will be
messy. Commercial equipments just use an LC circuit. I am looking for a
pointer to the design of this cicuit.

Thanks,
Bernard

I don't know what commercial designs use, but you can simulate a 50 ohm
quarter wavelength line with a lowpass pi network (shunt C, series L,
shunt C). The reactance of each component should be equal to the Z0 of
the "line" (e.g., 50 ohms). This will behave just like a quarter
wavelength line at the design frequency, and will be a good
approximation over a typical amateur band.

Roy Lewallen, W7EL

Bernard wrote:
I am trying to design a solid state TX/RX switch for VHF (2-meter).

DC (when TX)
|
| pin diode
TX-------|---------- Antenna
|
delay line
|
|-------- RX
V (pin diode)
GND

(I am not good at ASCII drwaings !)

My problem is the design of the 1/4 wave line that allows the RX input to be
grounded while showing an open circuit at the antenna. One possible
implementation would be a length of coax but at VHF frequencies that will be
messy. Commercial equipments just use an LC circuit. I am looking for a
pointer to the design of this cicuit.

Thanks,
Bernard

Thanks this helps. I went into the wrong track because I ignored the shunt C
at the antenna point. Looking at it as a pi network is definitly helping
(not that I know the formula in top of my head, but I will find it !).

Now, the next problem will be to test the efficiency/tuning of the network.
I will need to correctly evaluate the value of the equivalent C's a the
antenna point and at the grounded diode point...

Thanks again,
Bernard


"Roy Lewallen" wrote in message
...
I don't know what commercial designs use, but you can simulate a 50 ohm
quarter wavelength line with a lowpass pi network (shunt C, series L, shunt
C). The reactance of each component should be equal to the Z0 of the "line"
(e.g., 50 ohms). This will behave just like a quarter wavelength line at
the design frequency, and will be a good approximation over a typical
amateur band.

Roy Lewallen, W7EL

Bernard wrote:
I am trying to design a solid state TX/RX switch for VHF (2-meter).

DC (when TX)
|
| pin diode
TX-------|---------- Antenna
|
delay line
|
|-------- RX
V (pin diode)
GND

(I am not good at ASCII drwaings !)

My problem is the design of the 1/4 wave line that allows the RX input to
be grounded while showing an open circuit at the antenna. One possible
implementation would be a length of coax but at VHF frequencies that will
be messy. Commercial equipments just use an LC circuit. I am looking for
a pointer to the design of this cicuit.

Thanks,
Bernard

On Thu, 5 May 2005 09:50:53 -0700, "Bernard"
wrote:

Thanks this helps. I went into the wrong track because I ignored the shunt C
at the antenna point. Looking at it as a pi network is definitly helping
(not that I know the formula in top of my head, but I will find it !).

Now, the next problem will be to test the efficiency/tuning of the network.
I will need to correctly evaluate the value of the equivalent C's a the
antenna point and at the grounded diode point...

Suggest that you visit he

http://www.microsemi.com/brochures/p...hapter%202.pdf

in particular page 10.



Thanks again,
Bernard


"Roy Lewallen" wrote in message
...
I don't know what commercial designs use, but you can simulate a 50 ohm
quarter wavelength line with a lowpass pi network (shunt C, series L, shunt
C). The reactance of each component should be equal to the Z0 of the "line"
(e.g., 50 ohms). This will behave just like a quarter wavelength line at
the design frequency, and will be a good approximation over a typical
amateur band.

Roy Lewallen, W7EL

Bernard wrote:
I am trying to design a solid state TX/RX switch for VHF (2-meter).

DC (when TX)
|
| pin diode
TX-------|---------- Antenna
|
delay line
|
|-------- RX
V (pin diode)
GND

(I am not good at ASCII drwaings !)

My problem is the design of the 1/4 wave line that allows the RX input to
be grounded while showing an open circuit at the antenna. One possible
implementation would be a length of coax but at VHF frequencies that will
be messy. Commercial equipments just use an LC circuit. I am looking for
a pointer to the design of this cicuit.

Thanks,
Bernard

Very good article. My google search had a hard time with it because they
spell out Transmit, Receive, Quarter Wave (instead of TX, RX, 1/4.wave...)

Thanks,
Bernard

"Wes Stewart" wrote in message
...
On Thu, 5 May 2005 09:50:53 -0700, "Bernard"
wrote:

Thanks this helps. I went into the wrong track because I ignored the shunt
C
at the antenna point. Looking at it as a pi network is definitly helping
(not that I know the formula in top of my head, but I will find it !).

Now, the next problem will be to test the efficiency/tuning of the
network.
I will need to correctly evaluate the value of the equivalent C's a the
antenna point and at the grounded diode point...

Suggest that you visit he

http://www.microsemi.com/brochures/p...hapter%202.pdf

in particular page 10.



Thanks again,
Bernard


"Roy Lewallen" wrote in message
...
I don't know what commercial designs use, but you can simulate a 50 ohm
quarter wavelength line with a lowpass pi network (shunt C, series L,
shunt
C). The reactance of each component should be equal to the Z0 of the
"line"
(e.g., 50 ohms). This will behave just like a quarter wavelength line at
the design frequency, and will be a good approximation over a typical
amateur band.

Roy Lewallen, W7EL

Bernard wrote:
I am trying to design a solid state TX/RX switch for VHF (2-meter).

DC (when TX)
|
| pin diode
TX-------|---------- Antenna
|
delay line
|
|-------- RX
V (pin diode)
GND

(I am not good at ASCII drwaings !)

My problem is the design of the 1/4 wave line that allows the RX input
to
be grounded while showing an open circuit at the antenna. One possible
implementation would be a length of coax but at VHF frequencies that
will
be messy. Commercial equipments just use an LC circuit. I am looking
for
a pointer to the design of this cicuit.

Thanks,
Bernard

I'm gathering from what you've posted that you don't have many
design/analysis tools available. Have a look for "RFSim99"--a Google
search should find it for you. It's freeware. It will let you make a
model of many linear RF circuits quite easily and get a decent feel for
what to expect in the way of performance. You can model a PIN diode
quite reasonably as a low-value resistance when "on" and as a
capacitance when "off". The data sheet(s) for your diode(s) should
list reasonable values to use.

Then you can easily see what sort of isolation you'll get versus
frequency, for example, and get an idea about the losses. If you have
any question about losses in inductors, estimate their Q at the
operating frequency and put in a resistor to account for that.
Capacitors, at least decent RF caps, have low enough loss that it can
usually be ignored.

RFSim99, in addition to the simulation stuff, has some "tools"
accessible from pulldown menus that will design simple filters for you,
calculate inductance, capacitance, frequency or reactance--enter two of
those and see the other two--and some other useful RF things.

Hope this helps some!

Cheers,
Tom


Bernard wrote:
Thanks this helps. I went into the wrong track because I ignored the
shunt C
at the antenna point. Looking at it as a pi network is definitly
helping
(not that I know the formula in top of my head, but I will find it
!).

Now, the next problem will be to test the efficiency/tuning of the
network.
I will need to correctly evaluate the value of the equivalent C's a
the
antenna point and at the grounded diode point...

Thanks again,
Bernard


"Roy Lewallen" wrote in message
...
I don't know what commercial designs use, but you can simulate a 50
ohm
quarter wavelength line with a lowpass pi network (shunt C, series
L, shunt
C). The reactance of each component should be equal to the Z0 of the
"line"
(e.g., 50 ohms). This will behave just like a quarter wavelength
line at
the design frequency, and will be a good approximation over a
typical
amateur band.

Roy Lewallen, W7EL

Bernard wrote:
I am trying to design a solid state TX/RX switch for VHF
(2-meter).

DC (when TX)
|
| pin diode
TX-------|---------- Antenna
|
delay line
|
|-------- RX
V (pin diode)
GND

(I am not good at ASCII drwaings !)

My problem is the design of the 1/4 wave line that allows the RX
input to
be grounded while showing an open circuit at the antenna. One
possible
implementation would be a length of coax but at VHF frequencies
that will
be messy. Commercial equipments just use an LC circuit. I am
looking for
a pointer to the design of this cicuit.

Thanks,
Bernard

Very nice package. Thanks for the tip ! I agree that this is the right way
to do what I am trying to do but I was not willing to invest time an money
on this. Luckily the price is right and the package seems to be very easy to
use (from the 5 minutes I played with).

Bernard


"K7ITM" wrote in message
oups.com...
I'm gathering from what you've posted that you don't have many
design/analysis tools available. Have a look for "RFSim99"--a Google
search should find it for you. It's freeware. It will let you make a
model of many linear RF circuits quite easily and get a decent feel for
what to expect in the way of performance. You can model a PIN diode
quite reasonably as a low-value resistance when "on" and as a
capacitance when "off". The data sheet(s) for your diode(s) should
list reasonable values to use.

Then you can easily see what sort of isolation you'll get versus
frequency, for example, and get an idea about the losses. If you have
any question about losses in inductors, estimate their Q at the
operating frequency and put in a resistor to account for that.
Capacitors, at least decent RF caps, have low enough loss that it can
usually be ignored.

RFSim99, in addition to the simulation stuff, has some "tools"
accessible from pulldown menus that will design simple filters for you,
calculate inductance, capacitance, frequency or reactance--enter two of
those and see the other two--and some other useful RF things.

Hope this helps some!

Cheers,
Tom


Bernard wrote:
Thanks this helps. I went into the wrong track because I ignored the
shunt C
at the antenna point. Looking at it as a pi network is definitly
helping
(not that I know the formula in top of my head, but I will find it
!).

Now, the next problem will be to test the efficiency/tuning of the
network.
I will need to correctly evaluate the value of the equivalent C's a
the
antenna point and at the grounded diode point...

Thanks again,
Bernard


"Roy Lewallen" wrote in message
...
I don't know what commercial designs use, but you can simulate a 50
ohm
quarter wavelength line with a lowpass pi network (shunt C, series
L, shunt
C). The reactance of each component should be equal to the Z0 of the
"line"
(e.g., 50 ohms). This will behave just like a quarter wavelength
line at
the design frequency, and will be a good approximation over a
typical
amateur band.

Roy Lewallen, W7EL

Bernard wrote:
I am trying to design a solid state TX/RX switch for VHF
(2-meter).

DC (when TX)
|
| pin diode
TX-------|---------- Antenna
|
delay line
|
|-------- RX
V (pin diode)
GND

(I am not good at ASCII drwaings !)

My problem is the design of the 1/4 wave line that allows the RX
input to
be grounded while showing an open circuit at the antenna. One
possible
implementation would be a length of coax but at VHF frequencies
that will
be messy. Commercial equipments just use an LC circuit. I am
looking for
a pointer to the design of this cicuit.

Thanks,
Bernard

One of the characteristics of the lowpass pi network Roy suggested is
that it provides a DC path between input and output. (Well, it IS a
lowpass, after all!) That characteristic can be a blessing at times,
in that it lets you use the same DC bias current for both the TX path
series PIN diode and the RX path shunt diode. But some other
arrangement might benefit from DC isolation between the two nodes, and
you can get that by changing to a highpass configuration. You have a
choice of a couple of them using three parts: a PI configuration or a
T configuration. In both, the shunt branches are inductors and the
series branches are capacitors. So a PI gives you DC-grounded input
and output nodes, and a T gives you DC-isolated nodes. And you can
also use a T lowpass with series inductors and a shunt capacitor. In
each case, all the Xls and Xcs are equal to the resistive reference
impedance. In other words, for example, at 150MHz and in a 50 ohm
system, C = 21.2pF and L = 53.1nH. Each of the four configurations (pi
and T, lowpass and highpass) behaves at the design frequency
essentially like a 1/4 wave transmission line.

Cheers,
Tom

Roy pointed out to me in email that the highpass versions have a
positive phase shift, which is different behavior than a transmission
line. However, they do have the characteristic that they reflect an
open circuit as a short, and a short as an open, and an Ro load as Ro,
which is the behavior you want for the PIN switch. The phase shift in
that application should not be important. In general, both the pi and
T versions, and for both highpass and lowpass configuration, reflect a
load Z as (Ro^2)/Z, I believe, where Ro is the reactance of each
component.

Cheers,
Tom