High-Power RF Multiplexer - Page 2

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
Re: High-Power RF Multiplexer


Quoted text here. Click to load it

6 dB of what, or where?

Re: High-Power RF Multiplexer



Quoted text here. Click to load it

46%eed your rectifier with about 6 dB less signal than comes out of the =
transmitter20%
(6dB directional coupler).  Or if you are space or parts limited, just a =
2:2:120%
transformer.

Re: High-Power RF Multiplexer


Quoted text here. Click to load it

Thanks!  But only 6 dB down seems like I will be leeching-off too much
power.  That would be about one-fourth of my 25 Watts, or about 6.28
Watts.  Will I need to take that much, to end up with 1.4 Watts?  On
the other hand, if it was out of 50 Ohms that would give about 17.7
volts, which would be handy for getting 15 volts.  Is that why you
said 6 dB?

Tom

Re: High-Power RF Multiplexer




[...]

Quoted text here. Click to load it
transmitter
Quoted text here. Click to load it

Don't rely on stable voltage levels here. A wee change in the antenna
(bird sits down on it, etc.) or whatever is connected can change that
voltage substantially. You really need switchers here if you want to get
by with a minimum of leeched power.

--
Regards, Joerg

http://www.analogconsultants.com /

We've slightly trimmed the long signature. Click to see the full one.
Re: High-Power RF Multiplexer



Tom Gootee wrote:
Quoted text here. Click to load it


   Is the RF on Coax, or in a waveguide?  If it's on coax, why don't you
insert some DC at the transmitter and pick it off at the switch?  A
coupling capacitor at each end and coils with a SRF below the frequency
used by the transmitter to insert and remove the DC.  That way you don't
lose less than 1% of the RF.


--
Offworld checks no longer accepted!

Re: High-Power RF Multiplexer


Quoted text here. Click to load it

Thanks, Phil!  Yes, it's on coax.  Unfortunately, I can't modify the
transmitters, or that might be a great way to go.

V/r,

Tom

Re: High-Power RF Multiplexer


Quoted text here. Click to load it

Did I say Phil?!  So sorry, Michael.  I used to know a Phil Terrell.

Re: High-Power RF Multiplexer



Tom Gootee wrote:
Quoted text here. Click to load it


   I used to know another Michael Terrell.  He was two grades ahead of
me in elementary school, and always starting fights. :)

   Years later another moved to town, and he even had the same middle
initial.  Unfortunately, he was a con man who wrote lots of bad checks.
The other one was in a bar fight two or three times a month.  The two of
them caused me lots of problems while trying to own & operate my
electronics business.

--
Offworld checks no longer accepted!

Re: High-Power RF Multiplexer



Tom Gootee wrote:
Quoted text here. Click to load it


   If it has an output connector it doesn't need to be modified.  You
build the DC inserter to connect to the transmitter, then connect the
coax to the DC inserter. Just make sure to connect it so the power goes
up the coax instead of the transmitter. :)

   BTW, my name is not Phil.

--
Offworld checks no longer accepted!

Re: High-Power RF Multiplexer


On Fri, 18 Dec 2009 23:21:51 -0500, "Michael A. Terrell" =

Quoted text here. Click to load it

I have to agree with Michael here, bias tees are likely to be the way to =
go.

http://www.umcc111.com/BiasT-DcBlock.htm

http://ams.aeroflex.com/ProductFiles/Datasheets/RFMicrowave/AeroflexInmet =
Shortform.pdf

http://www.minicircuits.com/products/bias_tees_main.html

Re: High-Power RF Multiplexer


Quoted text here. Click to load it
rthlink.net> wrote:
Quoted text here. Click to load it
A

I will be able to put the switch very near to the transmitter and run
two separate coax lines out from there.  The problem isn't actually
how to get the DC out to the switch, it's how to get the DC at all.  I
was just hoping to be able to have a self-contained switch unit, with
its own DC supply derived from the RF.

Re: High-Power RF Multiplexer


Quoted text here. Click to load it


IMHO the problem in a "harvesting" scenario is mainly what happens the
instant the RF comes on while the DC stuff still needs to settle. What
does the switch do when it has do come on "hot" (under RF exposure) and
what does the TX think when the load situation isn't defined for a split
second?

Some things can be simulated (just send it over :-) ) but other things
would require a chat with the manufacturers of the TX and of the switch
module.

--
Regards, Joerg

http://www.analogconsultants.com /

We've slightly trimmed the long signature. Click to see the full one.
Re: High-Power RF Multiplexer



Quoted text here. Click to load it

Oh.  That mule is ornery in a different way.

Re: High-Power RF Multiplexer



Quoted text here. Click to load it

<Snip>
Quoted text here. Click to load it

That was my thinking.  Doesn't seem so smart now.

Re: High-Power RF Multiplexer


Quoted text here. Click to load it


Well, you essentially need a fast rectifier diode with very low
capacitance. Maybe two in series of this type (with leakage resistors
across each):

http://www.avagotech.com/docs/AV02-0533EN

John Larkin is quite the expert on fast diodes, maybe he knows a better
one off the top of his head.

You can simulate that in SPICE. The diode needs to see a DC path,
meaning there should be a choke to GND on the RF or some other sort of
DC path. The other side of the diode dumps into a small ceramic cap.
Make it large enough so the ripple is acceptable at max load and min
frequency but not much larger than needed. This allows a fast ramp-up.
Again, make sure your RF source doesn't fry when the switch hasn't
reached a stable state yet.

--
Regards, Joerg

http://www.analogconsultants.com /

We've slightly trimmed the long signature. Click to see the full one.
Re: High-Power RF Multiplexer


Quoted text here. Click to load it

Oh wow!  It simulates brilliantly.

Is part of the idea to use the right number and type of diodes in
series to get a 50 Ohm input impedance at a frequency of interest?

Just playing around, I used some zener models that were already in my
LT-Spice library (maybe altogether unsuitable for this application,
some big 10A MBR20100CT, Cjo3D%10p) and found that putting three in
series got the impedance around 50 Ohms at 1 GHz (but only about 25
Ohms at 2 GHz).   And with a .01uF to gnd downstream, and then a 1 Ohm
series R and a .022uF to gnd, then a 1k to gnd to simulate a 2.33-Watt
load (plus 100uH to gnd, before the diodes), and with input of 50V
peak at 1 GHz, I get 48.3 Vdc out with 63 uV ripple, with a rise time
to 90% of that in 70 ns.  (I had no idea what value of leakage
resistor to use across each diode, but tried 10k, 100k, and 1 Meg and
didn't see much difference.)

If I could find a good way to make the input peak voltage a little
over 16 V, instead of the 50 V from the 25W of RF in 50 Ohms, this way
would quite fast in getting to the needed DC voltage, with the hope of
not frying the pin diodes in the switch.

Thanks, so much, Joerg.  It looks like I'm getting closer to being on
the right track.

Regards,

Tom

Re: High-Power RF Multiplexer


Quoted text here. Click to load it
switch.
Quoted text here. Click to load it

But don't forget that in real life capacitances are higher, inductances
creep in, and the cookie never tastes as good as the TV ad promised (but
brandy beans do!).


Quoted text here. Click to load it

No, no, the goal is to keep it as high as possible and the capacitive
load as low as possible. Anything that you connect in parallel will
disturb your RF path and mainly do two things: Add capacitance and add
real load (because you are tapping off real power). So the total Z the
transmit stage sees drops below 50ohms meaning there could be reflected
power.


Quoted text here. Click to load it

Zeners? They would not be suitable as rectifiers here. You need real
RF-diodes. Low in capacitance yet with enough current rating. If you use
buck converters the current won't be so bad because you can easily get
70-80% conversion efficiencies. RF diodes with enough voltage rating are
sometimes difficult to find so you may need two in series (often can be
had in the same SOT-package) plus a few ten k of resistance across each
to equalize the leakage (resistor values depend on leakage spec).

0.01uF plus 0.022uF seems highish, you don't need very low ripple but I
assume the voltage must come on fast. Place an inductor between the
first and the second cap to reduce modulation sidebands from the buck
converters getting onto your RF path. That could cause the federales to
come out if  it spills outside the spec'd frequency band.

For leakage you have to consult the spec of whatever diode you select
and make sure the resistors handle the worst case value, where one diode
shows worst case leakage and the other none.


Quoted text here. Click to load it

50V is quite comfy, it's in the realm of automotive switch mode
regulators and because of that market they are often quite cheap. Many
of those are spec'd for 80V max input, some even 100V. As an example,
check out whether this one would fit the bill:

http://www.national.com/ds/LM/LM5009.pdf

You'd need two, one set for 5V and one for 15V. They are hysteretic,
meaning the loop compensation efforts will be zilch and the trade-off is
ripple on the output. But you can either filter that our via LC or the
extra ripple may not even matter.


Quoted text here. Click to load it

Glad to help. Whatever you do think the following through:

a. Is it ok that the RF switch will be in some sort of limbo for the
brief period after turn-on before the regulators deliver 5V and 15V?

b. Will the RF source behave itself while the switch is thrown in the
other direction?

c. Any other condition that could cause the RF source to go bzzzt ...
*PHUT* ?

--
Regards, Joerg

http://www.analogconsultants.com /

We've slightly trimmed the long signature. Click to see the full one.
Re: High-Power RF Multiplexer


Quoted text here. Click to load it

Thanks again, Joerg!  Sorry I couldn't reply sooner.  I had to go on
travel for work, unexpectedly, this week.  I just got home, finally,
and need sleep.

OK.  I don't know why I was thinking so stupidly about the impedance,
since it's in parallel.

Yeah, I know I will need real RF diodes.  I was just playing around
and was also still thinking wrongly about the input impedance.

I'll give it another go after I've had some rest.

Thanks again,

Tom

Re: High-Power RF Multiplexer


<snipped>
Quoted text here. Click to load it

Hi Joerg,

How high is it possible to get the impedance of the input of the RF-to-
DC converter, at 1-2 GHz, do you think?  Putting it another way, when
using either one or two of those HSMS-280X diodes, in LT-Spice, about
the best I could do was around 300 Ohms at 1 GHz to 165 Ohms at 2 GHz.

That was with, for example, a large inductor (1-10 uH) to ground
across the input, two diodes in series (with 33k across each), 2 pF to
gnd, 10 uH in series, 1 Ohm in series with 3300 pF to gnd, and 150
Ohms load.  And that just barely gives me 15V across the load, with
about 1.53 Watts being dissipated in the load.  Those impedances seem
awfully low.  Is there some trick to getting them much higher?  Or
maybe I should be asking what might be acceptable, in terms of
impedance.

I also tried simulating with some directional couplers made with
transformers and a few resistors, but couldn't do much better, except
for maybe a little bit higher output voltage.

Of course, in either case, I was always able to trade a higher output
voltage for a lower input impedance.

I guess I should look for some other diodes to try.

Also, I'm not so sure about the switch-mode power supplies.  I'm
afraid they will take too long to come up, and the pin diodes in the
RF switch might fry.  About the best I could do was around 200 us,
with some tens of mV of ripple.  Is there some simple way to power the
RF switches directly (a "linear supply mode" for startup), until the
switchers come up?  It seems like there ought to be a way to have a
diode circuit dump current into the caps at the switchers' outputs,
until the switchers take over, or, probably better yet, just switch
from the linear supply to the switcher when the switcher comes up high-
enough.

I hope I'm missing something.  Otherwise it looks like it might be
time to think about just running some wires.  But that'll still need
some switch-mode supplies.   And then I'll have to worry about people
turning off the transmitter but leaving the RF switches on.

Thanks,

Tom

Re: High-Power RF Multiplexer


Quoted text here. Click to load it

Send me the LTSpice *.asc file, the reply-to address in the header works
by just clicking on it. If you placed the diode data in a lib file send
that along. I won't get around to it until Monday though because I've
got to do ministry work today.


Quoted text here. Click to load it

Transformers are ghastly up there, it'll be microstrip or stripline.
Directional couplers, ok, can be done but then the switcher would have
to be a SEPIC because a 6dB coupler drops the voltage to half. 10dB down
to a third and so on.


Quoted text here. Click to load it

Yep, it's all in the diodes. The ones I showed were just a quick pick. I
am sure there will be some with lower capacitance out there. But let's
try the LTSpice file first. Just remember, simulation and the real
circuit, two different things up at those frequencies.


Quoted text here. Click to load it


I got one down to less than 100usec (had to) but even that may not be
good enough in this case.


Quoted text here. Click to load it

Many ways to do that. One is a pass FET from raw DC to output, with
diode and resistor. A TL431 set slightly above target voltage (but
within abs max limits of the RF switch) caps and regulated, and before
all that fries up into a plume of black smoke the switcher comes on line
and turns the pass FET off.

But: Take one of those RF switches and hang it onto a bench supply with
100ohm resistor in series for +5V and then +15V. Turn it on by plugging
in the banana plug fast and hang a DSO in trigger mode "normal" onto it.
If the electronics in the RF switch have largish bypass caps chances are
the voltages won't come up fast enough anyhow. Because all those bypass
caps need to be charged up. Then you'd be back to square one, having to
supply a permanent power rail.


Quoted text here. Click to load it

Switcher supplies are easy. Then you could even use ye olde and cheap
MC34063 because you can guarantee that it won't see more than 30V.
What's the problem with leaving the RF switches on? Battery drain? Maybe
some protection mechanism could be put in place.

Action items that would make a lot of sense here: Ask the manufacturer
or the RF switch what happens when RF power is applied while it does not
have any supply voltages. Also, ask them how long a ramp-up for those
supplies is considered safe while under RF power. Then, inquire whether
any sort of sequencing is recommended (like 5V first, 15V after xxx usec
delay).

--
Regards, Joerg

http://www.analogconsultants.com /

We've slightly trimmed the long signature. Click to see the full one.

Site Timeline