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Author Topic: How to add an RF protection device to amplifier?  (Read 14920 times)
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K1JJ
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« on: February 24, 2011, 11:38:46 AM »

I wonder if one of the engineers here can suggest a solution.

I have a solid state 200W FT-1000D RF PA unit (final amplifier) that comes with a "protection board."   Normally the protection board samples forward and reverse swr and feeds "ALC" back to the I.F. to fold back the power during faults.  The PA will be driven with 100mW maximum.

In my case I will not have an I.F. to work with, but want to add a device at the low level RF drive INPUT to the PA Unit that will produce a gradual higher resistance pad when the swr rises.  From zero to 1000 ohms might work?

I don't want to use an on/off relay there, but some kind of linear device. A voltage controlled resistor, VCA, or other type device that can handle 100mW of RF would be suitable.

I included a schematic of the protection board below that samples RF voltage from two areas. The resultant ALC output at the left socket is the signal produced for the power fold back signal. (middle left socket, pin 4)

Any ideas?


T


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« Reply #1 on: February 24, 2011, 12:00:03 PM »

Check out Mini Circuits. I'm using something similar to provide AGC in a UHF transmitter with a retrofit exciter.

Here's one

ZX73-2500-S+

You might have to do it at I.F. somewhere. Anyway that points in the right direction and it's relatively cheap. A surplus instrumentation amplifier would work too.
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K1JJ
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« Reply #2 on: February 24, 2011, 01:19:09 PM »

http://www.minicircuits.com/pdfs/ZX73-2500.pdf

That looks pretty good, Dave. A 50 ohm variable voltage attenuator for $49.

I see this particular one is good  starting at 10MHZ, and the IP3 is at -43db. Maybe I can do better with some looking.

A good start anyway.    Why do you think I will need to go to I.F?   That would be back into the Penny HPSDR exciter and I don't want to go there.  Why can't the protection board drive the VVA directly or with a buffer at the PA input?  The exciter is already protected with a 50 ohm pad.

T
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« Reply #3 on: February 24, 2011, 02:27:15 PM »

http://www.minicircuits.com/pdfs/ZX73-2500.pdf

That looks pretty good, Dave. A 50 ohm variable voltage attenuator for $49.

I see this particular one is good  starting at 10MHZ, and the IP3 is at -43db. Maybe I can do better with some looking.

A good start anyway.    Why do you think I will need to go to I.F?   That would be back into the Penny HPSDR exciter and I don't want to go there.  Why can't the protection board drive the VVA directly or with a buffer at the PA input?  The exciter is already protected with a 50 ohm pad.

T

The freq coverage of this particular unit would dictate using it @ I.F. I'm sure there's something out there which will work on H.F.

Those IP3 figures are worst case and done with a lot more bandwidth than any barn door AM person would ever use.
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« Reply #4 on: February 24, 2011, 05:50:54 PM »


 I would start any RF PA protection scheme by making a adjustable current limited DC supply feeding the 'brick'. I once made a CC amp, one of those 50v amps that would do 200W AM with 5 w drive. It worked great until I dropped my microphone going to my 100W exciter. The burst of RF allowed me to watch two expensive zitters turn bright orange as my 50V brute supply provided junction fuzing currents... Embarrassed

I don't think an ALC circuit would have prevented this kind of failure since the reduction in drive would occur too late.

Jim
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K1JJ
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« Reply #5 on: February 24, 2011, 05:59:55 PM »

The current limiting PS is a good idea, Jim, tnx.

Yes, an swr circuit would not be aware that a drive surge has occured. I know that the FT-1000D can be turned up to 250W++ and some guys have blown them doing so. Evidently there is no protection there when done so.

This power adjustment is on the I.F. board and probably limits drive power. However, I have no IF board in this system, so your PS shutdown idea is good.

The swr ALC wud still be needed cuz at full power, a shorted antenna may cause problems that the PS wud not be aware of.

I still haven't located a suitable variable voltage attenuator.

 I wonder if there is a block that can be put in series with the 30V PA line that can be set to shut down at a given fault current?

T
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« Reply #6 on: February 24, 2011, 06:09:36 PM »

Tom,

Could you not use the output from the RVS power sensor to throttle back a voltage regulator?

Seems like the job for LM723MAN!!!  (or insert Vreg superhero of choice).

You might also be able to incorporate the current regulator.

I once modulated a Lambda supply this way, can't see why you wouldn't be able to do the same..... Although as with ALC, timing constants may creep up.


--Shane
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Steve - K4HX
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« Reply #7 on: February 24, 2011, 09:40:00 PM »

I was thinking along the same lines. Since you are running the amp as such a low level (far from it's peak output capability) you don't really need anything like ALC. Just limit the PS current above some level and you should be safe.



 I would start any RF PA protection scheme by making a adjustable current limited DC supply feeding the 'brick'. I once made a CC amp, one of those 50v amps that would do 200W AM with 5 w drive. It worked great until I dropped my microphone going to my 100W exciter. The burst of RF allowed me to watch two expensive zitters turn bright orange as my 50V brute supply provided junction fuzing currents... Embarrassed

I don't think an ALC circuit would have prevented this kind of failure since the reduction in drive would occur too late.

Jim
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K1JJ
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« Reply #8 on: February 24, 2011, 11:53:04 PM »

Well, I plan to run the unit barefoot at full 200W power sometimes, so will need the full protection package.

If I cud find a simple series current block that can be preset, this wud be FB.  I know the QIX Hall Effect board wud probably work with some mods, but that may be too complex for the job.    And a voltage controlled attn from the ALC board to handle swr issues.

I'll keep looking.

   
T

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« Reply #9 on: February 25, 2011, 05:36:30 AM »

If you can live at the 10 mW level, here's a cheapo solution, similar to the VVA that Dave posted, but it a SOT package.  Handles 13 dBm, up to 16 dB attenuation.  Twenny for $13 bux, how can ya go wrong.

http://cgi.ebay.com/20-MA-AT-259-2GHz-Voltage-Variable-Attenuators-/190423033064?pt=LH_DefaultDomain_0&hash=item2c5618e4e8

Data sheet is here http://www.macomtech.com/DataSheets/AT-259-PIN.pdf
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« Reply #10 on: February 25, 2011, 10:48:07 AM »

Well, I plan to run the unit barefoot at full 200W power sometimes, so will need the full protection package.

If I cud find a simple series current block that can be preset, this wud be FB.  I know the QIX Hall Effect board wud probably work with some mods, but that may be too complex for the job.    And a voltage controlled attn from the ALC board to handle swr issues.

I'll keep looking.

I found this, Tom...

http://radiofrequencycircuit.blogspot.com/2009/01/swr-protection-circuit.html

And, still looking, for I have a similar interest with a mobile amplifier.

Gotta protect my PILLS ;-)
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K1JJ
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« Reply #11 on: February 25, 2011, 11:14:14 AM »

Geoff,

That's a pretty cool circuit with the relay and LEDS. 3:1 swr puts a load on the driver and disconnects the PA input. I wonder if it is fast enuff compared to a SS attn?   

Johnny, I'll need X10 attn ratings for 100mw. Maybe there are some out there.

Well, at least the PA has a thermal sensor and switch on it. That's a good start.

If youse guys spot a simple DC sensor that will preset limit current, let me know. Same for a suitable attn. Heck, you'd think there wud be something like this out there as a ham or CB kit for guys wanting to run those cheap pill amps that have no protection other than a fuse.

BTW, Geoff, are these the pills you are looking to protect?  Grin

T


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« Reply #12 on: February 25, 2011, 11:57:36 AM »

Geoff,

That's a pretty cool circuit with the relay and LEDS. 3:1 swr puts a load on the driver and disconnects the PA input. I wonder if it is fast enuff compared to a SS attn?   

 

I don't know.  I would hope so.  Solid state devices protecting solid state devices.  Depends on which is quicker to respond.  In the timing of things, RF drive forward biases the final, it produces power, the power out is compared to the power back in (meaning it's already produced RF and any high SWR [or not] has been detected to kick in the protection circuit or not).  I dunno, Tom.  You can't detect high SWR until you produce RF into a possibly high SWR environment.

Quote

BTW, Geoff, are these the pills you are looking to protect?  Grin


Uhm, yeah... -NOT-.   Wink  That's the home-brewed rig - push-pull 250TH's modulated by a pair. I don't have to worry about them going no where, or being bothered by SWR!

The MOBILE rig is a plastic yaecomwood into a (4) 2SC2879 ~600w amplifier, into a Texas Bug-Catcher.  Brother, lemme tell ya, going from a (2) transistor (~300w) to =the (4) transistor (~600w) has made the difference in working AM while mobile.  Running ~150w of carrier vs ~75w (when mobile), of carrier is like going from a single 5763 modulated by a pair of 12AX7's, to a single 813 modulated by a pair of 811s!  It's been -that dramatic- of a change!

First, I make sure the antenna is tapped 'right on' on the frequency I want to operate (bandwidth on 75m is ~7.5kHz at the 3:1 points requiring different taps for frequencies as close as 3.870, 3.875, 3.880 and 3.885.  4 freqs, 4 taps required), then place the plastic radio (Kenwood TS-450S) in AM, crank up the Power output level to MAX, Carrier level to -min-, then place the xcvr in xmit and -slowly- increase the carrier level until there's enough RF to trigger the RF detection/relay circuit. then, at some >20w level, check SWR again (with closed doors) then raise the carrier level to the point where the amp makes no more power out, make that reading, then back it off to 1/4 of -that- output. 
I love that feature of the 450, that it has a separate Power out and Carrier level pot.

Typically ~125/~150w of carrier from the amp requires only about 2w of drive. Having the Power-out pot a full scrote, and very little carrier to drive the amp requires 'not-a-lot' of audio from the MC-60 Low-Z microphone. I usually leave the mic-gain control (from 7 o'clock minimum / 5 o'clock maximum) around 8:30.
The Hi-Q of the Bugcatcher seems to radiate effectively.  I get glowing reports all the time. 

I want to protect those 4 transistors from silly things, like driving under an over-pass, crossing through an intersection, or accidentally transmitting on a frequency the antenna isn't resonant at...

As complicated as all that sounds, that's the -easy- part.  The HARD part about HF mobile on the lower bands, is being able to flippin' HEAR the returning signals, due to the (sometimes) 60 year old power lines and crappy insulators that are CONSTANTLY breaking down.  Noise, noise, noise!

One Qso, on SSB, I was accused of running and "alligator" station.  Station (on 75m) in Washington State (near Portland) said "well, yer mo-bile, and I'm on a fixed station, running a hunnert-nfitty watts!  Why can't you hear me?"  I said "...because I'm running Four Times the power you are into a more EFFICIENT antenna?"

Here's yer sign!  Grin

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w5omr
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« Reply #13 on: February 25, 2011, 12:07:58 PM »

btw, Tom, here's a link to another explanation of a protection circuit design in another amplifier.

http://www.rf-amplifiers.com/index.php?topic=load_protection
 
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K1JJ
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« Reply #14 on: February 26, 2011, 11:18:21 AM »

Geoff,
Tnx for the extra info.

Yes, mobile reception has always been the issue on 75M. The roadways are filled with man-made QRN.  The second issue is the poor groundplane presented by a vehicle on 75M, thus big ant losses no matter how big and efficient the whip is.

Looks like you have a good procedure for protecting and tuning up the amp. Hopefully I'll get something going that handles both swr and over current situations. I know what needs to be done - it's just doing it in the simplest way using ready-made parts or modules that is the challenge now..

Have fun with the mobile, OM!

T

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« Reply #15 on: February 26, 2011, 11:32:46 AM »

Just get the rest of the FT-1000D except the I.F. board of course  Grin
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« Reply #16 on: February 26, 2011, 01:57:18 PM »

Looks like you have a good procedure for protecting and tuning up the amp. Hopefully I'll get something going that handles both swr and over current situations. I know what needs to be done - it's just doing it in the simplest way using ready-made parts or modules that is the challenge now..

Have fun with the mobile, OM!

I get compliments all the time.  Because I'm down in the area, often times I do act as a relay for the net controls on the traffic nets.  I don't know how it is I can hear some stations better than someone with an antenna a half-wave above ground, but it happens.

Solar flux is down to 88.  higher bands are not as good as they were today as they were on the 18th of Feb when the SF # was upwards of 125.

I'm headed off to the laundromat... is there anything happening on 14.286? Grin

Wanna see how many slop-buckets we can ... uhm.. well.. You know... ;-)
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« Reply #17 on: March 01, 2011, 05:43:34 PM »

Geoff,

One of the ways I protected transistors back in the cb days was to install zeners on the collectors.  24-30 volt zeners worked fine across 12 volt devices.  The theory is the zener will crowbar if the reflected voltage causes it to conduct. 

It's been argued up one side and down the other that this either will or won't work.  I've always held that if you size the zener correctly, then you will blow the fuse / crowbar / whatever I protection you've incorporated....  HOWEVER, it does NOT blow the fuse on the first half-cycle..  This lead CBers to say it didn't work, running 4 2879s at 1000+++ watts PEP.  Yeah, run at the ragged edge, the zeners didn't work, but when I ran them like your talking about (I also have a 4X2879 amp, and a 16 x 2879..  The 4 has 28 volt zeners from collector to ground, and I've never blown a transistor since) it seemed to work fine.

Of course, YMMV, but this was the EASIEST way I found to protect "pills" from the consumers Smiley


If you've made the jump to a 4 transistor amp, forget the Henry, etc. 8 device amps.  Legal limit is the best step Smiley

--Shane
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w5omr
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« Reply #18 on: March 02, 2011, 12:20:29 PM »

Geoff,

One of the ways I protected transistors back in the cb days was to install zeners on the collectors.  24-30 volt zeners worked fine across 12 volt devices.  The theory is the zener will crowbar if the reflected voltage causes it to conduct. 

It's been argued up one side and down the other that this either will or won't work.  I've always held that if you size the zener correctly, then you will blow the fuse / crowbar / whatever I protection you've incorporated....  HOWEVER, it does NOT blow the fuse on the first half-cycle..  This lead CBers to say it didn't work, running 4 2879s at 1000+++ watts PEP.  Yeah, run at the ragged edge, the zeners didn't work, but when I ran them like your talking about (I also have a 4X2879 amp, and a 16 x 2879..  The 4 has 28 volt zeners from collector to ground, and I've never blown a transistor since) it seemed to work fine.

Of course, YMMV, but this was the EASIEST way I found to protect "pills" from the consumers Smiley


If you've made the jump to a 4 transistor amp, forget the Henry, etc. 8 device amps.  Legal limit is the best step Smiley

--Shane
KD6VXI

the amp seems to be ruggedly built.  I've got 6ga wire feeding the amp directly from the battery and the amp has survived multiple zorch-outs by high circulating current in the coil.

What I -need- to do, is re-locate the antenna on the truck.  Currently, it's sitting right behind the cab, on the drivers side.  The feed point is about 4' off the ground.  The total length of the antenna is ~ 9' (whip and coil) and I -really- need more whip. 

I need to move the feed-point down a couple of feet and use an 8' whip, or build up a capacity hat for the current installation.  The added apparent length of the radiating whip would negate 10m operation, as currently the highest frequency I can work is the low-end of 28MHz.

I think the capacity hat might be the better solution, as it would leave the antenna in it's current position (and away from any load I might be carrying in the truck for work).

I've heard of the transistor protection circuit, but with regular diodes, and not Zeners.
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K1JJ
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« Reply #19 on: March 02, 2011, 12:45:44 PM »

Thanks for the info, Shawn, Geoff -

On the class E rigs we use "transorbs" to protect both the gates and drains of the final MOSFETS.  Mousier or DigiKey should have them.


They are 2-way zeners, good at RF freqs and ramp/clamp at a certain peak voltage. A Transient Voltage Suppressor (Transorb) is a zener diode that is engineered for high power operation. Some transorbs are "bidirectional". In other words, they consist of two zeners in one package.

We use 18V devices on the gates and about 540V? devices on the drains, IIRC.  When they fry, they usually short, which is a good thang.  

A little too much drive and they get warm, but still good.  They are excellent protection.

T
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Jim KF2SY
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« Reply #20 on: March 03, 2011, 12:04:45 PM »

The current limiting PS is a good idea, Jim, tnx.

Yes, an swr circuit would not be aware that a drive surge has occured. I know that the FT-1000D can be turned up to 250W++ and some guys have blown them doing so. Evidently there is no protection there when done so.

This power adjustment is on the I.F. board and probably limits drive power. However, I have no IF board in this system, so your PS shutdown idea is good.

The swr ALC wud still be needed cuz at full power, a shorted antenna may cause problems that the PS wud not be aware of.

I still haven't located a suitable variable voltage attenuator.

 I wonder if there is a block that can be put in series with the 30V PA line that can be set to shut down at a given fault current?

T

Tom,
Echoing what some of the others mentioned, you could use the B+ line with a series resistor that would provide a voltage drop.  This voltage drop could be either scaled by an external opamp and/or used with a reference voltage to a comparator IC with its output providing a signal for perhaps
inhibiting the xmit, interfacing with the ALC or decreasing bias to the driver section.  Lotsa ways I suppose, depending on what/how you want to do it....

http://analogtalk.com/?p=663
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Mark


« Reply #21 on: March 13, 2011, 08:59:51 PM »

Tom,

The schematic you show (Protection Board) is only a small part of the overall load monitoring circuitry from the FT transceiver. Specifically, this board develops a voltage proportional to RF line current (magnitude only) and then sums that voltage with other voltages VR and VF from another location.  So this board may not develop the control signal you desire as it does not distinguish forward from reflected current, or moreover, power.   

Fortunately, the desired control signal should be relatively easy to develop with very little additional circuitry added to this board.  There's lot of information available for developing a Bruene type bridge for this purpose. Some info is available on my web page as well. See: http://home.comcast.net/~msed01/RFpickup.html Figure 3.

Assuming that hurdle is surpassed, I would consider using PIN diodes for simply shunting the 100mW input or better yet, an appropriately biased FET should serve as series RF attenuator without much difficulty.

Mark
 
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« Reply #22 on: March 16, 2011, 11:59:08 PM »

Mark and Jim,

Thanks for the suggestions.

After a lot of thought I decided to go with one of Steve/QIX's  overload detection kit boards - the same one we all use for the class E rigs.  It has SWR inputs as well as the Hall effect device to sense final overload current and shut down.  This will be the easiest and most effective solution.

Schematic:
http://www.classeradio.com/easy_e_efficiency_meter.pdf

I'll post the finished amplifier when completed and tested.

T
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« Reply #23 on: March 17, 2011, 12:41:26 PM »

What do you plan to control ?? PTT line, bias??
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K1JJ
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« Reply #24 on: March 17, 2011, 12:56:48 PM »

I plan to kill the voltage that turns on the various 1000D amplifier stages in the PTT or bias circuit. No mechanical relays involved. It will be super fast to respond, whatever I do.   Just like the class E PDM pulse kill I hope.

The voltage controlled attenuator idea seems too hard to find for HF and they add some pin diode IMD I would think.   So QIX board for swr, final DC current sensing and an 8 amp fuse as backup shud be enuff.  I will also add a tiny 1/8A fuse to the RF input for good measure.   It wil be hardwired to the HPSDR, but ya never know. The HPSDR has a variable pad that will be mechanically limited to 10mW out.

I will also need a sequencer board, maybe another Jay/W1VD version to keep the receiver ant relay happy. Right now the amp is generating 15V spikes on the scope during unkey to the RX.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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There's nothing like an old dog.
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