3GHz RF Limiter with DC block and Attenuator for GBP30 !

[Fraser]

For the RF techs on the forum I have news of a neat device to help protect the front ends of your receivers and spectrum analysers 

I purchased a pair of RF Explorers some time ago and regularly receive updates from the designer, Ariel. The latest news release was that Ariel had designed and produced a 3GHz RF Limiter with DC block and attenuator all in one small quality package.

The specs are: 3GHz BW, 30dBm Max input, 15V DC block, 6dB attenuation. Output Max = 4dBm.

I have discussed the design with Arial and he indicated that it has taken many months of prototyping to get the design right. I can believe this as getting a decent response from DC to 3GHz is a challenge. He is now happy with the design and so it has been released for use with the RF Explorer. The RF Explorer can have an attenuation factor entered to offset the 6dB attenuator.

I believe that this device would be useful to anyone who wishes to protect the delicate RF front end of their receivers or spectrum analysers, and for GBP30 delivered, I consider it a bargain purchase. HP products of a similar spec cost several hundred pounds. I have not seen the spectrum plot of the unit yet but may be able to sort that out this week with my SA and tracking generator.

I attach a picture of my unit and the web site details. I purchased my two limiters from Seeed Studio without any difficulties and they were delivered quickly.

I have no affiliation with Ariel, or his projects, but he has kindly detailed the contents of this limiter design and it does follow best practice. I will not be opening my units and photographing the PCB as that, IMHO, could damage Arial's intellectual property rights. He has worked very hard to create the right PCB layout and it would be wrong to show it on this forum as it would surely be copied.

At GBP30 in a quality SMA case, this is cheaper than I could build it anyway.

http://micro.arocholl.com/index.php?option=com_content&view=category&layout=blog&id=3&Itemid=50

[PA0PBZ]

Alright, that was a nice teaser but since you opened it, where are the pictures? 

[Fraser]

The components in the unit are 'commercial off the shelf' parts available to those who wish to source them and know about uW RF. I cannot (will not) disclose the exact components used, or the PCB layout, as they are the fruits of Ariel's efforts and are not 'open source'. Ariel is not some large company, 'just' an RF Tech who has kindly shared his projects at reasonable prices. It would be very wrong indeed to disclose the fine detail of his design. I hope you understand.

Ariel is a really great person to have designing RF tools as he offers fast help via email and is a person who is not inflating his products prices as they become popular. I hope that he gets the custom that he deserves. He intends to build other useful add-ons for the RF explorer but that will take funds. Unlike 'kickstarter' projects, and the like, Ariel developed the product from his own pocket, released it, supported it and is now using any profits to improve the capabilities...as I said a really good guy 


$45 for this RF limiter is a bargain IMHO. I will generate a Bode Plot of the units frequency response this week so there should be no need to see the PCB track design to know that it works OK.

[Fraser]

Ariel has kindly supplied me with some more information on the Limiter:


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From Ariel -

Yes I of course produced a lot of tests with all RF supported band, you will find it has a max of 1.5dB insertion loss deviation towards the higher frequency end, so attenuation can vary from 6 to about 7.5dB max in all my tests. The protection works fine well above 3GHZ but insertion loss grown about 2-3dB per GHZ so I consider that a reasonable limit. In other words it works as expected by protecting your circuit beyond 3GHZ but measurement accuracy beyond that frequency will not be good. Return loss was -20dB or better (actually well under -30dB in the lower band region).
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I did say he was a helpful chap. He intends to produce full documentation on the unit, but currently has a lot on the to-do list.

Ariel has confirmed that he would like me to protect his IPR, so I will not divulge the finer details of the design unless he tells me I may at some point in the future. As I have stated, its not just the components that matter, its how you deploy them 

I will produce a bode plot to 8Ghz for those interested.

[fpliuzzi]

I will not be opening my units and photographing the PCB as that, IMHO, could damage Ariel's intellectual property rights. He has worked very hard to create the right PCB layout and it would be wrong to show it on this forum as it would surely be copied.

Aurora, I agree with you on this one. Ariel has developed some reasonably priced, very useful devices and does deserve a lot of support for his efforts.

I noticed that the right side SMA connector on my RF Explorer-3G Combo is already protected up to +30dBm (and +16V dc), so I didn't need to purchase this external protection device, but I'm sure that it will be be a welcome, inexpensive addition to many test benches.

Your Bode Plot out to 8GHz would be interesting to see when you get the time. I'm curious to see how low of a frequency the device can pass (-3dB point). If it goes reasonably low I'll  purchase one of them to use it as a bit of added protection for my HP461A (pre)amplifier (1Vac, 2Vdc max input) and a few other somewhat fragile devices on my bench.

Interestingly, Ariel is also currently working on an external tracking generator for the RF Explorer.

Regards,
Frank

[Fraser]

I am taking the limiters into the office today, so will hopefully find time over lunch for a quick bode plot. I will include detail of the low end -3dB point as well.

[amyk]

To put things into perspective... anyone who wants to clone this would certainly have a budget > $45.

[KJDS]

To put things into perspective... anyone who wants to clone this would certainly have a budget > $45.

I disagree, two SMAs at £1 each, a decent RF cap is another £1. £2 max for the limiter diodes and three resistors. £1 for a small peice of FR4 and a little time with a scalpel to cut a 50 ohm track, then a bit of bent copper for a case. I also suspect that as most equipment will handle +30dBm in then it won't find too many applications. If it could cope with 20W in and limit around +20dBm then it would have a much bigger market.

[Fraser]

I did a quick series of Bode Plots on the limiter. Unfortunately I couldn't save the files in a format for publishing here but I am working on that.

I attach the numerical data that I took from the plots using the marker. The unit performs as expected to 3GHz and has useful performance above that frequency all the way to the limit of the test jig I was using (7.5GHz) There is a distinct dip just above 3GHz and also evidence of resonance and attenuator bypass at some frequencies in the range 3Ghz to 7.5GHz

The low end is excellent with a -3dB point of 60kHz.

Limiting level has yet to be confirmed but should be +4dB maximum output.

[Fraser]

With regard to the usefulness of this unit, I own quite a bit of kit that wouldn't appreciate inputs higher than 13dBm so I have a definite use for an RF limiter. My very expensive communications analyser has a low level RF input that will definitely be destroyed if more than +13dBm is applied to it. A repair costing around GBP5k results !
 
Ariel produced this for his RF Explorer because some of the RF modules can only cope with a +5dBm maximum input.

[fpliuzzi]

Aurora,
Thanks for posting the pdf of your test results. I can definitely use one of these on my bench.

Regards,
Frank

EDIT: The additional 10 screen shots that you've supplied below are very helpful, thanks.  You're right; a picture is worth a thousand words.

[Fraser]

I will be finding a way to convert the Advantest .dat files to a jpg to post here so that you can see the response curves as a picture speaks a thousand words. Watch this space.

[Fraser]

OK I have retested the limiters and saved the images as .png files

The bode plots are attached

The TG was set to 0dBm and the test jig was normalised to compensate for its frequency characteristics. The Advantest SA bottoms out at 50kHz and the Test Jig tops out at around 7.5GHz

Further tests will be carried out with an RF amplifier inserted in the jig to generate adequate level to cause limiting. Breakthrough may be an issue above 3GHz.

[G0HZU]

Some time ago I designed and built my own limiters to protect my test gear.

The one I use the most only protects up to about 750MHz but it will withstand a 5W strike and it only cost me the price of the connectors plus four £0.01p diodes.

I designed it to have very low insertion loss and also very good return loss to allow me to use it with a spectrum analyser or a thermocouple power meter with minimal measurement uncertainty.

The insertion loss is below 0.1dB at 750MHz and the return loss is better than 35dB across the effective limiting range of LF through to 750MHz.




I also made a version with a DC block and inline fuse but this degraded the insertion loss to about 0.5dB by 500MHz. The idea is that the fuse blows in a tiny fraction of a second for high power strikes (eg >10W) and this prevents failure of the limiter diodes.


Basically, you can make a 5W rated DC to VHF limiter with negligible insertion loss for a few pence if you build it yourself.

[Fraser]

I have tested the overload protection performance of the Ariel RF Limiter and it does what it says on the tin. Producing the specified limited +4dBm output power when driven by +30dBm (1W). The response is excellent to the specified 3GHz and the limiter continues to limit the input signal all the way to 7.5GHz. This units design is optimised for use up to 3GHz and so it lacks the elements needed for the demanding higher microwave frequencies. Such higher frequency microwave limiters are normally quite narrow band in nature and so not suitable for Ariel's application.

Ariel has very kindly sent me an un-built limiter and a filter PCB to experiment with. These items are not normally available to buy, but if enough interest is shown maybe Ariel will consider selling experimentation kits as well as built modules.

The case used for the limiter modules is very neat indeed and perfect dimensions for experiments in the microwave frequencies, as it is terminated in SMA connectors and very compact. Perfect for small RF PCB's.

The module case offers the RF experimenter many neat options of which I list a few below:

1. RF Limiter with user selectable attenuation and limiter diodes. The PCB is already designed by Ariel.
2. RF Filters. Low Pass, High Pass or Band Pass. Ariel has sent me a universal PCB for such use.
3. RF Notch Filters.
4. RF Fuse protection for Attenuators.
5. Power Inserter (BIAS 'T') for powering electronics via the coaxial cable.
6. Custom RF amplifiers using MMIC technology or discrete FET/Bipolar technology.
7. DC Block to protect equipment from DC offsets.
8. RF Matching transformer or Pad to change coaxial impedance from say 50 Ohms to 75 Ohms.
9. Static Dissipator to protect equipment from static electricity. (60V Gas Discharge Tube plus non inductive 2K2 resistor)
10. Custom Attenuator Pad.
11. Pocket Noise source (Biased Zener or genuine Noise diode feeding MMIC amplifier).
12. RF Detector probe head containing a microwave Diode.   
13. FET high impedance converter for 50 Ohm equipment. 1M Ohm to 50 Ohms.
14. EMC E & H field probe head electronics modules with selectable probe sensors
15. Microwave SWR bridge
16. Microwave diode power sensor

And there are likely many more uses. I have often hunted for such a small and neat RF module case but most are designed for larger connectors and are unsuitable due to dimensions.

I like the fact that Ariel has used a 3D printer to create a build JIG that assures correct spacing and alignment of the SMA connectors during PCB assembly. He was kind enough to include the alignment JIG with my module parts.

We will have to see what Ariel decides to release in the future as accessories for his superb RF Explorer but he may also have a customer base for experimentation kits like that which he has kindly given to me. This Gentlemen is worth supporting.

http://micro.arocholl.com/index.php?option=com_content&view=article&id=48:introducing-rf-explorer&catid=40:article&Itemid=64

I have deliberately not photographed the details of the Ariel's PCB's to protect his IPR.