1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

RF amplifier (200 MHz to 1GHz), JFET biasing design question

Discussion in 'Radio and Communications' started by Clint Mclean, Sep 25, 2016.

  1. Clint Mclean

    Clint Mclean New Member

    Joined:
    Sep 25, 2016
    Messages:
    8
    Likes:
    0
    Location:
    Port Alfred, South Africa
    Does anyone know how I can get the best out of this JFET for an RF amplifier between 200 MHz and 1GHz:

    http://www.mantech.co.za/Datasheets/Products/J30X.pdf

    The manufacturer says that it has excellent high frequency power gain (Gps) of 11 dB at 400 MHz.

    They mention that's with:

    VDS = 15 V

    ID = 5 mA

    in a common source configuration.


    So that should produce a Voltage or Current Gain (Vout/Vin) > 3?


    I'm not sure of how to bias it to achieve that. If someone could show that it's possible with LTSpice I'd really appreciate it.


    Here's the LTSpice specification for the J304 from http://ltwiki.org/?title=Standard.jft

    .MODEL J304 NJF(Beta=729u Betatce=-500m Rd=1 Rs=1 Lambda=10m Vto=-3.52 Vtotc=-2.5m Is=33.57f Isr=322.4f N=1 Nr=2 Xti=3 Alpha=311.7 Vk=243.6 Cgd=2.17p M=362.2m Pb=1 Fc=500m Cgs=2.205p Kf=5.58e-003f Af=1)


    Alternatively does anyone know of a better way of building an amplifier for the 200 MHz to 1 GHz range. Quality of the signal is not required, I just want to detect that it's there. I'm going to amplify it and then convert it to a DC voltage using the circuit in the attached image.
    rfenergydetector.jpg

    Thanks
     
  2. spec

    spec Well-Known Member Most Helpful Member

    Joined:
    Oct 27, 2015
    Messages:
    5,828
    Likes:
    709
    Location:
    Somerset UK GMT
    Hy Clint,

    Welcome to ETO,

    It is no mean task to build a 400MHz to 1 GHz amplifier, especially if you do not have specialist test equipment. It simply is not worth the effort.

    If I were you I would go for one of these ready-built RF amplifiers: https://www.aliexpress.com/item/0-1...BbQ_K44DrCQsoq8Hz21ZyR74iNufz-epV5RoC_nDw_wcB

    Once you have got that going OK we can sort out the detection.

    spec

    (PS: I see you are from SA: care to put your location next to 'Location' in your user page so that it displays in the window at the left of your posts)
     
    • Like Like x 1
    • Agree Agree x 1
  3. Mikebits

    Mikebits Well-Known Member

    Joined:
    May 24, 2008
    Messages:
    6,214
    Likes:
    174
    Location:
    San Diego, Ca
    I would not mess with the FET amp, more trouble than it is worth. Just get a monolithic amp such as those made by Avago, or Mini-Circuits. The Mini-Ckts ERA-1A is very simple to use and it can be had for a few bucks. If you look on Ebay you can even find PCB's for them. Check out this link. http://www.minicircuits.com/products/amplifiers_smt_gpw.shtml
    http://www.minicircuits.com/MCLStore/dashboardPdf?model=ERA-1+
    Here is application ckt for amp:
    eraAmp.PNG
    Looks like your trying to make a RF detector. You should have a look at Analog Devices, they make quite a few RF detector IC's which will perform much better than your proposed circuit. The AD8361 might do what you want and it is very simple to connect. https://www.google.com/url?sa=t&rct...4wTuV35qMlagNGbxMV9RUg&bvm=bv.133700528,d.cGc

    AD8361.PNG

    If you go over to the Analog Devices web site, you can do a parametric search on RF detectors, they have a whole slew of parts to choose from.
     
    • Like Like x 1
    • Agree Agree x 1
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. JimB

    JimB Super Moderator Most Helpful Member

    Joined:
    Sep 11, 2004
    Messages:
    6,329
    Likes:
    585
    Location:
    Peterhead, Scotland
    ONLINE

    Would you care to tell us what you are trying to detect?
    There may be an easier way to do it.

    I also agree with Mikebits, Mini-Circuits and Analog Devices have some easy to use components which can take away a lot of difficulty for RF applications.

    JimB
     
    • Like Like x 1
  6. Mikebits

    Mikebits Well-Known Member

    Joined:
    May 24, 2008
    Messages:
    6,214
    Likes:
    174
    Location:
    San Diego, Ca
    It seems that you responded while I was still typing my response. If I had read your post prior to my posting, I probably would not have bothered posting as you had already said what needed to be said. I guess great minds think alike :)
     
  7. spec

    spec Well-Known Member Most Helpful Member

    Joined:
    Oct 27, 2015
    Messages:
    5,828
    Likes:
    709
    Location:
    Somerset UK GMT
    :p I do the same thing all the time Mike. Anyway there is more information in your post and you have gone for the Rolls Royce MiniCircuits approach whereas I have gone for the cheapskate AliExpress approach. Great minds think alike.:D

    There are certain members on ETO who post exactly what I am thinking time and time again.

    spec
     
  8. Clint Mclean

    Clint Mclean New Member

    Joined:
    Sep 25, 2016
    Messages:
    8
    Likes:
    0
    Location:
    Port Alfred, South Africa
    Thanks guys. I most likely will take your advice and just buy a pre-built amplifier.

    This is a great forum. I may get in contact with you guys about some other design questions later.
     
  9. spec

    spec Well-Known Member Most Helpful Member

    Joined:
    Oct 27, 2015
    Messages:
    5,828
    Likes:
    709
    Location:
    Somerset UK GMT
    That would be good- we always like a challenge on ETO, and good information with questions is appreciated.:)

    spec
     
    • Like Like x 1
  10. Mikebits

    Mikebits Well-Known Member

    Joined:
    May 24, 2008
    Messages:
    6,214
    Likes:
    174
    Location:
    San Diego, Ca
    You never said, what is the project that you are using the amp/detector for?
    One issue if you plan on tapping off the antenna is maintaining a 50 ohm match.
    You might want to look into something like a directional coupler.
     
  11. Clint Mclean

    Clint Mclean New Member

    Joined:
    Sep 25, 2016
    Messages:
    8
    Likes:
    0
    Location:
    Port Alfred, South Africa
    I'm very interested in detecting biological electromagnetic signals guys.

    I want to determine whether EEG signals can be detected remotely. Most EEG systems use electrodes that have to be on the surface of the skin. So I want to determine whether such signals can be detected from further away.

    I've got a masters degree in computer science and am thinking of doing a PhD in human computer interfaces where someone does not need to be connected with wires.

    I'm certain that this is already possible. The Italian scientist F. Cazzamali achieved the remote reading of EEG data in the 1920's.

    He transmitted radio waves towards someone which interacted with the electrical activity in their brain to produce a modulated waveform that could then be received by a detector further away.

    So what I want to find out is, what normal biological electromagnetic activity occurs that could be detected and whether experiments such as Cazzamali's could be used to receive EEG data without the use of electrodes.

    From experiments that I've been doing using the basic RF detector that I mentioned, I've determined that it can detect electromagnetic energy from myself from reasonably far away, further than a meter. So it seems to be detecting a far field energy.

    The way I determined this was to move closer and further away from the detector, noticing the change in energy levels detected.

    I find this fascinating that energy is being radiated from us that can do that.

    The electromagnetic energy of house current seems to make this detector more sensitive. I'm thinking that it may get the diodes to their working point, that is so that the energy radiated from myself can exceed the voltage drop of the diodes and be detected.

    So I want to connect amplifiers to it so that house current or any other electromagnetic energy is not a factor.

    If you guys know a way of biasing the diodes to also achieve that (voltage divider?) then let me know.

    There's also something called a resonant frequency that I would like to do experiments with. The idea is that different organs have an electrical resonant frequency. The brain for adults is around 450 Mhz.

    A radio wave then at your resonant frequency would create oscillating electrical currents in you, this would be affected by the electrical activity in your mind, EEG signals. Those oscillating currents would in turn create another radio wave modulated with this EEG data.

    So a signal broadcast at your resonant frequency would effectively be re radiated containing EEG data that could be detected from further away.

    So I would like to research these things and to do that I need to build an effective RF detector.
     
  12. alec_t

    alec_t Well-Known Member Most Helpful Member

    Joined:
    Jul 10, 2011
    Messages:
    9,259
    Likes:
    1,218
    Location:
    Cardiff, Wales
    How did you distinguish that change from the change you would expect as your body blocked or reflected radiation from other sources in the environment, e.g. radio and TV signals?
     
  13. Clint Mclean

    Clint Mclean New Member

    Joined:
    Sep 25, 2016
    Messages:
    8
    Likes:
    0
    Location:
    Port Alfred, South Africa
    Hi Alec

    The detector will tell me if the energy detected is increasing or decreasing.

    I have noticed it decreasing at times, so RF was being blocked there.

    The increase in energy is fascinating, though, because normal radio, television...etc signals should not reflect off of the human body. As far as I know most radio frequencies should be absorbed or pass straight through.

    It's actually a reflected or I prefer to describe it as a re radiated signal that I'm looking for because that could very likely be modulated with biological electrical activity.
     
  14. Mikebits

    Mikebits Well-Known Member

    Joined:
    May 24, 2008
    Messages:
    6,214
    Likes:
    174
    Location:
    San Diego, Ca
    Oh brother :rolleyes:
     
    • Agree Agree x 1
  15. alec_t

    alec_t Well-Known Member Most Helpful Member

    Joined:
    Jul 10, 2011
    Messages:
    9,259
    Likes:
    1,218
    Location:
    Cardiff, Wales
    Why not? The human body presents an impedance discontinuity in the path of electromagnetic signals; therefore some reflection will occur.
    You would ideally need to repeat your experiment inside a Faraday cage to eliminate extraneous radiation which would otherwise swamp any body radiation.
     
    Last edited: Sep 27, 2016
  16. Clint Mclean

    Clint Mclean New Member

    Joined:
    Sep 25, 2016
    Messages:
    8
    Likes:
    0
    Location:
    Port Alfred, South Africa
    Thanks Alec. Would I not need to be very near the antenna for that to occur?

    I've got an SDR (software defined radio) usb dongle that can work as a spectrum analyzer and have written some code for it to record frequencies both when I'm near the antenna and further away and compare them. So far I haven't found any frequencies there in it's range (30 Mhz to 1.7 Ghz) that are stronger when I'm closer to the antenna.

    I also used to think that a lot of frequencies would reflect off of the human body until I started experimenting with this sdr device. There just doesn't seem to be any effect of reflection.

    That's why I want to determine why this RF energy detector device, which is essentially just diodes and a capacitor connected to a voltmeter is showing a significant effect based on how far I am from it.
     
  17. Rich D.

    Rich D. Member

    Joined:
    Feb 3, 2014
    Messages:
    210
    Likes:
    15
    Location:
    West Chester, PA
    Just speculating...an experiment in the 1920's was very likely done in a fairly clean electromagnetic environment. Now, we have to worry about cordless phones, cell phones, cell towers, blue tooth, wifi, automobile security keys, garage door openers, laptops, HD televisions, autos with electronic ignition, quad copters, microwave ovens...you get the idea. While the European standards are trying to clean up this environment, I'm certain it is a lot more "polluted" than it was 1oo years ago. You might very well need to be in a Faraday cage these days.
     
  18. Mikebits

    Mikebits Well-Known Member

    Joined:
    May 24, 2008
    Messages:
    6,214
    Likes:
    174
    Location:
    San Diego, Ca
    Apparently the original experiment was done in a Faraday Cage.
    Image Source: https://borderlandsciences.org/project/bio-icomm/lg.lawrence/Electronics_and_Parapsychology.html
    [​IMG]
     
    • Like Like x 1
  19. alec_t

    alec_t Well-Known Member Most Helpful Member

    Joined:
    Jul 10, 2011
    Messages:
    9,259
    Likes:
    1,218
    Location:
    Cardiff, Wales
    No. The reflection coefficient would be constant, although the incident and reflected signal levels would be weaker at greater distance from the transmitter.
    This article may be of interest.
     
    Last edited: Sep 28, 2016
    • Informative Informative x 1
  20. Clint Mclean

    Clint Mclean New Member

    Joined:
    Sep 25, 2016
    Messages:
    8
    Likes:
    0
    Location:
    Port Alfred, South Africa
    A faraday cage could be very useful.

    From that article: "...Currents are also induced in tissue (as well as any other conducting or dielectric materials) upon exposure to electromagnetic fields. Although hands and bodies are not particularly efficient radiators, they will scatter a fraction of the power incident on them, depending on the RF frequency involved."

    I'm very interested in the idea that the human body has a unique resonant frequency for different organs. Just like an antenna, organs should have a unique frequency and will re-radiate energy more effectively at that frequency.

    So a resonant frequency would reflect or re-radiate energy a lot further than other frequencies.

    If the brain does that then the re-radiated energy should be modulated with EEG electrical activity which would provide a way of transmitting such data wirelessly. All that would be required is the use of a transmitter broadcasting at their unique resonant frequency.

    It seems as though it is a fact that human organs do have resonant frequencies:

    Lin, JC (1989) Electromagnetic interaction with biological systems. Plenum Press, New York.

    Any biological electrical activity should then affect or modulate such a frequency.
     
  21. RadioRon

    RadioRon Well-Known Member

    Joined:
    Oct 7, 2005
    Messages:
    1,755
    Likes:
    53
    Location:
    BC, Canada
    This research is fraught with technical difficulties. I agree that it is likely that a shielded enclosure (Faraday cage) will be required, and I think it likely that it would have to have an anechoic interior. My work in testing undesired EM emissions showed me that reflective internal surfaces in a shielded room or box destroys the accuracy of your measurement, and that anechoic or open space is required. Finding a suitable test site is challenging.

    I also wonder about whether electric currents in the body that originate biologically would indeed modulate RF current coupled into the body from outside. Normally, we must have a non-linearity of some sort for modulation to occur. Without such non-linearity in the medium, the biological electrical activity would not mix with currents to create new spectrum, and this is essentially what modulation is, the creation of new spectrum around a carrier (for example). This begs the question of whether there is any human tissue that behaves non-linearly in its reactance to small RF currents. I have some doubts about this, although, with many media, you can get non-linear effects when the field strengths (induced currents) imposed on the media are high enough. This suggests that the previous experimenter was using some pretty strong RF on his test subject. You noted that someone believes they have already detected re-radiation of modulated RF. Do you have a copy of the publication for review?

    In your previous experiments, you were using a diode detector. These are notoriously insensitive compared to a good radio receiver, so if you did indeed measure radiation from the body, then the field levels must have been fairly high. But others don't find such high field strengths as you do, so I wonder whether your detector was, in fact, responding to the ability of the body to affect the field strength of electro-magnetic fields that originate elsewhere. I am interested to know what kind of antenna you used for those initial tests with your detector. Can you provide a photo or description? I think the effect you see is most likely that your body is distorting the EM field around your detector antenna when it is close, simply by virtue of having a very different dielectric constant and conductivity than air.

    It is worth mentioning that many spectrum analyzers have relatively poor noise figure and require external pre-amps for sensitive work.

    It is commonly done to bias diodes with a bit of DC to improve their sensitivity as a detector, so you are perhaps on the right track thinking that house AC may be helping you. On the other hand, by adding an amplifier in front of your diodes you may be creating new trouble unless there is some sort of band limiting filters to help exclude undesired EM from outside sources. Adding an amplifier to a diode detector is the first step in creating a radio receiver, and I wonder whether you want to re-trace the steps of many radio pioneers, as this would be a long road to follow.
     
    Last edited: Sep 28, 2016
    • Informative Informative x 1

Share This Page