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My Workshop .... Radio Frequency mod...

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John_kent
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Posted on Tuesday, 09 January, 2007 - 03:35 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Radio Frequency design has always been regarded as a black art, so it came as a relief to me when these cute little VHF receivers and transmitter modules first appeared on the market. I thought we might have a look at them as they may be of interest to beginners.

Radio modules fall into two types, amplitude modulated and frequency modulated. They can also work on the superheterodyne principle or a much simpler and inferior super-regenerative principle. Although super regen modules have applications I am only going to consider the superhet types as these generally have much greater ranges and more suitable for an application I have in mind. A superheterodyne receiver is one that converts the received signal and steps it down to a much lower frequency where most of the signal amplification takes place. In some SRD receiver modules this frequency conversion may take place twice (dual conversion) or even three times (triple conversion}. Most of the modules we will be considering will be of the dual conversion type. It does not matter to us what is happening inside the module because we are only interested in the overall specification and the types and parameters of the functions a module makes available to us.

One of the pins on both receiver and transmitter modules is for a 50 ohm impedance aerial. As we are dealing with very low transmitter powers efficient aerials ar essential. If I live long
enough we could cover aerials in depth at a later date. The aerial pin is normally on one end of the module and normally has an earth pin next to it. At this end of the module we have to stick to RF design rules concerning screening and ground planes.

There will also be a pin for the supply voltage + and often several pins connected to ground or supply voltage -.

Typically one might expect other pins to make available some or all of the following fuctions .....


CD (Carrier detect)
This can be used to drive an external transisor and LED to indicate the the receiver has locked onto a signal.

AF.
This is a buffered and filtered output from the FM demodulator and could typically be used to drive linear decoders etc.

RXD
Received data which is a squared up version of AF and again can be used to drive external digital decoders.

RSSI (Radio Signal Strength Indicator)
Produces a DC voltage that is proportional to the strength of the received signal.

That is about as complicated as it gets !. If there is any interest, perhaps we can follow this through to considering the 'design'of a Dual Diversity Radio Direction Finder for use in tracking lost children, animals and stolen goods ?.

Good Heavens ... did I use that word 'direction' again !
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John_kent
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Posted on Wednesday, 10 January, 2007 - 12:54 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Dual diversity means direction detection means that we use two aerials and two receivers and in this case measure the phase difference beteen their outputs. It can be done with two aerials and a single receiver, but this complicates circuit design. I am a great believer in the KISS principle (Keep It Simple Stupid), especially since receiver modules are now so cheap.

Simple Phase angle detection theory.
Imagine there is a remote carrier wave transmitter. In our own location we set up two quarter wave vertical whip aerials spaced about a quarter wavelength apart. At 433.92 mHz a quarter of a wavelength is about 6.82" or 173mm (compensated). This is one of the advantages of working at VHF, as aerial sizes become smaller and more suitable for handheld portable applications. If both of these aerials are EXACTLY the same distance from the transmitter, they will both receive the transmitter signal in phase. If we move one of the aerials ... the phase of the signal picked up by the aerial nearest the transmitter will lead the phase of the signal received in the other. Of course the signal at the aerials is very small and it is also difficult to measure phase angles at 443.92 mHz, so we need to amplify it in a receiver and then we can measure the phase difference between the last IF stages. If you do not understand any of the terms I use, look them up on the internet (IF stands for intermediate Frequency).

So far our block diagram for the direction finder consists of two aerial and two receiver modules. If you remember the list of available functions on the module, one was RXD and this is the perfect access point to the phase signal on both modules. All we need to do is feed these two square wave signals into a phase sensitive detector and with a little processing end up with a center reading meter that will tell us which way to turn the aerial to get perfect lignment with the transmitter. A few extra components will actually measure the phase difference in degree's.

Phase sensitive detectors.
They sound complicated but in reality can be very simple. A nor gate is a phase sensitive detector if followed by an integrator, so we could use a CD4001 for our phase sensitive detector along with an R and a C. However as this is meant an education project I can sneak in Phase lock loops here such as the CD 4046. This chip has a very good phase sensitive detector in it as well as an integrator and a cute little Voltage Controlled Oscilator that is worth using on it's own in other projects. What makes this PLL really useful is the fact that we can open the loop and use it for other purposes. For example we could use the VCO instead of a meter to allow blind people to use the direction finder !. Next we will go back and have another look at aerials for this suggested project.
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John_becker
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Posted on Wednesday, 10 January, 2007 - 01:10 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

John, we have sometimes used them in EPE constructional projects, but not to the extensive extent you mention. I've also played with the modules, notably some of those from RF Solutions (also available via RS) and found that they are not necessarily too easy to manage, needing careful PCB design to be reliable. I have as yet failed to use them for serial control between say a PC and PIC, nominally at 9600 Baud.

Certainly I can see merit in EPE publishing a feature/tutorial article on them sometime, written by someone who really knows them.

J
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John_kent
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Posted on Wednesday, 10 January, 2007 - 01:51 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

For the purposes of explanation we used simple whip aerials in our example. Whip aerials do have an impedance that is a close match to the 50 ohms impedance of the transmitter and receiver modules, but they have limitations.The first is that a true unipole needs to have a good earth plane beneath it, which is not really practical in most hand held applications. Another is a that unipoles can have high radiation angles, which can reduce range. A better solution is to use a half wave dipole, which theoretically transfers energy at right angle to the aerial elements (ie. what we want). The only problem is that the impedance of a half wave dipole is about 72 ohms (compensated). So it is not a perfect match to the modules. The vertical dipole aerial is also omnidirection (we say it has a circular polar diagram) and it really would be useful to use an aerial that is directional. A YAGI is a direction aerial.

If we take our vertical dipole and place another element behind it, a few percent longer than the dipole and about 0.2 of a wave length from it ....it acts as a REFLECTOR and turns the circular polar diagram of the dipole into a cardiod or heart shape, which is directional. A disadvantage is that it causes the impedance of the dipole to approximately halve.

If we place another element, a few percent shorter than the dipole, in front of the dipole it makes the aerial even more directional, which of course is a good thing. A disadvantage of doing this is that it causes the impedance of the dipole to halve yet again ... so it is now about a quarter of what it was as a simple dipole. However if we use a 'folded dipole' it has four times the impedance of a simple dipole .... we end up with a resultant impedance of 72 ohms. we have just re-inventted the YAGI AERIAL ARRAY which is what most TV aerials use.

This is still not a good match to the module impedance and can reuce the overall efficiency of the aerial system. This is because we get maximum energy transfer when the impedance (Z) of the load matches the impedance of the source. In a transmitter (not really applicable in this case because the rules for 433.92 says that we are not allowed 'fancy' aerials on our transmitter modules) an impedance mismatch will cause some of the transmitter energy to be relacted back from the aerial to the transmitter where it gets lost as heat etc. The ratio of energy sent to the aerial to the amount of energy reflected is called the 'standing wave ratio'. Now you do not have to remember any of this except that in a maximised system we would do something about correcting this mismatch. (If you want any search keywords, try gamma match, delta match, element diameter ratios in folded dipoles etc).

So I would suggest that we use two three element yagi aerials for our direction finder. Why would we bother with phase angle detection when we could simply use a directional Yagi ?. The answer is they are OK if you only want an accuracy of several degrees. The phase angle method will give accuracies better than one degree and coupled with the yagis can resolve bearing ambiguity.

(Message edited by john_kent on 10 January, 2007)

(Message edited by john_kent on 10 January, 2007)
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John_kent
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Posted on Wednesday, 10 January, 2007 - 03:36 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Searching

The RSSI output. This is generally referred to as a linear DC voltage that is proportional to signal strength. On many modules it is anything but linear , but it can give a useful indication of Signal strength if the response curve (not a line) is smoothed out a little. In fact if I were to build but one 433.92 receiver for general use, I would definitely include a signal strength meter. If nothing else it would give us the means of tuning our aerials. I would also like to include a small AF amp and connect it to the AF pin, but am not allowed to tell you why Hi!. Also I don't think that a general purpose receiver would be complete without a simple decoder built in. If you think about it there is no reason why one of the radio channels in the DF box could not also be used as the basis for a general purpose 433.92 receiver.

Search techniques.

433.92 is in the UHF band and as such is primarily a line of sight frequency. In buildings and built up areas, the range can be much reduced. This is why I place such importance in squeezing as much range as possible from the permitted 10 mW output of the transmitter as possible by using efficient aerials etc. and correct layout around the aerial pins on the PCB.

At the beginning of this posting I mentioned three possible applications ... finding lost children, tracking stolen goods and animals. Obviously detection range on the ground can be severly reduced in built up areas, woods etc.. Our best situation is where we have line of sight with no obstructions. Also with UHF we tend to get 'dead spots' where would should be able to receive a signal but cannot (search words Fresnel zones, destructive interference, refraction and reflection). So it pays to more around with reciver in order to avoid them.

In many cases the only way to get the required range is to put the receiver on an aerial platform. This could be the top of your local grain silo's or in a radio controlled drone aircraft. Since a half wave dipole is only 13.64 inches long the two yagi's should fit comfortably on a model aircraft airfame.

It is practical to consider fitting the direction finder to a real fixed wing or rotary wing aircraft to greatly extend the range. It might also be useful to include an SOT pic in the transmitter for signal identification purposes.

OK I chose the DF theme to illustrate the usefulness of radio modules. In the case of the children, stolen good and animal tracking, we may be drawn to other types of Direction finding, because of the way tracking transmitters HAVE to work. I will deal with that next.
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John_kent
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Posted on Wednesday, 10 January, 2007 - 04:10 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Tracking Transmitters and James Bond.
Anyone can tell you that battery powered radio bugs the size of a pinhead can transmit a whisper a thousand miles, for years on end. In the real world this cannot happen. If we want range, we have to have power and that means large batteries or shorter batttery life. It follows that any battery operated CCW tracking transmitter has a finite operating life. To extend battery life practical Tracking transmitters only transmit a very short signal at long intervals. This makes manual direction finding a little difficult !.

Doppler DF.
The easiest way we can solve this problem is to move from a manual syestem to a fast automatic electronic system. The one I want to mention is the Doppler Direction finding principle. Here we have a ring of dipoles to detect phase doppler differences. The system uses a minimum of four aerials. You can see this type on police cars. Some systems may use many dipoles in a ring to increase resolution. (search words, CADF, commutated aerial direction finding, quasi doppler, doppler).

The advantage of a Doppler system is that it can automatically capture and resolve a bearing on a pulsed tracking transmitter, store and present the bearing on a display in a fraction of a second. Throw in a PIC and it can even present track information. Accuracy is proportional to the number of dipoles used but with a four unipole set up you can get accuracies of a few degrees. It hardly makes for a hand held system, but does easily fit onto a car or aircraft. If I was looking for a lost child I would prefer to use a Doppler system, mounted on a helicopter. Remember that with DF all errors resolve to zero as you approach the target !.

JK
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Zeitghost
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Posted on Thursday, 11 January, 2007 - 08:43 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Please bear in mind that in the UK there are no restrictions on the antenna connected to the receiver, but to meet the licensing criteria, there are restrictions on the antenna connected to the transmitter. (i.e. the tx antenna is likely to be a simple omnidirectional one, not a highly directional yagi). This is due to the ERP limits allowed.
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John_kent
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Posted on Thursday, 11 January, 2007 - 10:24 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

A good starting point for regulations concerning the use of SRD's can be found at ....

http://www.ofcom.org.uk/static/archive/ra/publication/ra_info/ra114.htm

It also covers the 418 mHz band situation in UK.
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Zeitghost
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Posted on Thursday, 11 January, 2007 - 03:46 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The now gone but not forgotten 418MHz band...
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John_kent
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Posted on Friday, 12 January, 2007 - 12:39 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Well .... Almost gone Hi!. The relevant part of the regulations say ....


"After 31 December 2007, we expect to reassign the 418 MHz band for use by TETRA. New SRD equipment or systems using the 418 MHz band will not be able to be placed on the UK market. However, we will allow existing SRDs using the 418 MHz band to continue operating in that band, provided they do not cause interference to the TETRA network".


Because of the impending change suppliers are now off loading 418 mHz chips at low prices and it might be worth buying up a few superhet transmitter / receiver pairs for special purposes. Like what ?.

One of the big problems with SRD's is that they are short range, partly because of the low power and inefficient transmitter aerials allowed. Another reason is that we generally employ them in the worst locations.

One way to extend the working range is to use a repeater (search terms repeater, re-broadcast, re-transmission).

The design of repeaters is fairly straight forward so let us consider how we would design a simple one, using SRD chips. First it might help if you go back and refresh your memory about the typical output functions available on SRD receivers and transmitter modules.

In short, a 'repeater' receives a signal on one frequency and automatically re-transmits it on another frequency, thus increasing the range of the communication link.

The first transmitter sends it's signal to the repeater. Think of a repeater as being a receiver and a transmitter in a small box. The reciver has it's own aerial and so does the transmitter. To stop the output of the repeater transmitter breaking through to the repeater receiver .... we have to keep the two aerials apart .... by at least a few wave lengths. It also follows that the greater the difference between the repeaters reciver and transmitter frequencies, the less chance of break through. So we perhaps could operate the first part of the link on 433 mHz and the second on 418 mhz. We have to use superhet receivers because super regen receivers are wide band devices, and could probably pick up both frequencies.

The repeater receiver then switches the repeater transmitter on and re-transmits the data in real time to the remote 418 mhz receiver. So how can we acheive this in practice ?. First we connect the data output of the repeater receiver to the data input of the repeater transmitter. We can then use the the 'carrier detect' switch of the receiver to actually switch on the repeater transmitter. When the repeater stops receiving at the end of a transmission, the repeater transmitter is automatically switched off.

The above is a single one way link and we normally call it "Simplex". For two way data transmission we need two such systems and the end result we call "Duplex". For most applications a simplex link is all we need.

How usefuul is a repeater ?. If we can site the repeater in a good line of site and high position we can increase the working range a lot. Line of site with 10 mW could give us a range of up to 2 kM. and since we effectively have two links this could give us up to about 4 kM. To get this kind of range we need to use good directional aerials, such as matched Yagi's on the receivers. We also need to minimise transmitter losses, so if I had to move one of the aerials to get seperation, I would move the repater receiver aerial and use very low loss UHF co-ax cable to make the connection.

We can of course use a string of repeaters, to extend range, if we have enough available frequencies.

}
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Miked
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Posted on Saturday, 13 January, 2007 - 02:23 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Just a couple of comments, John. IF you were going to make a repeater and use separate transmit and receive antennas instead of a duplexer (cavity) and single antenna, you probably need to point out that the separation between antennas should be vertical separation and not horizontal.

The theory of using a repeater is correct but I would question the practicality. Using a repeater for digital transmission would require a delay at the source between key-down and transmitting the actual data stream. This would allow the repeater transmitter to come to full power output before "repeating" the data stream. Use of multiple repeaters would add to the delay time required and would be subject to any interference or signal loss in the entire linked system. A more practical approach might be a "store and forward" system. With this type of system, no consideration need be given to desensing the receiver since the transmitter would only key up after the data stream had been received. The quality of the signal received at the final point could be the same as the original transmission, since each 'store and forward' could regenerate the data stream, instead of being degraded as it passed through the entire system.

Just my thoughts on the subject. And, just to mention, I don't think this type of system would be legal here on this side of the pond. Commercial transmitters of the type being referenced for unlicensed operation in the U.S. are required to have the type of antenna permitted for use with the transmitter specified by the manufacturer. Only those antenna that have been tested and certified may be used and the antenna must be permanently attached to the transmitter. In other words, no coax connectors. But again, that is on this side of the pond and probably isn't applicable there.

I'm enjoying following the discussion.
Mike
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Terrym
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Posted on Saturday, 13 January, 2007 - 04:12 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Another type of repeater is the passive type. Basically two yagis back to back, one pointed at the Tx and the other at the RX. Used here in Oz to get TV signals to people in gullys and reasonably successful too. They are of course much stronger signals but should work with these small Tx/Rx pairs if sited correctly.

TM
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John_kent
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Posted on Saturday, 13 January, 2007 - 10:35 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Miked,
I agree with your comments. A lot of people have helped me with electronics over the years, so this was my feeble way of giving something back. It also keeps me off the streets, out of trouble and out of jail Hi!.


I doubt if a beginner who read it would actually make a repeater because of the cost of the modules, but it would no do any harm for them to know to know that they exist and a little about them. As far as beginners are concerned everything has to be simplified and only the basics can be covered due to space. Sometimes even I wince at what may be an over simplification Hi!.

It would be interesting to actually measure the response time of a single link using these modules. It ought to be quite fast, especially since we are not using mechanical relays. On a single link a short delay in the order of a few milli-seconds should be enough to stop any data loss.

I guess that if a beginner wanted to know more about things like cavity filters I would just give him an old ARRL handbook and let him get on with it. He could then tell me all about it Hi!

Ae connectors. In the case of the DF aerials we have to have them separated and that means coax. I do not feel comfortable about introducing losses into an SRD layout, which we would probably have to compensate for in some way. In the case of the DF setup, I did think it would be an idea to insert, an RF pre-amp at the aerial. I could not see any justification for using BNC or N connectors, so having the aerial permanently wired wasn't a problem for me. (Note: When I was sorting out all my typo's I realised I was wrong. There is no reason why the receivers should not be installed at the aerials and only the power, etc and two phase signals fed back to the central processing board).

I did post a link for UK regulations concerning the use of SRD's but although it dealt with duty cycles etc, there was no mention about transmitter aerials. If anyone has a relevant link, could you post it under this theme please.

I like your suggestion concerning 'store and forward' is interesting. It did cross my tiny mind that if we throw in a PIC there are lots of interesting possibilities there, checksums and the like. Thanks for the conversation Miked, it was interesting.

Terrym.
I have just come back from Malta and near to our hotel was a holiday home chalet site. The site was in a narrow valley and looked like a dense forrest of TV aerial masts. I wondered if they had thought about trying a passive repeater. I have never used one myself but I guess it is a juggling act between signal strength and system losses. Have you tried these yourself ?.

jk.
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Terrym
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Posted on Saturday, 13 January, 2007 - 01:41 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I've never set one up myself, but a person I once worked with did. He lived in a gully with the highest point between him and the transmitter. Apparently it worked well and gave him clear reception (I should point out that all our TV transmitters are in the one location for all channels here). All he used were two TV antennas, back to back, connected with a short feed with one angled down towards his house.

I have heard of various other people around the country that have done the same thing with satisfactory results.

TM
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Zeitghost
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Posted on Saturday, 13 January, 2007 - 05:31 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

When I was working in radio telemetry way back in the 90s, the modules came with a datasheet that showed the type of antenna that MUST be used to comply with the license free regs. Nothing else was permitted.

And the ERP was 250uW.

458.5 to 458.8MHz with 500mW was much more capable.

And expensive.

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