DIY Narrow band FM RX (+ tx mods?)

[peterbennett26]

As suggested by JMP, because it was going off track, a new thread devloping a diy narrow band FM micro-receiver, but not in modelling science. My reasons are that this appears a much more active forum, and there is great interest in going lighter and smaller here.

The starting point is the circuit for this little beauty:-
which is from this site, which welcomes DIY making of the circuit etc., but not for profit use etc.
http://home.nordnet.fr/~fthobois/rx22.htm

Benchmarks, are the falcon, JMP, micromag Ruijsink system,
see the thread on microprocessors http://www.rcgroups.com/forums/showthread.php?s=&threadid=195327&perpage=15&pagenumber=1

for just how small and light its possible to go! Also the receiver circuit for the RX22 is given there.

Pete

[excaliber]

Pete,

I've a quick look at the RX22 what I found surprising was the openness of the layout. I realise this might not be too critical for this application, but can I offer some advice in your endeavours, rx and tx. Probably more relevant to the TX.

If you are an expert in this I apologise, But as you are asking:-

Do not layout RF circuits as they are drawn!
Look at the circuit and try and identify which are the RF components, and which are only involved with low frequency or DC.

Also ask yourself:-
Which components are in the main RF signal path?
which components are in the ground return paths?

When tryiing your initial layout:-
Layout the RF path in almost a straight line as possible.

Ground and and ground return paths are those points in the circuit which need to be kept at RF ground potential. The nearest you will get to this true gound potential on a PC board is a copper ground plane covering the whole of one side. Points in the circuit which cannot be connected directly to ground for DC reasons must be bypassed ('decoupled') to ground by capacitors which provide ground return paths for RF.

The values of the RF bypass capacitors are chosen to present a low reactance at the frequency in use, e.g. 100nF at LF 10 nF at HF and 1 nF or less at VHF.

Almost every RF circuit has an input, an output and a common ground connection. It is imperative that the input and output ground connections are kept distinct, and to place a low impedance common ground between them.

never let the rf path double back on itself
keep stages at different frequenceis well separated to avoid
breakthrough

use plenty of RF bypassing especially on the DC supply rails.

try to keep RF and DC wiring on the opposite sides of the board so that the DC wiring is in an RF dead zone.

If you go SMD then this will not be applicable but keep leads as short (especially on capacitors) as short as possible.

A good introduction to the basics

'Solid state for the radio amateur' Doug De Maw and Wes Hayward (ARRL)

[JohnnyB]

Do all the micro-receivers use crystals for channel selection? Is that why these are called narrow band, because when they are added (sorry mixed) with the carrier they are very specific to the transmitter frequency?

If you used LC tuning, perhaps to save weight (would it save weight?) would this become a wide band receiver? then would a further filtering stage be necessary?

Johnny

[excaliber]

Johnny,

One big advantage of crystals is they are very frequency stable.

Peter,

JR have a patent for their (I think single conversion) receiver, which is designed to stop interference between adjacent channels.

I do not have the number to hand. A look at it might give you a few ideas.

Good luck with your work!

[JohnnyB]

Pete,

you should have this in your image collection it is made by dynamics unlimited

[Graham Stabler]

That is based on a Philips TDS70... and cannot be made narrow band.

Ceramic filters are what gives the "tuning" the crystal is part of the local oscillator LO that feeds into the mixer along with the input RF. The mixed signal is then at the correct frequency for the filter. A bit like moving the mountain to Mohamed. So because the filter is exact then if your oscillator is not spot on you block the mixed signal and probably let some other channel through.

Or words to that effect.

Word of warning, I designed a narrow band RX that I would eventually have put into production (if JMP hadn't well and truly beaten me too it) and it did work. The main problem is getting components, you can get components but not always the lightest ones. For example I could only get filters of the sort used by JMP, Ruijsink and Falcon as samples. These were from Murata and Toko, even these were made especially.

Cheers,

Graham

[JMP_blackfoot]

Graham is right about obtaining components :
The receiver IC must be bought in 1,300 lots,
The filters in 1,000 lots,
The discriminators in 500 lots,
The JST connectors in 2,000 lots.
Takes a brave man to jump in :confused: :confused:

[peterbennett26]

However,

Its not my intention to try to provide a commercial rx/tx (go into production ala Graham's input), or even a 'beer money' alternative (pcbs available etc.) that some amateur developers do to offset some of their hobby costs (I have no problem with that) when they have made something like this.

My motivation I just want to be able to say "I built that all myself, its based on such and such, credit to ..... who designed the original circuit, and it works, doesn't knock Joe Bloggs adjacent channel plane out of the sky, or career off and wack somebody"

Perhaps having obtained ICs, etc in their 1000s JMP might consider selling them to people who just want to play. What about it? a kit?

Some people would always buy the ready built item, they just want to get them in their planes and get flying.
They would not want to waste their time doing this.
BTW Many of the would be diyers would fail, but then there is always the ready-built item available

Would this destroy the market for the micromag, Falcon JMP RFFS100 etc.?

I don't know, and I'm not really interested in what the worldwide market is either.

Peter

[Tim Wolff]

Originally posted by JMP_blackfoot
Graham is right about obtaining components :
The receiver IC must be bought in 1,300 lots,
The filters in 1,000 lots,
The discriminators in 500 lots,
The JST connectors in 2,000 lots.
Takes a brave man to jump in :confused: :confused:

Thanks for being brave :)
I like my first JMP Combo RX so much, I ordered a second last week. Works great in the same enviroment that grounded my Hitec Feather.

[Graham Stabler]

My meaning is that you can only get small quantities as samples. As a home constructor project it is hard for more than one or two to do it. Samples will soon become hard to get. It took me about 3 months.

In my experience it is possible to get IF chips in smallish quantities but only from dealers who buy end of lines etc. The filters/discriminators are the real problem unless you are OK to go heavier.

Graham

[excaliber]

Pete,

Look in your private messages, IC similar to that used in the JMP can be bought for £0.67, a superior (but heavier unfortunately, but as used in one of your 'target' receivers) IC about £2.40, available as one offs (not samples)

Also have a line on the quad coils about £2.20, available as one offs, (not samples).

Crystal filters are more tricky, you need to get a few more people interested, then you can get them for £0.77 each. If my contact turns up a source, I'll help you out with details.

If you want a top notch sensitive receiver and weight was not a problem check out the Sanwa Rx515 (see test carried out by Dave McQue http://www.norcim.fsnet.co.uk/Radio5/Radio5.htm)

That's probably another design worth looking at for ideas.

[Graham Stabler]

You don't need a quad coil specificaly you can save weight by using small inductors and capacitors or for convenience go for a ceramic discriminator (no tuning), a ceramic unit is about 0.2g using an inductor and capacitor should be much lighter.

The filters should be ceramic not crystal.

Graham

[peterbennett26]

Thanks Excaliber, just the ceramic filters now!

Pete

[excaliber]

What about laying a small lead type one its back, (possibly CA on top of another component)

The component is HCFM2455D

You can get these for 0.44 pence each.

Unfortunately you have to buy ten, but think of poor old JMP he has to be brave enough to buy 1000 and hope that he is not left with them on his hands.

[JMP_blackfoot]

Maybe that's why there are two in each receiver ?:D
No, in this case, two is definitely much better than one.

[Graham Stabler]

Leaded components will work fine but most are rather heavy. The ceramic filter found on the GWS 4Rp weighs 1.6g. There are lighter ones of course but they tend to be fairly chunky. I think didikey has some that might be passable.

Who makes the HCFM2455D? Need details as the bandwidth has to be right. It is also a good idea to follow some of the advice on the IF chip datasheet as to filter selection.

Cheers,

Graham

[excaliber]

Toko ceramic filter, 455 kHz centre frequency the D tells you 10 khz bandwidth at 6 dB attenuation

[JohnnyB]

The micromag appears to be using using filters with a narrower band width than the JMP, see table in pdf ECFM3

filehttp://www.tokoam.com/passives/filters/ceramic/.

The JMP appears to use D, i.e. 10 Khz bandpass, whereas the Micromag uses B 5.5 Khz bandpass (http://www.rcgroups.com/forums/showthread.php?s=&threadid=195327&perpage=15&pagenumber=1)

How will this affect interference performance from models on adjacent channels?

[JMP_blackfoot]

The Micromag uses one Toko filter, The JMP uses two properly matched Murata filters of 5.5 kHz bandwidth, which provide the necessary adjacent channel (10 kHz) attenuation. What is seen in the picture attached to Post #3, (http://www.rcgroups.com/forums/show...15&pagenumber=1)
is not a filter, it is a ceramic discriminator.

[JohnnyB]

So what we see in the JMP photo is the Murata CDBM455C49 ceramic discriminator (CD). If you have a scrap cordless telephone, there is a small possibility that you already have an MC3361 and the matching CD in it.


JMP (I don't want to be impertinent but) are you sure about the bandwidth of your ceramic filters? Murata don't seem list a filter with those characteristics.

Johnny

[JMP_blackfoot]

Originally posted by JohnnyB
JMP, are you sure about the bandwidth of your ceramic filters? Murata don't seem list a filter with those characteristics.
Johnny
As an example, I trust Murata when they write that type SFZLA455KS2L-B0 is specified for a bandwith of 5.5 kHz (+/-1.0 kHz), as much as I also trust Toko when they write that type AECFM3-455BL is specified for a bandwith of 5.5 kHz minimum.
But maybe this thread should be moved to the new DIY Electronics forum ?

[Dave Wulff]

I am not too familiar with the new forum, but I would like to see these micro based electronics stay right here. I would not want to see this get lost amoung homebuilt glow drivers, wet cell battery chargers and the like. Just my 2 cents.

Dave

[JohnnyB]

Hi JMP,

Yes I think you can trust the figures from Toko and Murata
The Toko filter
AECFM3-455BL has a specified bandwidth of 5.5 Khz at 6dB attenuation


However, the Murata filter you mention has used a slightly different method to give the same bandwidth as the Toko filter, i.e.
SFZLA455KS2L-B0 is 5.5 khz at 3 dB attentuation


So the Murata figure for bandwidth is taken from higher up the 'skirt' than the Toko figure.

[JohnnyB]

Do you get a better defined pass band by having two filters in series as seems to be the case in the JMP?

Johnny

[mmormota]

Anybody considered direct conversion?

Zero IF, no mirror frequency, no ceramic filters. Double mixer with 90 degree local oscillator phase shift, RC active filter, a/d (built in the uC) and complicated firmware to further filtering and decoding the signal.

Millions of pagers are working this way.

[billystiltner]

I thought about it and thought that that was how the small rxs worked till I realized the PICs were 4MHz. It would take quite a fast processor to do it. Most of the old pagers I have seen have lots of filters. Is there a standard chip most of the pagers are using?
How big and heavy is it?

Billy

[bravokilo]

hi friends, this is BravoKilo, Manila. Been trying to obtain a full schematics of receivers, FM's , and tried google which led me to this thread. If anyone can direct me, pls. I read discussions on IC , quad coils, ceramic filters, etc...parts. may I suggest looking into old wireless phones. I have a few of them, old models and I found one for example using the older MC3357 complete with all the parts.Just change the local oscillator frequency and input coils I think. This one even have a 10.7mhz crytal in series to the input pin (pin16). In short, we take the 10.7 input create create our on LO and front end coil. someone brought me a "Mantua" receiver , made in Korea using the same IC but not utizing the dual conversion. It directly used the 455khz strip only all the way to using the scan pin for the clock fed into a CD4015, th e popular decoder used by Futaba (shift register). I would be glad to communicate with anyone via this forum or private messaging. My email addresses are bongdeleonko@yahoo.com, registered here at RCGroups and another one - rcdevices@yahoo.com.ph. I prefer the latter for electronics topics. Am an "oldtimer", 56, and built various receivers largely AMs. Built Heathkit,Royal E.,Ace kits and very familiar with various decoders, making own chokes, PCbs,coils, etc. Tried the NS 1871 and 1872 RC TX and RX chips etc...opened upservos and wrote down parts used, i.e H bridge for the Futaba S10. Homebuid "brute" servos for robots, large sailboats etc. Use the B and D power screwdrivers for example. kept circuits of the 60's pre IC era, Graupner, O.S. to solve problem of having to buy quantity(our common waterloo), uses discrete components to drive brute servos, O.S. circuitry of the 60's. tow transistors for the Local pulse generator, 2 for the summing junction, and 2 each for the schmitt triggger to drive the H bridge. No IC and no center tap. Good for learning. May not be as good as the NE544, ZN409, the BA..., but it works. Some of yo guys is developed countries have enormous junkyards and flea markets to pick up wireless phones. You have everything in there, all the inductors you need. Why, if not for the law, you can run your toys using the 49.+ Mhz channels of the wireless phones. (Many are now in the upper band anyway). No modification, in fact you have 2 rx and 2 Tx in a wireless set(full dup). If size is not a problem, you can get the RX and put in a big boat. Increase the Tx power to 500 MW. Use the Japanese C1957, C2314 etc..... You even have the antennas.


Thanks . I just registered and this is my first posts.

BravoKilo

[bravokilo]

ESTARTER, am not sure if you were the one with an RX with very short range. may I suggest that you check the front end coil. It has 4pins but only 2 pins are used cuz it is just a single coil. the two other pins are usually grounded. i just fixed a Futaba RX with this problem. The servos get all jiterry and range is short. Since I could not rewind this factory coil, I made my own using a regular coil form but the small one with a 1/8 inch bore. make sure you have the capacitor on the board which is parrallel to the coil. Hope this will solve your problem.


BravoKilo