DIY Narrow band FM RX (+ tx mods?) - RC Groups
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Feb 07, 2004, 09:14 PM
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DIY Narrow band FM RX (+ tx mods?)

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.

Benchmarks, are the falcon, JMP, micromag Ruijsink system,
see the thread on microprocessors

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

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Feb 09, 2004, 11:23 AM
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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

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)
Feb 10, 2004, 04:07 AM
Registered User

crystals or LC filters

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?

Feb 10, 2004, 07:46 AM
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One big advantage of crystals is they are very frequency stable.


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!
Feb 10, 2004, 11:35 AM
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you should have this in your image collection it is made by dynamics unlimited
Feb 10, 2004, 07:07 PM
Sticky Shepherd
Graham Stabler's Avatar
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.


Feb 11, 2004, 02:54 AM
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JMP_blackfoot's Avatar
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
Last edited by JMP_blackfoot; Feb 11, 2004 at 04:14 AM.
Feb 11, 2004, 08:25 AM
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Thanks for the warning


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.

Last edited by peterbennett26; Feb 11, 2004 at 08:34 AM.
Feb 11, 2004, 08:30 AM
Gravity is a harsh mistress.
Tim Wolff's Avatar
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
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.
Feb 11, 2004, 10:46 AM
Sticky Shepherd
Graham Stabler's Avatar
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.

Feb 11, 2004, 05:20 PM
Registered User

ICs & quad coils


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

That's probably another design worth looking at for ideas.
Feb 12, 2004, 07:04 AM
Sticky Shepherd
Graham Stabler's Avatar
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.

Feb 12, 2004, 07:17 AM
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Thanks Excaliber, just the ceramic filters now!

Feb 12, 2004, 09:00 AM
Registered User

Ceramic filters maybe suitable?

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.
Feb 12, 2004, 10:06 AM
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JMP_blackfoot's Avatar
Maybe that's why there are two in each receiver ?
No, in this case, two is definitely much better than one.

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