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Old Nov 30, 2004, 08:41 PM
Martin - AKA mr.sneezy
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Adelaide, Australia
Joined May 2004
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RC Reciever tuning procedure ?

Hi Guy's
With out having to explain myself too much, is there a common method of procedure for fine tuning a RC reciever. (In this case it's a Hitec HFS-04 single conversion on 36Mhz)

I normally work on more adavnced radio systems and use techniques like 12dB SINAD or 20 SN ratio, or even 3x10*6 error threshold (digital radio) while injecting a suitable test signal.

Is there a common point for measuring and a common method for tuning for RC recievers for best sensitivity?

I don't have any RX schematics and I 'could' re-invent the wheel here and go probe around the PCB till I work something out, but a few short cut tips would be great if anyone knows them. Feel free to be as technical as you can.

cheers
Martin
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Old Nov 30, 2004, 11:43 PM
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Regina, Sask. Canada
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Here is a simple method I have used very successfuly for quite a number of years. It is certainly not technical and I am sure the technical people will come up with many reasons why it won't work or shouldn't be done in this manner.
1. Unplug all servos.
2. On the receiver connect an earphone at one of the servo outputs.
Make the connection between the signal and negative pins
so that you can hear the pulses coming through from the decoder. On
some receivers connecting between the signal and positive pins seems
to work better. I might add that I use a 2,000 ohm earphone, but I
believe just about any earphone should work.
3. Turn the transmitter on, collapse the antenna and put transmitter down.
4. Turn the receiver on and listen to the pulses coming through.
5. Move the receiver further and further away from the transmitter and
simply tune for maximum range.

I have found this method simple and very effective.
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Old Dec 01, 2004, 08:06 AM
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I guess hackken procedure is correct for those who do not have equipment like Oscilloscope, I myself have not done the procedure yet, but I just talk to people who are expert this field, the procedure is the same as describe by hackken, but instead of using the Earphone to monitor the signal, you use an Oscilloscope to measure the signal, and just tune it for Maximum signal or maxinum signal deflection in the Oscilloscope, just tap an the Collector or Drain of a Transistor or FET. or might just tap at the Input of a Decoder Chips.....this is just tuning for maximum sensitivity.

I just wonder how they tune it for Selectivity, say converting a Hi-Band Receiver to Low-Band Receiver of 72 MHZ receiver and vice-versa.....I guess its just like the same procedure as describe above, except that you need a Signal Generator to set the operation frequency set at 72.250 (for LOW-BAND) or 72.750 (for High-Band) and just tune it for maximum signal....this is just me idea, But I have not done it, I do not like to experiment on my perfectly working receiver.

Love to hear, from the Real experts here.
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Old Dec 01, 2004, 08:51 AM
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I might add that I too have used an oscilloscope or a meter to tune a receiver in the past but I have found the earphone method I described above yields as good and in some cases I believe, even better results.
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Old Dec 01, 2004, 10:12 AM
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Near Albuquerque, NM USA
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For convenience when working with the receiver on the bench, put the transmitter, with the antenna down, in a large metal trash can. 25 to 35 gallon (100 L) works well. Sliding the metal lid on and off will vary the attenuation of the signal. If you have it, a piece of sheet metal is a little easier to adjust than the regular lid.
Putting the transmitter in a metal file drawer also works well as you can slide the drawer in and out to vary the signal. Put some papers in the drawer so the antenna does not touch the metal of the drawer.
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Old Dec 01, 2004, 10:13 AM
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East Anglia, UK
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Yes.

On a typical S/C receiver there are only really two things to tune - the dicsriminator coil, and the RF coil.

Everythung lese is preset by the ceramic filters.

Tuning the discriminator coil is probably not valuable, since it will have been preset correctly at the factory.

That leaves the RF coil which can - and does - have a large impact on sensitivity.

Because its usually coupled (albeit loosely) to the antenna, I wouldn't tune it without the antenna present. Its actually best done in the model itself, and the best procudeure for tuning up the whole assembly, its to leave the tranny parked on a table or chair, and retreat with the model, outdoors, to a point where servo buzz becomes noticeable, and simply tweak it for least buzz. If the buzz entirely goes, retreat further and repeat.

Usal caveats apply about not using metallic tools to tweak, and removing hand as far as possible after each tweak, because all these things affect the tuning.

Nevertheless, I would expect on average 50% more range from a set spot tuned to the tranny than a randomly tuned one.

I thmk hooking it all up to test gear is a waste of time. After all, what you are after is probably best range with least servo buzz. You have it.

The only other reason to tweak away from that setting would be to reduce the impact of e.g. the image frequency. If you could set up another transmitter on or near that, its a good thing to try, if you fly in company a lot.

Or even a signal generator. Its usually possible to montir te AGC output from teh chip to glean some kind of idea as to what strength of signal the receiver thinks it has, and it may well be that you can use a sweep generator , AGC and an X-Y scope to plot the characteristics of the passband. The image should clearly show up. Then maybe a discrete tweak of the input coil would help reduce it without adversely affecting the pass band.
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Old Dec 01, 2004, 07:16 PM
Martin - AKA mr.sneezy
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Adelaide, Australia
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Thank guys

The audio method sounds like a good thing. I also agree with Vintage1 that all the test gear should be un-neccesary and I think his idea of tuning in the model is very logical.
I'll try out both methods and see if the slug positions change much.

The Hitec HFS-04 has about 5 tuning coils if I can visualize it right from when I had it apart a few days back. So there is possibly more scope for improvement.

PS. I've read test reports in magazines that give the tested bandwidth of the RC reciever. I only guess they adjust the input frequency till responce drops off but I wonder how they detect the 'limit', given that a general radio test set usually won't modulate correctly to use the servo outputs as a guide. Unless they detune a real RCV transmitter while testing... Anyone know ?

Martin
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Old Dec 01, 2004, 10:31 PM
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Yes, tuning the receiver in the model will usually give slightly better results than tuning it on the workbench. There is usually not much of a noticeable difference.
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Old Dec 02, 2004, 05:20 AM
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Quote:
Originally Posted by PLMS
The audio method sounds like a good thing. I also agree with Vintage1 that all the test gear should be un-neccesary and I think his idea of tuning in the model is very logical.
I'll try out both methods and see if the slug positions change much.

The Hitec HFS-04 has about 5 tuning coils if I can visualize it right from when I had it apart a few days back. So there is possibly more scope for improvement.

PS. I've read test reports in magazines that give the tested bandwidth of the RC reciever. I only guess they adjust the input frequency till responce drops off but I wonder how they detect the 'limit', given that a general radio test set usually won't modulate correctly to use the servo outputs as a guide. Unless they detune a real RCV transmitter while testing... Anyone know ?

Martin

Normally you e.g. us e a sweep generator and vary the input power to get the same output at varying frequencies. Or pick up theoutput on a scope or analyser and do some sums.
Or simply read te specs off the filters
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Old Dec 02, 2004, 05:23 AM
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Quote:
Originally Posted by PLMS

The Hitec HFS-04 has about 5 tuning coils if I can visualize it right from when I had it apart a few days back. So there is possibly more scope for improvement.
Almost certainly those are IF coils. Its rare to have more than one RF coil - very near the antenna input.

A quality set might have two.

You don't need to mess with IF coils. They are usually preset to the IF rrequency, and will not need tweaking, and you can get in a mess doing it.
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Old Dec 02, 2004, 08:30 AM
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I always adjust the IF coils along with the RF coils. Using the earphone method they peak up nicely. Have never had a problem adjusting the IF coils.
There is a possibility they may have drifted slightly from the original setting.
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Old Dec 02, 2004, 11:51 AM
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Quote:
The Hitec HFS-04 has about 5 tuning coils
I can follow most of the discussion, however, how does one know which coil is which? I have never opened a receiver...


Fred
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Old Dec 02, 2004, 02:29 PM
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I agree with Vintage1. The only coil I would touch is the 1st RF, and then only after changing aerial length. Fiddling with the others is just asking for trouble.

The picture below shows the coils in a Hitec HFS04Mi. It has a triple-tuned front end, thus the three RF coils.

The LO coil is connected to the crystal. Adjusting this may appear to have no effect, but could cause problems such as poor oscillation or intermittant startup.

The IF coil feeds the 455kHz ceramic filter. It should have been properly aligned at the factory.

The QUAD coil adjusts the FM demodulator's center frequency. You could tweek it to account for TX/RX crystal tolerences, but it might be worse after changing crystals!
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Old Dec 02, 2004, 03:22 PM
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Schematics

PLMS,

Well... I do have diagrams for this RX, But I do not know where they are at and I am going for a trip for about a week.

The diagrams I think are for the 72Mhz version. I do fly all my small stuff on the AM version of this RX.

Martin
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Old Dec 02, 2004, 11:26 PM
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Manila, Philippines
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Guys, I hope somebody can guide/teach as how to tune those IF coil.
Here is my Idea, Say you what to tune it at 72.350 MHZ, just make a sweep generator, sweep from 72.000 to 72.990 at 10KHZ step of increment, and connected the Y input of the Oscillospcope to the Special Output of SWEEP Generator, that will Synchorinize with Sweep Generator and the X input of Oscilloscope connect to the output(Tap at output of Discriminator) of the Receiver under test.....and tweak the IF coil for miximum peak at 72.350......the Output of the Oscilloscope will display the whole Bandwide of the Receiver...its just a matter of optimizing at your operating Frequency......I've never done the above procedure...is this correct?

I have not seen a Sweep Generator, I do not know if you can sweep from 72.000 to 72.990 at 10Khz step.... I just have a simple RF or Audio Generator, Frequency counter and Oscilloscope.

To look at the characteristic Pass-band of an Audio filter, I usually do it manually, I adjust the Frequency of Audio Generator from 100HZ to 3KHZ step of 100 HZ e.i. 100,200,300...until I reach 3 KHZ, In every step I measure the output of the Audio Filter, then I enter all the data into EXCEL Spread sheet, then plot it into Graph to look at the characteristic of the Pass-band Filter, its a lot of work, Everytime I redesigned the Audio Filter, I have to plot it again manually......I just wonder how they do it in RF level....I guess I can do the same procedure on the RF level, 72.000 to 72.990 at 10KHZ, thats just 100 step.....I can treat the whole Receiver as a RF BAND PASS FILTER, then optimize or Tune-in at a particular frequency, with this procedure, you may also know how the recever respond to other frequency.
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