David Cutler

Maybe as replacements for faulty or broken ones without changing channels?

Put simply, a dual conversion crystal goes through the process of checking if the channel is the correct one twice. It has the effect of making the receiver less prone to interference. Dual conversion only applies to receivers, not transmitters.

You can change your receiver crystal to match your transmitter without worrying about the legal aspects as the FCC doesn't stipulate that you shouldn't change the receiver crystal. The question of cross talk being transmitted to other channels doesn't arise if you change a receiver crystal because, well, ummm, the receiver isn't transmitting!



-David C.

RC-Captain

.

hmmmm are you sure about this , I have been receiver shopping for a month now and I know with a dual conversion receiver you get 2 crystals . So if one goes in the receiver where in the world would I put the other one ? Also single conversion gives you only 1 crystal .....Just trying to understand

Luke 3D returns

In the UK we change our crystals all the time- its legal too.
It a common thing. Never heard of a problem caused by crystals clashing over here, even on adjacent frequencies.
To say that changing crystals detunes your radio and shoots down another guys plane is absolute rubbish. The channel i now mostly use did not come with my transmitter, and yet, when i book in to transmitter control at a show and get my frequency checked, it comes up with exactly the right frequency. So do you want to say again that my radio is de-tuned Glen?

I remember reading somewhere that there were other industrial stuff operating in bettween the channels in the USA. Is this true?
If it is then it may be possible that if you change to a defective crystal you end up getting interference from whatever is operating bettween the frequencies, but affecting another guy's plane?[:'(]

I cant imagine how difficult it must be for you american flyers not to change crystals. What happens when a beginner turns up at your field on your frequency?
In the UK its simple- the beginner changes to another frequency. Everyone has been a beginner at one point, so most people here will jave had to permenantly change to another frequency at some point.

Ask an average modeller over here if they have had their radio tuned they will not know what you are talking about, but i suspect your radio system may be slightly different to ours.


Anyway, someone please settle this matter. Get a frequency checker, change the crystals, test the signal, and see if the signal is slightly off.

tadawson

Technically, there is no such thing as a "Dual Conversion Crystal" there are crystals on frequencies to go into a "Dual Conversion Receiver", but other than being on a different base frequency than if that RX were single conversion, there is nothing special there. There ARE typically two crystals in a dual conversion RX, but the second one never changes and is hard mounted into the unit. To try and clear things up here, I will take just a second to explain dual and single conversion . . . .

In single conversion, the signal from the antenna is taken in and electronically mixed with the signal from the crystal (the one you put in) to give an output frequency, typically at 455 KHz. This is called the Intermediate Frequency, and will never change, no matter what channel you are on - the crystal you put in ensures that the desired channel mixed with the crystal frequency will always be 455 (or whatever - nothing magical about 455 - just typical.) This signal is then run through a series of filters to remove everything other than 455, and then decoded. The fact that this happens once is why it is called Single conversion. Note that any DSP or other "exotic" filtering done at the 455 KHz level is not reflected by the conversion status.

In dual conversion, things start out as above, but the crystal you plug in is a different frequency, and gives a much higher Intermediate Frequency output (typically in the 5-10 MHz range ) but as above, it is a constant in a given receiver. This is filtered and then mixed with the output of ANOTHER crystal, to give the 455 KHz output, which is filtered AGAIN prior to decoding. Since the input and output frequencies to this second mixer are always the same, you never change that
crystal, 'cuz there is no need.

Please note that the primary advantage of dual over single conversion receivers is what is called Image frequency rejection. On the mixers described above, any signal either ABOVE or BELOW the first crystal frequency by the first IF frequency will come through - the one you want, and the "Image". Since the first IF is higher in a dual conversion than in a single, the Image frequency can be filtered out more effectively in the front end of the receiver. Not that this cannot be done as well in a single conversion receiver, it just takes better quality parts, and as such, will cose more $$$, so it all depends on what the Mfg. wants to spend. This is why the really cheal single conversion part flyer receivers suck like no other, but a good JR, Berg, FMA, etc. single conversion receiver does not. It is not about the inherent design, but about the quality of the design and build. As such, those who claim that dual conversion is always better, pretty much display an ignorance of the subject in their comments, and are pretty much guaranteed to be wrong in most cases. Please note that I did not say that single conversion is better either! It all depends on where the interference is at your site. A strong signal at the image of a dual conversion RX will be ignored by a single, and vice versa - it all depends on what problem you are trying to solve! Keep in mind too, that in the communications world there are also triple conversion receivers, but I have never seen one in RC!

That, and the single/dual argument totally ignores other types of filtering such as DSP, JR's ABC&W, etc, which effectively can make the single conversion units work as well or better than dual conversion units. It all depends on the quality of the design and components, and not single/dual!

In the transmitter, there is a similar process, but the single/dual issue does not apply, since there is no external interference to deal with. Typically, the transmitter crystal frequency is a lot lower than the desired output, and then it is run through a series of frequency multiplyers, mixers, and filters to get to the desired output. Note that "multipliers" are not like mixers above, and can be 5x, 10x, 20x, etc. As such, the crystal frequency needed to give a certain channel output is almost never the same as in the receiver. Once again, like in the receiver, all Intermediate Frequencies are typically constant until the last stage, where the crystal which determines the channel in introduced, and the mixture of it's output and the last Intermediate Frequency gives the desired output, which is amplified and sent to the antenna. Note that since everything prior to this point is fixed, there is nothing to tune on a channel change! If the I/F frequency is clean, then the output of the final mixer should be too. About the only thing which can be done is to peak the power amp for maximum output, and to ensure that the combination of the crystal and it's associated circuitry is running at the correct frequency, although frequently, this is not adjustable either - the circuit components are chosen with tolerances that given a non-defective crystal, that the output will always be in specification.

Oh, and one last comment for the guy who commented

"Heck I don't even know why we have to have crystals in our receivers. I've got a perfectly good receiver in my car that I can tune to any station I want. All I need on my airplane receivers is a knob and a place to plug in a black box with a signal strength meter for goodness sakes !!!"

Sorry to burst your bubble, but yor car radio is STILL crystal based. What is going on there (as well as all RC synthesized gear) is that there is still a crystal which gives a stable, know, input frequency to the synthesizer and it changes multiplications, etc. to get the desired output for your channel/station. Still crystal based, but you don't have to change it!

And my apologies if this is too "deep" and long winded . . . . I'm just trying to add some "light to the darkness . . . "

- Tim

David Cutler

Thanks Tim. Excellent message if I may say so!

-David C.

RC-Captain

Im still confused with the wording :


the crystals mentioned above are from tower hobbies . I did a search for crytals only not receivers or transmitters so please explain to me what you mean by saying :

if Im buying a crystal and Im being asked do I need single or dual , I can only beleive the crytals are what represent single or dual conversion, keep in mind all I did was a search on crystals not receivers or transmitters.

tadawson

What is meant in your quote is that the crystals in question are meant to go into either a dual conversion or single conversion receiver. There is no physical difference in the crystals other than the frequency they are cut to. I may have added more confusion than clarity here, if so, I am sorry . . .

- Tim

RC-Captain

if thats the case,the retailers of the cryatals should explain this a little further than just single or dual , It may be simple for them,which is hard to beleive,but for a new RC scout its confusing No appology is needed you cleared it up for me . Thank You. One more question , I was asked if I had a dual or single conversion transmiter when try to by a receiver, please help me understand why.

David Cutler

\

My guess is that the person who tried to help you in the store didn't know what they were talking about! It does happen occasionally!



Dual or single conversion applies to the receiver, not the transmitter.

-David C.

tadawson

Exactly! The only thing which will determine receiver compatibility is modulation (FM/AM), encoding (PPM/PCM) and shift (modulation direction, on FM). A receiver is tuned to a given frequency, as is the transmitter. As long as that frequency (your channel number) matches, it matters not if it is a single conversion or sextuple - the net result is the same - they are on the same freq. and will work. I agree with the statement that the guy at the shop really didn't know what he was talking about.
AM/FM(PPM)/PCM and brand are all relevant questions, though.

- Tim

Lynx

Just don't put a single conversion crystal in a dual conversion receiver.

Phil Cole

Tim,

I've never seen a crystal controlled RC transmitter with an IF. They have all run the crystal at the output frequency, or had a single frequency multiplier stage.

tadawson

Phil -
I was speaking from a radio theory point of view, not necessarily authoritatively as to R/C gear. I am a ham licensee, and was speaking more towards VHF gear in general. In any case, at the low powers that our gear runs, the statement about not much in the way of tunable components in the transmitter is still true. At these low power levels, broadband transmitters are easy, and meet the emission specifications handily.

- Tim

quemazon

My Feedback: (1)

How do you break a xtal? Usually by dropping your TX on the ground or some other severe impact. This is one occasion when you really should be sending the TX back for a retune, yet it's the one occasion that most anti-xtal-swappers say it's OK to swap. Quite ironic.

Think about it...for some reason your TX stops working. Maybe you've dropped it on the ground or left it in the car on a hot day, or nothing at all. So you surmise by some act of total guess-work that your xtal is bad. But, what if it isn't the xtal? You'll be fooling around popping in a new xtal and testing it, while quite possible generating harmful interference because some part of the circuit, other than your xtal, is detuned.

If your TX stops working, you should send it in for repairs, not try new xtals. The argument that hobby shops sell TX xtals for when one breaks is totally bogus.

RC-Captain

I think you misunderstood my queston: I was simply trying show the hypocrytical reason why people ,including myself at one time, beleive its ok to swap there own TX crytals.

I AM TOTTALLY AGAINST THIS ACTION .......especially after I lost a plane which I think was due to someone splashing into my frequency.

ALbert.S

I think the whole issue is resolved by looking at the AMA website at the bottom of the application form where it has a place where you sign and promise to obey the rules of the AMA Saftey code It also says that your insurance is void if you have a problem and have not followed the rules.The code clearly states that you are not to change crystals Now if you are a AMA member and a person who keeps your word I don't see where there is a problem

pete913

Bingo! There is nothing in the LAW saying that any CB'er, or anyone else for that matter, can't go out and buy a 2 kw linear amplifier, ( or build one) . Just don't ever get caught using the thing on the 11 meter CB band, or on the ham bands if you're not licensed for it. That IS illegal, and will get you a serious big fat fine, last time I payed attention it was in the neighborhood of $10,000, possibly confiscation of ALL your equipment, and possibly some heavy duty jail time. No ethical business would ever sell an amp to someone not licensed to use it, but ethics and the law are two different things. Personally, I don't have a problem with anyone changing crystals. Just be prepared to show on a spectrum analyzer that the transmitter is in limits before flying it, each and every time. Cause if you shoot me down with it, the law better be there before I am.

3d-aholic

My Feedback: (7)

Sorry to burst your bubble Tim...but I never said there was no crystal. The fact is the crystal is soldered to the board and you can change the frequencies of reception with a knob. There is NO reason why this can't be done in our case except that if you don't do it right, you have a crash.

quemazon

My Feedback: (1)

Hmm...I'm not an AMA member, so I guess it's OK for me to swap xtals.

3d-aholic

My Feedback: (7)

It doesn't say that....though...your dreaming.

That has been reestablished in this thread....it clearly DOES NOT say that. Some folks have made some inference that it does...but it definetly does not say that CLEARLY anywhere.

JPMacG

My Feedback: (2)

Interesting discussion. I have an FCC General Radiotelephone License (once had First Class Commercial, but the FCC discontiued that). I also have a frequency counter and a spectrum analyzer. So am I authorized to change my crystals? Or do I need to have FCC approved test equipment? Or an FCC approved service business?

ALbert.S

I guess I didn't give exact instructions on how to get to the AMA rule on changing crystals.So here it is
1.Go to AMA website
2.Go to Publications
3.Click on 2003 membership manual
4.Scroll down to page 17

3d-aholic

My Feedback: (7)

Ok, that appears not be allowed by the AMA and that IS much more straight foward. However, the FCC regulations are a different thing.

In fact the AMA guidelines indicate it is illegal to use a HiTec module in a Futuba radio as well...and I know some people do that too.

ALbert.S

Yes I agree the FCC Regs are poorly written and thats being very kind. In all fairness I think some Mfg found the loophole in the rules and made the crystal easily accessible and plugable and then sell the crystals in sets.Why not get some extra bucks from the modeller when he just wants to change a receiver.Long ago I was stopped for a loud muffler and got a ticket I told the judge I bought the muffler in good faith and thought that if it was sold it was legal to use.He pounds his gavel and said "Selling is not illegal using is pay the Clerk $75"
I BOUGHT A TRACKER ll END OF PROBLEM I'll have some crystal controlled transmitter's for sale soon!!!!!!!!

3d-aholic

My Feedback: (7)

Yes, I have a 9C and I'm waiting for the long awaited Freq. Synth.

No doubt the manufacturers are making a killing on this stuff....Futuba is dragging their heels on the Freq. Synth at the expense of market share to Hitec...and now Tracker II. The crystal $$'s in the front-end is going to cost them dearly on the back end in lost market share....which they won't fully realize for another 2-4 years.

And Tracker II is really just a competitor they created out of thin air over this issue.