Crystals

[Foxfan]

I have a Code 4 set which I understand is a Hitec under licence. Trying to test it all with a Hitec TX and find the crystals are different numbers, so have ordered a channel 90 TX and RX crystals from different dealers (no match with any one dealer).

But I see there are some called "dual conversion" and some wot ain't.

Can anyone explain crystal differences please and why 2 identical looking objects are labelled Tx and Rx? Are they different inside that wee can?

Last time I had ought to do with R/C it was all 27 meg and didn't have changeable crystals. That was too hi-tech! (Forgive pun). Now saved for model boats, yachts, etc. Although my son bought me a plastic trainer last year which has 27 Meg R/C in it!

Cheers,

MrTin

[Foxfan]

Not sure if I'm supposed to do this, but erm....bump? Somebody must know crystals still!

MrTin

[FlyinBrian]

Dual Conversion crystals are for Dual Conversion receivers, I wont go into the technicalities but a single conversion rx wont work with a dual conversion xtal and vice versa.

[Phil Green]

... and although they look identical, the transmit crystal and the receive crystal are completely different.

For the standard Code-4 receiver you need a Hitec single-conversion receiver crystal.

Scoonies had plenty in stock but you have to ring them as they're not listed on their website.

Cheers
Phil

[Simon Chaddock]

Mr tin

Tx and Rx crystals are different and can only be used in their appropriate device.

So you need a Tx and Rx frequency pair (ideally a matched pair) and both must be same single or double conversion type.

Just an aside but there are also 'synthesised' receivers that have no crystal and 'tune' themselves to the frequency of the Tx but usually only to a limited range of channels.

[Phil Green]

Posted by Simon Chaddock on 11/08/2015 18:07:43:

Mr tin

Tx and Rx crystals are different and can only be used in their appropriate device.

So you need a Tx and Rx frequency pair (ideally a matched pair) and both must be same single or double conversion type.

Just an aside but there are also 'synthesised' receivers that have no crystal and 'tune' themselves to the frequency of the Tx but usually only to a limited range of channels.

Erm....

[Shaunie]

The Tx crystal is cut for the desired transmit frequency (techies, let's not get into second overtone or harmonics please).

The receiver crystal is different from the Tx crystal by the amount of the intermediate frequency or I.F. On a single conversion receiver this will be 455KHz. On a dual conversion receiver the first I.F will be 1.6MHz so the receiver crystal will be different by this amount. (A dual conversion receiver has two I.F frequencies 1.6MHz and 455KHZ). So single and dual conversion receiver crystals are different for this reason.

Dual conversion receivers are much better at rejecting interference from transmissions on neighbouring channels. with single conversion receivers there are certain channels you should avoid but I can't remember which ones they are at the moment.

You should not use crystals for equipment of a different make. So if you are using a transmitter of a different make to the receiver then the crystals will also be of different makes, this is not ideal as they are then not "matched" but to be honest crystals are cut to very close tolerances and I'm not entirely sure if manufacturers actually match thir crystals anyway.

All the above from memory as I'm almost entirely on 2.4GHz now.

Shaunie.

[Foxfan]

I have just ordered a Hitec Rx crystal and a set of three HitecTx crystals (cos that's how they came!) based on the info you gave me a while back, Phil, about the Code 4 being Hitec and those being different from others. I hope that was the right thing to do, but I don't know if the Rx is dual or not.

Thanks for the info, chaps. I'll report when I get them this week.

I got the gear very cheaply so am trying to save some costs along the way, although selling my Austin 7 Special has released a bit of "play" money for airyplanes for once, like club membership at last!

Cheers,

MrTin

[Former Member]

[This posting has been removed]

[Phil Green]

To repeat, the standard Code-4 receiver is single conversion MrTin
A genuine Hitec single-conversion crystal should have the label "35.xxx RA"

Posted by Shaunie on 11/08/2015 21:16:39:
On a dual conversion receiver the first I.F will be 1.6MHz

Erm...

[Foxfan]

Sorry Shaunie,your post hadn't appeared on my machine before I typed my last post. Not that I understand any of it, but thanks for the reassurances. I wasn't sure we'd mentioned anywhere if the Hitec(Code 4) was single or dual.

I take it Rxs like Futabas in 35 meg days were dual? I have 2 35 meg Futaba Txs, so I guess I should try to find Futaba Rxs to go with them and genuine Futaba crystals.

Thanks again, Dave.

Cheers,

MrTin

 

Edited By Foxfan on 12/08/2015 00:07:34

[John Privett]

Posted by Foxfan on 12/08/2015 00:06:54:

I take it Rxs like Futabas in 35 meg days were dual? I have 2 35 meg Futaba Txs, so I guess I should try to find Futaba Rxs to go with them and genuine Futaba crystals.

 

Some were dual conversion, some (most?) weren't. The dual conversion ones usually had "Dual Conversion" displayed prominently on the label.  Eg;

Your 35MHz Futaba transmitters will work fine (in PPM, not PCM mode) with just about any 'ordinary' (not PCM or equivalent) 35MHz receiver. Use the appropriate crystal for whatever the crystal is plugged in to. So if you use a JR single-conversion receiver then you need a JR single-conversion crystal in that receiver even if you use a Futaba transmitter with it. Similarly a Futaba transmitter needs a Futaba transmitter crystal regardless of the make of receiver you use it with.

 

Edited By John Privett on 12/08/2015 00:43:12

[Peter Miller]

Posted by Simon Chaddock on 11/08/2015 18:07:43:

Mr tin

Tx and Rx crystals are different and can only be used in their appropriate device.

So you need a Tx and Rx frequency pair (ideally a matched pair) and both must be same single or double conversion type.

Just an aside but there are also 'synthesised' receivers that have no crystal and 'tune' themselves to the frequency of the Tx but usually only to a limited range of channels.

You do not need a double conversion crystal in the transmitter. In fact you don't get "double conversion" crystals for transmitters.

[Engine Doctor]

Back in the crystal days , not that long ago really , you had to know what crystals would work in what . Jr in Jr . Sanwa didn't know what worked in what ! Futaba crystals were the standard and would work in a lot of generic/aftermarket Rx's .They could also be used in Hitec sets .Dual conversion were for dual conversion Rx's only and make specific .

Only use dual conversion Xtal if the Rx has Dual conversion written on the case It wont work with normal xtal . Tx's used normal TX crystal Marked TX on the tab . If.you do find your set works with different make crystals , do a thorough range check with motor running as vibes can badly effect dodgy crystals and sets have been known to stop working at certain ranges then work again when further away . If you fly electric then think seriously about using 35 or 27 meg sets as they can be seriously affected by electrical noise from the power train . For safety and reliability better to use the old set once converted to 2.4 with a Fr-sky module . No crystals ,no fuss no, crashes .

[Phil Green]

Just to comment on a couple of points made earlier...

>>So you need a Tx and Rx frequency pair (ideally a matched pair) and both must be same single or double conversion type.
In our context theres no such animal as a dual conversion tx crystal. Yes there are heterodyne tx designs, but not in conventional R/C model radios.

>>there are also 'synthesised' receivers that have no crystal
Synth receivers do have a crystal, dual conversion synth receivers have two.

>>On a dual conversion receiver the first I.F will be 1.6MHz
first IF is 10.7mhz, second is usually 455khz but some are 465 (Macgregor etc)

>>Dual conversion receivers are much better at rejecting interference from transmissions on neighbouring channels.
Not really, they give better image rejection, its the 2nd IF at 455khz that gives channel selectivity

>>Tx and Rx crystals are different and can only be used in their appropriate device.
True in this particular context but as a generality, this isnt the case. For example 27mhz AM sets Tx & Rx crystals can be interchanged as they are of the same spec, to give a 'new' (albeit illegal) operating frequency

>>You should not use crystals for equipment of a different make.
A popular misconception. Hitec, Futaba, Multiplex, etc do not make crystals, they buy them in, to a spec. If you order to the same spec, you can use crystals from any reputable quartz company.

>>I'm not entirely sure if manufacturers actually match their crystals anyway.
No they dont, but each one is accurately cut for the specified load capacitance by the crystal manufacturer (ie the crystal manufacturer, not Hitec or Futaba)

Sorry to be pedantic, just trying to help

Cheers
Phil

[Foxfan]

Wow, nw I'm getting more than I can cope with, but many thanks, anyway, chaps. I really don't get all the number stuff, but I'm sure others do.

Doc, My old chum only has 35 Meg and flies with that without a problem, including 1/4 scales and some superb scale models he designed and built.

I would use my never used 2.4 gear if I went electric, which I may do on an Ugly Stik and a Skystreak 32, when I am a bit more proficient a pilot. I am just going through the radio gear I happen to have to see what "sets" I can put together. The Code 4 will go in a Super 60 I am building, with an Enya 35. The 2 Futaba Txs in 35 Meg, I shall attempt to match to suitable Rxs. The 27 meg stuff will have to be kept for my various boats. They are old enough not to have changeable crystals.

The 27 meg Commander trainer is a bit of a mystery. I will have to replace the built in "brick" with some lightweight 2.4 stuff, I think. Not sure I trust the fishing line and elastic band return flying surfaces actuation!

The 27 Meg brick can be put in a tiny model boat I have.

I take it I will need Futaba Rxs for the 2 Futaba TXs I have.

Many thanks for your help.

Cheers,

MrTin

But I have

[kc]

Shaunie's explanation is one of the clearest I have seen. To make it complete though the frequencies which can interfere are those with 23 channels diffference when using only single conversion 35 mhz (if I remember correctly. ) So for example channel 61 possibly interefers with ch 84 on single conversion but not with dual conversion. But not the other way round e.g ch 84 does not affect 61. Again from memory I cannot find it in writing!

 

If you are a club member then asking other members if they have any 35 Mhz stuff going spare should result in someone donating working stuff to you if you are lucky.  Just put out a rumuor that you are looking!   Most of us have some good stuff going unused now.

Edited By kc on 12/08/2015 12:09:05

[PatMc]

Posted by Foxfan on 12/08/2015 11:55:24:

 

I take it I will need Futaba Rxs for the 2 Futaba TXs I have.

No, virtualy any other make of Rx will work so long as both Tx & Rx are on the same channel. The rare exceptions are 35AM Rxs & Non Futaba PCM Rxs.

With so few people using 35meg nowadays dual convert Rxs have virtually no advantage over single convert, the problem DC solved was pretty rare even when 35meg was king.

If you're joining a club don't bother buying any 35meg Rxs or Xtls but have a word with some of the other members. You'll probably be offered some no longer needed ones for nowt or for next to nowt.

Crossed post advice on other club members.

Edited By PatMc on 12/08/2015 12:14:44

[Foxfan]

I've ordered a pair of Hitec type crystals of number 90, because the suppliers had only a few of some frequencies. 90 tied in as a a Hitec pair from different suppliers.

I do hope it all works after all this!

Cheers,

MrTin

[Foxfan]

I don't know what is wrong with this machine, but it doesn't post replies very quickly and I miss a couple, so apologies for the apparent disjointedness.

I will ask around at the club about unused stuff. Perfectly happy to pay for it, though.

I might make up a few "sets" after all.

Cheers,

MrTin

[PatMc]

BTW you do realise that you could just use a single Tx of any number of Rxs don't you ? If you prefer to use one particular frequency then you don't need to change the Tx Xtl when for each model just have several Rx Xtls all on the same frequency, I used to use ch 60 for all my models - still do when I'm not on 2.4GHz.

[kc]

I suppose we should also stress that anyone using 35 mhz needs to keep a sharp lookout for any other person using 35 mhz - i.e. anyone with a Tx with an aerial. Frequency control is now very lax or nonexistant at many clubs now so beware! Find out who still uses 35 at your site and keep a lookout for them sneaking in when you are flying. Let everyone present know you are on 35. And of course remember to extend your aerial .....nobody else will notice and warn you if you dont! Nearly everyone uses 2.4.so its not obvious to bystanders,

[Foxfan]

Pat and kc,

very good advice. As far as I know I would be the only one on 35 Meg, but I would certainly check thoroughly. There's never more than 3 or 4 flying anyway. Most seem to go for a chat. Since I'm a beginner, they'd probably give me the space to start with!

Pat, that's a very good point I hadn't considered, thanks.

Cheers,

MrTin

[Mike Blandford]

Dual conversion solves a specific problem of interference. Take single conversion with an IF (intermediate frequency) of 455kHz (0.455MHz).

Suppose your transmitter crystal frequency is exactly 35.000 MHz (channel 60). Your receiver crystal frequency is likely to be 34.545 MHz (35.000-0.455). The received transmitter signal and this receiver frequency are mixed together. When you do this, you create, amongst other things, a signal at the difference of the two frequencies, 0.455MHz = 455kHz. This is then picked out. The problem is that the difference between 34.100MHz and 34.545MHz is also 0.455MHz, so the receiver will also respond to ANY transmission on 34.100MHz. Because 34.100 and 35.000 are not very different, the receiver cannot be designed to reject the 34.100MHz signal.

Now if you use dual conversion, the IF is 10.7MHz, so at a Tx frequency of 35.000, the receiver crystal would be 24.300 MHz (35.000-10.700). The possible interfering frequency is now 13.600 MHz (24.300-10.700). This is far enough away from 35.000 that the receiver "front end" may be designed to reject it, thus significantly reducing the possibility of interference.

Mike.

[kc]

Whilst it is normally good manners to offer to pay for anything, I think in relation to 35mhz gear it is different. Ordinary 35 mhz gear is so low in value most people would sooner see it go to a good home for nothing Frankly it is now a form of recycling and decluttering! Same with computers we would all like to find a good home for the unused stuff rather than dump it. If you get given any good stuff then just remember you owe a favour

However beware the old nicads will likely be too poor for reliability and will be a liability. replacing a TX nicad might cost nearly the cost of a basic 2.4 set Test extensively before trusting old nicads.

If you are going to spend money put it into 2.4 stuff.