Topic: Cheap 27Mhz crystals (Read 9413 times)

barriew · « **on:** July 29, 2006, 05:40:11 pm »

Unlikely, but true! According to a fellow cub member, Halfords have sets of 6 pairs of crystals for £15.99.

Barrie

Adzam · « **Reply #1 on:** July 29, 2006, 11:17:20 pm »

Hi barriew
it sounds like these may be the sets available with the Xmods range of model rc cars ( 1/43 scale) i`m unsure of their compatibility with our common 27 Mhz system, i`ve a funny feeling they MAY be FM.if anyone could shed light on this it would be great

Eddy Matthews · « **Reply #2 on:** July 29, 2006, 11:24:23 pm »

27Mhz FM Chris? I thought ALL 27Mhz sets were AM?

I don't doubt what your saying, but I've never heard of 27Mhz FM before, so yes - Clarification would be appreciated if anyone knows?

Regards
Eddy

Youngat65 · « **Reply #3 on:** July 30, 2006, 08:42:29 am »

many years ago ..........The Old Man Said ......there was a Futaba M 27mhz set which was FM looked exactly the same as Futaba M Am sets but the box was silver

Adzam · « **Reply #4 on:** July 30, 2006, 08:54:43 am »

Eddy, i`m trying to find out about these crystals.

Im sure there is something about them that isnt run of teh mill, thats if it is XMODS crystasls we are referring to here, Xmods by the way are small model cars that come complete with their own RC gear, not the standard stuff you buy, and the crystals are purely so you can race other cars at the same time,

if i find out about these i`ll post here

barriew · « **Reply #5 on:** July 30, 2006, 10:57:56 am »

I've seen them working in an old Futaba M series set! I don't do 27Mhz myself, all 40Mhz, but two members now have these sets and report they are OK.

Barrie

meechingman · « **Reply #6 on:** July 31, 2006, 09:27:06 am »

OT slightly, but could someone explain the 'difference' between AM crystals and FM crystals. Is it just the frequencies we're talking about, as the job of a crystal is just to act as prime part of the oscillator for the carrier, with one or the other types modulation then applied to the carrier. The type of modulation is of no concern to the oscillator itself, presumably, as it's done its job by then.

Andy G

DavieTait · « **Reply #7 on:** July 31, 2006, 12:10:23 pm »

In AM you modulate the wave by increasing and decreasing the power so you have spikes in the Tx signal that can be decoded by the Rx. In FM you are sending out a series of pulses ( like morse code ) but NOT changing the amplification on the carrier wave so there is a difference in the needs of the crystals and thats why you can't use one type in the other.

Davie

meechingman · « **Reply #8 on:** July 31, 2006, 02:00:24 pm »

Interesting comments. However, the modulation in both cases is applied after the carrier frequency has been created. In AM by applying the modulating signal to the output level of the carrier, thus varying the amplitude, as you say, and in FM by applying the modulating signal to the original, in analogue form, not actually as a series of pulses. I have ham radios that offer both types of modulation. I can't quite see why the 'needs of a crystal' would be different in both cases.

Any more thoughts?

Youngat65 · « **Reply #9 on:** July 31, 2006, 04:53:38 pm »

Your Ham radio most likely has synthesized frequency not individual crystals for each frequency ??? ???
Cheers
Bob B (G4UBM.....Usless Brainless and Mindless)

wombat · « **Reply #10 on:** July 31, 2006, 05:07:05 pm »

Thinking about this one, and I could be wrong (frequent occurrence)

For AM transmission, you have a fixed carrier frequency - this will be centred on the middle of the channel band.
FOr FM transmission, the carrier frequency is varied by the control signal. Because of the narrow channel spacing, for FM, I suspect that SSB or SSB-SC transmission is used. This would mean that the frequency is placed at one end of the channel band. So FM crystals will work in an AM set and vice versa, but the "positioning" of the signal in the channel will be wrong and you will end up spanning two adjecent channels.

Tim the Wombat

Peterm · « **Reply #11 on:** July 31, 2006, 05:19:21 pm »

The last statement on modulation is correct. Crystal frequencies are nothing to do with modulation. Trying to keep it simple, the transmitter crystal determines the frequency of the transmission on which control information is `imprinted`. The receiver crystal is used to control the frequency of the receiver oscillator against which the incoming transmission is compared. In a single conversion receiver the difference between a matched pair of crystals is approximately 455 KHz. This `difference frequency` is processed by the receiver to obtain the transmitted control information. In a dual conversion system the transmitter frequency can be the same, but the receiver has two oscillators. The first one is crystal controlled and the `difference frequency` is approximately 10.7 MHz. The second oscillator produces a second `frequency difference` of 455 KHz which is then processed. Hopefully this explains why it is the receiver type and crystal frequency which is important. If I haven`t made a very good job of the explanation. apologies. Pete M

Aston · « **Reply #12 on:** August 01, 2006, 12:03:34 am »

for what it's worth (not much !!) Sanwa used to do 27 FM gear, I had some, and I can assure you that it don't work with AM crystals, and the FM crystals don't work with AM sets. I found the first one out by accident :-[, and the second by design ::)

Aston

RickF · « **Reply #13 on:** August 01, 2006, 12:11:55 am »

Interesting technical discussions guys, but the bottom line is still "Will these bargain crystals work with your average R/C kit?" Has anyone bought a set and tried them?

Rick

Youngat65 · « **Reply #14 on:** August 01, 2006, 08:37:15 am »

Worked for me ::) ::) ::)

barriew · « **Reply #15 on:** August 01, 2006, 10:37:33 am »

As I said,

I've seen them working in an old Futaba M series set! I don't do 27Mhz myself, all 40Mhz, but two members now have these sets and report they are OK.

Barrie

ambernblu · « **Reply #16 on:** August 01, 2006, 05:49:41 pm »

.... Ok, I'll go down to Halfords tomorrow and get a set - and give 'em a fair trial in my two boats this weekend...... just don't all rush at once to offer to rescue them from the middle of the lake.

Youngat65 · « **Reply #17 on:** August 01, 2006, 05:53:11 pm »

Got a pair of swimmimg truncks haven't you , water should be quite warm after all this weather we been haveing , good R.C.,s usualy do a range check as well ::) ::)
Happy Swimming
Bob B

ambernblu · « **Reply #18 on:** August 01, 2006, 10:12:46 pm »

.... hey Bob, and I thought you sounded confident that these crystals will work... :-\

Can't say I'm keen on having an impromptu dip though even if the temps have been sky high - I think I'd rather walk around the lake with a big ball of string!

OMK · « **Reply #19 on:** August 01, 2006, 11:49:25 pm »

The spotty kid in the Halford's around our way didn't have clue. The manager wasn't much better. Says he's not aware of the company selling any such crystals.
How did this subject get around to AM / FM anyway? All BarrieW did was mention that Halford's were selling them.
Shame I couldn't buy some though. You can bet they'd be bundled "For AM use Only". Ideal for FM transmitters.
If you stuff the pulses from the decoder up the crystal, each pulse will deviate the crystal oscillator from its resonant frequency. It's this shifting of frequency that gives us frequency modulation. Someone here said that the modulation in both case (AM and FM) is applied after the carrier frequency has been created. Not so. In the case of AM, yes. FM, no.
Not me of course, but I know people who've been resonating old 27MHz xtals on 81MHz FM for years.

A Model World · « **Reply #20 on:** August 02, 2006, 08:26:35 am »

Yes PMK Im with you on this, called in yesterday to get some bulbs and thought Id ask, Wished later i hadnt though I was maybe in the same branch as a spotty kid replied "crystals for washing your car with" mmm tried explaining but from the look on the face it wasnt going in Manager wasnt much different (less spots) similar gaunt look. Back to you BarrieW to track down club member I think

Martin (Admin) · « **Reply #21 on:** August 02, 2006, 09:13:43 am »

http://www.xmodsrc.com/

barriew · « **Reply #22 on:** August 02, 2006, 11:59:41 am »

The crystals were bought in the Burton 0n Trent branch of that esteemed organisation(??????), I believe. I'll ask at club night tonight to make sure.

Barrie

BlueWotsit · « **Reply #23 on:** August 04, 2006, 09:58:30 am »

I had months previously bought a set and they did work fine with my bog standard Futaba TX

Theres a guy selling them on Ebay and when I asked if they were totally suitable for using with boats he said no - cars only, which didnt really make sense as I have used my normal 27mhz crystals with cars as well

dougal99 · « **Reply #24 on:** August 04, 2006, 10:42:06 am »

If crystals, of whatever frequency, work with your equipment (of whatever frequency) then they will work in boats or cars (or aircraft). The limitations of 40mhz to ground, 35Mhz to air and 27Mhz to either are an administration decision not a technical limitation.

HTH

Doug

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Author Topic: Cheap 27Mhz crystals (Read 9413 times)

Adzam

Eddy Matthews

Adzam

Aston

ambernblu

ambernblu

OMK