ddang

Hi,

I have a JR 10 SX transmitter and recently bought a feather-weight single conversion receiver. When I try to put my JR 945 receiver crystal into the new receiver, it did not work.

Is it b/c I am using a dual conversion crystal in a single conversion receiver? If so, can I just buy a single conversion crystal for my new receiver without changing the crystal on the transmitter?

Please help.

david

Peter Khor

My Feedback: (4)

What single conversion receiver did you buy? (no brand/model mentioned). You will need to get a xtal that fits the receiver you bought. Weather or not your your RX is single or dual conversion doesn't matter to your TX.

Mighty Mik

You need a SINGLE conversion crystal for that Rx.

ddang

Thank you guys for your quick help.

NitroWoman

What is the difference between a single conversion and a dual conversion crystal? I never knew the number of conversions in a super-het receiver had anything to do with the crystal .

A.T.

There are two types of transmitter crystals for RC use = AM & FM.
There are three types of receivers crystals for RC use. = Single conversion [single conversion FM and AM receivers use the same crystal - refer to Hitec xtyal packets which show AM/FM single conversion.] and Dual Conversion for which only FM receivers are available.
The TX does not care whether you are using a single or dual conversion RX.
To quote Max Fell =
The crystal frequency required is determined by the local
oscillator frequency at each stage, plus or minus the first intermediate frequency. For single conversion the IF is 455 kHz but for dual conversion it is usually 10.7 MHz which explains the big difference in crystals.
For example if you are on 72.250 MHz, then the crystal frequency will be 72.250 MHz +- 455 kHz for single conversion but 72.250 MHz +- 10.7 MHz for dual conversion. The plus or minus depends on whether the rx is designed for high or low side injection. The second stage of a dual conversion rx always is going from 10.7 MHz to 455 kHz so thecrystal is fixed and never needs to be swapped.
For full details refer to the many posts on Advanced Google search of rec.models.rc.air =
http://groups.google.com/groups?as_o...=&num=30&hl=en
(copy and paste entire URL)
regards
Alan T.

Steve Lewin

Sorry, but you have no idea what you're talking about. The difference IS the crystal frequency ! Receiver crystals DO NOT work at the notional channel frequency but at a frequency which is different by the internal local oscillator. For SC this is a 455KHz difference. For DC it is always 10.7MHz. That's easily enough difference to be sure that one will never work where the other is expected.

Steve

Rodney

Whirly Bird, better listen to Steve as he is correct. Maybe you need to check the accuracy of your freqency meter.

Steve Lewin

What "counter" is this and how exactly are you doing the measurement ? When you say "XTAL frequency" do you mean the nominal channel frequency ? Naturally you can measure the crystal frequency, it's just not the same as the channel frequency and a DC crystal is not the same as an SC crystal. A basic grasp of radio theory will tell you that and I've checked enough crystals to be 100% confident that there's nothing wrong with the theory .

If you've got a working Dual Conversion radio with a first LO running at CHANNEL frequency you've just broken several laws of physics.

Steve

Ladyflyer

My Feedback: (2)

Quote:

" If thats the case then why do I get the XTAL frequency reading on my counter when I have a XTAL inserted in my dual conversion RX.
The XTAL is a par of the LO circuit"

Just WHERE are you coupling you frequency counter in the receiver ?
Are you reading the local oscillator or one of the IFs ? If an IF which one ?
The receive crystal is cut for a different frequency than the marked channel frequency for the reasons explained. In fact if you put the crystal in an oscillator circuit and measure you will see it's FUNDAMENTAL frequency. The fundamental freq is multiplied in an RC transmitter to the intended operating freq.
The receiver crystal has an IF offset freq and it too is cut for a fraction of the intended freq.


Some folks have tried putting the receiver crystal in the transmitter. Unfortunately they have no idea where they are transmitting. Hopefully not where any interference is generated.

A.T.

Previous posts have given good clear information with regard to how RC circuits operate but obviously more information is required, please read "Everything You NEVER Wanted To Know About Radios ! by Ian Hirschsohn =
http://www.torreypinesgulls.org/Radios.htm which has an in depth explanation, which I endorse and saves me typing out. Iin particular note =
quote
"Bottom line: Dual Conversion is needed for FM to eliminate in-band interference from other transmitter.
AM receivers don't need Dual Conversion which explains why they are generally cheaper, lighter and more robust.
The frequency of the receiver crystal is NOT the channel frequency e.g., the receiver crystal for channel 14 (72.070 MHz) is NOT 72.070 MHz but displaced by the IF frequency. The exact IF frequency is vendor dependant so DON'T MIX CRYSTALS FOR DIFFERENT VENDORS. (e.g., don't plug a Futaba crystal into a Hitec receiver and expect it to work.)
Transmitters have no Superhet mixing so transmitter and receiver crystals CANNOT BE INTERCHANGED (transmitter xtals also use "5th overtone" so they are a totally different animal"
It would appear that your "counter" <which make and model number?> is defective or you are not using it correctly.
a LOT more info is also available under both
"Radio Systems, Accessories, Alterations and FAQ"
at http://homepages.ihug.co.nz/~atong/
and please never, never turn you TX on with a RX xtal in the TX within less than 4 miles of where our club is flying.
regards
Alan T.

tiggerinmk

ddang hasn't replied in a while, maybe it's because you guys have gone slightly off the topic.

I don't think "more information is required". It was a simple question and just requires a simple, to the point, correct answer.

The theories and technicalities of how radios work can be discussed elsewhere.

Incedentally, dual conversion is not absolutely required to eliminate interference. My JR single conversion receivers have worked perfectly well so far with no interference.


Back to the question.....

In another thread I asked about some JR dual conversion crystals I had mistakenly purchased. The following is the answer to mine and another question from the HH tech:

"Yes, PCM and FM crystals are the exact same thing, same part number. JR's dual conversion crystals work in the 940 and 945 receivers. All other current JR receivers use single conversion crystals.".


Bottom line is, the RED JR RX dual conversion crystals are for the 940 & 945 receivers only. All the other JR receiers use the GREY FM crystals.

If you use any other receiver (such as Hitech) use the correct crystal for that receiver from the same manufacturer.

Simple innit?

Gary Retterbush

ddang probably hasn't come back in because he got the correct answer in post #3.

More information was then required to assure that all concerned did in fact realize that there are differences in single and dual conversion crytals. Post #5 brought this requirement to light.

I was under the impression that this forum was exactly for gaining knowledge about radios and how they work. From the looks of things, some sure could use the help.

Dual conversion can be a real saver in some cases, especially here in Germany with 35MHz and commercial FM stations.

Happy New Year to all

Gary Retterbush

WB,

Please read what I wrote. I did not say anything about it being off topic. In fact, if you read it I think you will see that I said exactly the opposite.

StarskiPZ

Hi all!

My understanding is thus:

You have brand 'x' transmitter & you buy a crystal which shows the CHANNEL No you want, and on it are the frequencies.

The trany will transmitt on the frequency relevant to the CHANNEL No, & not nesseccarily on the crystals stated freq numbers.
Or in other words its how brand 'x' uses its crystals compared to other brands.

So its a shame you cant reliably change crystals between brands.

So, I can't wait for the synthesised crystal-less gear thats just hit the market..........!

Gary Retterbush

We all seem to think this way at first. But, think a little deeper to what it means as far as frequency control is concerned. Without some type of safety feature (such as the MPX "channel checker") you never know who's on first base or how long he is going to stay there! It could very well end up being a total disaster! It was bad enough when folks forgot what crystal they had installed but this might be a lot worse.

Steve Lewin

It looks like you are saying that you swapped crystals over and everything still worked. By swapping or switching I take it that you mean you put the transmitter crystal in the receiver and the receiver crystal in the transmitter. Yes ?

I'd be interested to know exactly what R/C equipment you are using, makes and model numbers please. I've only used JR, Futaba, Hitec and Airtonics/Sanwa equipment. I know for a fact that swapping Tx and Rx crystals will not work in any of those (and not just because the crystal frequencies are different, they are different in other ways too).

So based on my experience there are only 2 possibilities. Either that isn't what you are saying or you are lying for some odd reason of your own. Which is it ?

BTW picking frequencies up "out of the air" does not measure anything in the receiver at all, just the transmitted frequency.

Steve

StarskiPZ

Your quite right!

First one down chooses his/her lucky number!

Its a good job the sets are expensive !

Its got to be better than now; I've turned up at my local to find only two other fliers - BOTH on MY channel

Ladyflyer

My Feedback: (2)

Thank You Steve.
And of course a receive crystal for a superhet and a transmit crystal for any of the gear we are using today are cut for DIFFERENT frequencies to operate on the same channel

. I don't think we ever did get a response to exactly what method was being used to check the tuned frequency of the receiver did we?


I think you summed it up Steve ,all the way around.

I know you don't agree with me 100% on home crystal swapping but I think you may get a clue from what is going on here. If it fits the socket ,someone will put it in and turn it on !

Steve Lewin

Hi Ladyflyer

The home crystal swapping is just a "cultural" difference, nothing personal. Because we do it all the time over here I know there are no real technical problems. But because it's common over here most people know how to do it correctly, i.e. the different types of crystal, not using crystals from another manufacturer, doing a careful range check when you've swapped crystals etc. It's not foolproof but then nothing is .

Steve

Ladyflyer

My Feedback: (2)

Hi again Steve,

MOST folks that swap crystals here have no problem either.

You are right the only diff here is the fact we share the band with industrial controls. As you have seen some people think a channel 14 crystal is a channel 14 crystal . You know what happens when a DC RX rock is plugged in to a transmitter, some others are cluless. Some even swear it "works" Being off freq here could cause hazardous interference with licensed operations.
The added safety at flying fields is a side benefit .

As I have said before problems are not very likely but there have been problems.

BTW, Gary R. Everything yuou said was accurate and I, at least appreciated it.