Dual Conversion Crystals

[Erfolg]

Can I simply replace my std. crystals in a 35 set, with a pair of Dual Conversion (matched) crystals in my Tx and Rx?

[Martin Harris - Moderator]

Only the receiver crystal is dual/single conversion and the correct type needs to be used for either a single or dual conversion receiver.

 

Unless your receiver is marked as dual conversion it’s almost certainly single conversion - in the case of Futaba, SC normally used orange aerials and DC had white ones. 

[Paul De Tourtoulon]

As said, duel receivers need a dual quartz.

[Simon Chaddock]

As Martin says only if the receiver is marked "dual conversion" then it must use a dual conversion crystal but it cannot use a single conversion one.

Dual conversion gives better reception between closely spaced channels so unless you are flying at a busy 35 mag site there are normally so few users that the problem has virtually disappeared

 

The last time I was at an "open day" at a club with over 100 potential flyers there were only two of us on 35 meg and our channels were well apart.

[kc]

13 hours ago, Simon Chaddock said:

our channels were well apart.

Isn't the point that channels a large distance apart interfere - wasn't it said that low '60's  could have interferred with 80 something when extra channels were introduced?   That was a reason for using dual conversion.   Anyway a bit theoretical now - the main problem is likely to be forgetting to extend the Tx  aerial or not spotting another 35Mhz user.

[Geoff S]

4 hours ago, kc said:

Isn't the point that channels a large distance apart interfere - wasn't it said that low '60's  could have interferred with 80 something when extra channels were introduced?   That was a reason for using dual conversion.   Anyway a bit theoretical now - the main problem is likely to be forgetting to extend the Tx  aerial or not spotting another 35Mhz user.

 

4 hours ago, kc said:

Isn't the point that channels a large distance apart interfere - wasn't it said that low '60's  could have interferred with 80 something when extra channels were introduced?   That was a reason for using dual conversion.   Anyway a bit theoretical now - the main problem is likely to be forgetting to extend the Tx  aerial or not spotting another 35Mhz user.

 

 

No, a high IF means so-called 2nd channel interference (another RF signal can beat with the local oscillator and potentially create a weaker IF which responds to the unwanted signal) is reduced because it comes from an RF frequency 2x the IF away (so 2x10.7 Mhz = 21.4 Mhz away).  With dual conversion superhet receivers, they have both a high IF for 2nd channel and a low IF for remote RF interference.  Because they have 2 local oscillators and 2 sets of IF transformers it makes them more expensive and unnecessary in the modern context of few 35Mhz users.

[Erfolg]

I think I have the issues resolved now. I have inherited a largish numbers of Futaba Receivers. Why I am using 35 is another story, a bit boring. As I do have 2.4 Rx available.

 

I did not initially (for 5 minutes) that they are all Dual Conversion. I then recognised that I had no DC crystals. Although the Rx all had Rx crystals, except one that pots to selected channel.  One that appears to have non (that it matters). 

 

I decided to buy a pair, which began the subsequent saga.

 

Nothing seemed to work. Which I thought in itself was unlikely. I slowly realised that I needed to change the Tx to PPM. Still nothing worked.

 

It was then I discovered the Rx that you could select a Rx Channel. Now I knew that the (new) Rx crystal was the problem.

 

I was going to complain to the original Vendor about the faulty crystal. Looking to purchase from another vendor, I did a search, noticing that on EBay I could buy 10 on the same frequency. That is what i have done, I await the delivery of the package.

 

It is Hmmm a lot of years since I used 35, two 2.4 transmitters later, and it seems a poor memory of the 35 past, 27 is a foggy bit of history.

 

Thanks for all your efforts, I hope to finally finish this trilogy with a final message all is now in order. 

 

 

[gangster]

I believe that dual conversion was of no advantage at all in this country on 35MHz. It was not to improve adjacent channel immunity and offers no advance there. I have long forgotten the maths on this but there was a significant change of interference on 72MHz . Modellers in this country were paying extra for dual conversion receivers in the erroneous belief that they were gaining further immunity. Can anyone remember the math of the 72MHz issue. I may have posted this on here years ago

[Martin Harris - Moderator]

I've pasted this exceptionally clear explanation (I can understand it!) of the technical differences from another thread - thanks Pete:

 

On 09/06/2013 at 17:31, Peter Christy said:

Any superhet receiver will respond to two frequencies, equally spaced either side of the receiver crystal frequency. A single conversion receiver normally uses a 455 KHz intermediate frequency, so it will respond not only to its own transmitter, but equally one 910 KHz (2*455) on the other side of the receiver crystal frequency. For example, a receiver on 35.070 MHz will actually use a crystal of 34.615 MHz. The difference between the two frequencies is 455 KHz, and it is this frequency that is amplified and detected in the receiver. However, it will also respond to a transmitter on 34.160MHz, which is also 455 KHz away from the receiver crystal frequency. The unwanted transmitter should be filtered out by the front-end tuning coils in the receiver, which are tuned to 35 MHz, not 34 MHz. However, the level of selectivity is slight at these frequencies, and the receiver response is probably only about 3dB down.

This usually isn'y a problem as there are no model control systems on the 34 MHz band - I think its used by the military.

However, by increasing the IF frequency to 10.7 MHz, the "image" frequency is now 21.4 MHz away from the wanted one rather than just under 1MHz, and can easily be filtered out. However, selectivity (10 KHz channel spacing) is difficult to achieve at 10.7 MHz, so a second mixer and IF is added to step it down again to 455 KHz again. (Hence "double-conversion")😉

So, a single conversion receiver will have an image frequency 910 KHz away, and a double conversion will be 21.4 MHz away. A double conversion receiver will be almost totally immune to interference on the image frequency, at the cost of extra complication in the receiver.

The receiver crystal for a single conversion receiver will normally be 455 KHz less that the Tx crystal (34.615 MHz in our example). A double conversion will be 10.7 MHz below, or 24.37 MHz for a transmitter on 35.070MHz. Clearly, one will not work in the other!

(Some receivers use "high-side injection", where the local oscillator is above the transmitted frequency, moving the image frequency above the wanted transmitter frequency, but I have ignored this for clarity! The principle is still the same.....)

Pete

[Alan Hilton]

On 12/03/2024 at 12:26, Erfolg said:

I think I have the issues resolved now. I have inherited a largish numbers of Futaba Receivers. Why I am using 35 is another story, a bit boring. As I do have 2.4 Rx available.

 

I did not initially (for 5 minutes) that they are all Dual Conversion. I then recognised that I had no DC crystals. Although the Rx all had Rx crystals, except one that pots to selected channel.  One that appears to have non (that it matters). 

 

I decided to buy a pair, which began the subsequent saga.

 

Nothing seemed to work. Which I thought in itself was unlikely. I slowly realised that I needed to change the Tx to PPM. Still nothing worked.

 

It was then I discovered the Rx that you could select a Rx Channel. Now I knew that the (new) Rx crystal was the problem.

 

I was going to complain to the original Vendor about the faulty crystal. Looking to purchase from another vendor, I did a search, noticing that on EBay I could buy 10 on the same frequency. That is what i have done, I await the delivery of the package.

 

It is Hmmm a lot of years since I used 35, two 2.4 transmitters later, and it seems a poor memory of the 35 past, 27 is a foggy bit of history.

 

Thanks for all your efforts, I hope to finally finish this trilogy with a final message all is now in order. 

 

 

Have you tried setting the tx to pcm ?

[Andy Stephenson]

I believe the reason for dual conversion receivers was to prevent something called half IF interference but I can't quite remember the theory behind it. Channels close to 227.5kHz above the lower channels could be interfered due to the phenomenon. When the 35Mhz channels were expanded up to ch 90 there was enough frequency difference to allow this to happen. Although this doesn't add up to an exact number of channels it's close enough to be a potential problem.

[Andy Stephenson]

This is a known problem in single conversion receivers.

Excerpt from...

Understand Electronics (Second Edition), 2001

8.2.2.2.2 Half-IF and image rejection

...The degradation is due to fourth order nonlinearity manifesting itself as second order nonlinearity. More precisely, any interference occurring at Fc±1[/]2FIF will overlap with the desired signal after mixing. In this case, the design trade centered on the filtering strategy, the choice of the IF, and linearity. This is an interesting trade; it entails the classical balancing act between design complexity and performance degradation. Certain knowledge of the level of interference at ½ IF will dictate the design parameters. Recall that the ½ IF problem is nonexistent for direct-conversion receivers since the IF for this architecture is zero...