dasintex

My Feedback: (10)

What exactly is the difference; is one better; less interference, etc

JRexA

http://www.torreypinesgulls.org/radios.htm


Link is Sniped from another thread

Dual Conversion is better than Single.

BarracudaHockey

My Feedback: (11)

You have to compare apples to apples, JR makes great single conversion recievers they just use another method to reject inband interference.

onewasp

Read this: www.bergent.net/SC-DC.pdf

Assuming good up to date designs either should operate well. You still can pick certain conditions where one will out perform the other----------my personal preference is Dual Conversion.

If you are flying JR then you'll find (I'm NOT a JR authority) that you'll come close to having the best of both worlds as the first stage is still A,B,C and W---- then it goes to the second stage etc.

CrashGaalaas

My Feedback: (8)

I just read the torrypinesgulls site. If what they say is correct, I have to change my entire concept of how our RC signals are really modulated.

Does anyone else agree with this?
"Likewise, true Frequency Modulation implies servo position proportional to the carrier frequency moved up or down a little, so full left on 72.070 MHz carrier (channel 14) would be 72.075 (carrier + 5 Khz), neutral at 72.070 and full right 72.695 (carrier - 5 Khz). "

XJet

That's a load of bollocks (excuse my French) :-)

There's no direct correlation between carrier frequency and servo position in modern RC gear.

I've not read the article concerned but I really wonder where people get this stuff from.

HarryC

My Feedback: (1)

Xjet, the torrey pines statement is not bollocks, it is true. That is how FM works. The real culprit is our radios claiming to be FM when they are not FM in the full sense.

The full paragraph from torrey pines actually reads
"Likewise, true Frequency Modulation implies servo position proportional to the carrier frequency moved up or down a little, so full left on 72.070 MHz carrier (channel 14) would be 72.075 (carrier + 5 Khz), neutral at 72.070 and full right 72.695 (carrier - 5 Khz). In R/C "FM" there is only full modulation i.e., carrier shifted by 5 Khz or NO modulation (carrier only). R/C FM is identical to wireless data transmission "frequency shift keying" (FSK), at least the computer people don't pretend to be FM."



H.

saitofreak

Irrespective of the technical feedback above, the fact is that Dual Conversion is an improvement, by design. A well made DC receiver will always have better specs than a well made SC receiver, when using the common superheterodyne topology (there are other ways eg: PLL). Whether this is significant to the user is debateable as already stated. I use DC where I can, for peace of mind only.

Rodney

Sorry Saitofreak, SC can be better than DC under some conditions and in others DC is better than SC. Either method of detection can be very good or very poor depending on the exact design. Cases in point, some of the JR and Berg receivers work as reliably as other receivers do and they are SC. The biggest advantage of DC over SC is their immunity to some intermodulation distortion.

CrashGaalaas

My Feedback: (8)

Rodney,
I totally agree that comparing SC to DC is totally dependent on the design of each, and in what conditions they are used.
Do we really have to be confined to a 455KHz IF?

I don't agree the Dual Conversion is necesarily more imune to IMD.

dirtybird

My Feedback: (5)

I do not agree that SC can be as good as DC provided both are carefully designed. The purpose of DC is to eliminate the image frequency. A SC receiver can be made to not respond to the image frequency through coding tricks but the existance of the image frequency,if present. will reduce the gain of the SC receiver. It will not reduce the gain of the DC receiver because it is eliminated in the first mixer stage.
In the US a 455KHZ IF is required to provide the necessary selectivity. You may not need this high selectivity

CrashGaalaas

My Feedback: (8)

Who requires us to put the IF on 455KHz?

Selectivity can be obtained at other frequencies, even at 10.7 MHz if desired.

dirtybird

My Feedback: (5)

You could probably do it with digital filters but a what cost? (5KHZ is needed)
Its cheap and easy at 455KHz.

XJet

It's just semantics.

FSK = digital (binary) frequency modulation of a carrier.

FM doesn't always equal FSK but FSK always equals FM.

In fact, another reason to consider RC FM as a true form of FM is because it only uses *one* oscillator. FSK is sometimes implemented with two oscillators that are turned off/on or switched between. Our RC transmitters have a single oscillator that has its frequency modulated by a varicap - and the limited (slew) rate at which it can change frequencies means that it doesn't just jump from 72.070 to 72.075 but *sweeps* from one frequency to another and back again. If it jumped then it would create all manner of sidebands that would pollute the remainder of the band.

saitofreak

Thats fine, take your pick. The advent of DSP in Rx's has muddied the water somewhat, but hey as long as it works. Enjoy.

Mac_Man_UK

455 KHz & 10.7 MHz are 2 Industry Standard IF's and as such the crystal/ceramic filters for these frequencies are in abundance and therefore cheap. You could go ahead and manufacture using your own IF's but then getting the filters made would be horrendously expensive.