TManiaci

My Feedback: (6)

Good Thread. Since there is some brain power in this forum, I thought I might ask for a summary clarification of the Key points:

Dual Conversion utilizes a secondary internal crytal, or digital equivalent, right?

If the secondary internal xstal determines Shift, then how do you have different Shift in Single Conversion Rx's?

Is there a differnece between Dual Conversion and Single Conversion primary xstal's?

How is this different from the ABC&W methods in JR? Airtronics?

Does JR/Airtronics have a Single/Dual Conversion equivalent?

If Conversion type or Shift are not factor is Range capability, then what are the primary factors?

XJet

While the frequency of the crystal may affect the way shift is interpreted -- this is a side-effect and not usually the way that the shift setting is designed into receivers.

It's a red herring.

Yes, SC crystals will be 455KHz higher or lower than the required frequency, DC crystals will be 10.7 (or whatever the first IF frequency is) higher or lower.[/quote]

JR used a comination of strong AGC and good filtering to try and overcome the limitations of SC in their receivers.


Sensitivity. In effect, it's the minimum level of signal that the receiver can accurately distinguish from the background noise. Most short-range parkflier receivers have a sensitivity of around 10uV (microvolts) wheras a full-range unit (SC or DC) will probably be able to work at signal levels as low as 2uV or less.

nwagers

I can say from experience that SC and DC receivers are not even comparable when it comes to noise rejection. It's fairly simple to see if you have a signal generator and an oscilloscope. The idea is know as "Superheteroyne" receivers. The reason that DSP is not widely used as a second IF stage is the cost of implementing such a design.

XJet

DSP is incredibly cheap to implement (software development costs notwithstanding).

In most cases it simply involves replacing a simple shift-counter (often a 4017) with a small microcontroller (PIC or Atmel). The component cost difference is about $1 in manufacturing quantities.

I suspect you'll see DSP-based decoders on most PPM receivers within a year or so -- because it actually works out simpler and cheaper to do things this way than to mess around making two versions of a receiver (pos/neg shift).

DSP is *no* substitute for good RF design but is a very useful companion to it when designing a good receiver.

JPMacG

My Feedback: (2)

I think NWAGERS was saying that it is expensive to A/D convert the first IF frequency (10.7 MHz?) and do the second conversion and filtering digitally. That would be a sampling rate of at least 22 MHz. I don't think you can do that for only $1. XJET, I think you are speaking of doing the DSP after the signal has been converted to baseband?

I can understand that a single conversion receiver with DSP applied to the baseband signal could not offer the filtering selectivity or image rejection of an analog dual-conversion receiver. I don't see why DSP applied at the first IF (e.g. 10 MHz) would not provide performance equal or superior to an analog dual-conversion receiver.

XJet

I can't see why anyone would want to apply DSP at the IF level of a single-conversion receiver -- the processing power and software complexity required to do so would be significantly greater than the current technique of applying DSP to the demodulated signal.

Given how well most moderl dual conversion receivers work in respect to sensitivity, selectivity and image rejection, there seems little point in complicating things just to get an equally good result.

Modern logic-level DSP processing is there to simply mitigate some of the effects of any in-band signal. In this way it's the RC-receiver equivalent of the old noise-limiting systems found in AM communications receivers. Moving the DSP to the IF section would be the equivalent of the old noise-blanking systems -- slightly better but far more complicated.

When it comes to DC receivers I say -- if it's not broken, don't fix it :-)

paulsen

My Feedback: (2)

Well, this discussion over my head, but I would like to know what Rx on the market to day would be recommended for a 1.20 size 4 ch aircraft with dependability and price a factor? I had a FM DC 7 ch futaba standard issue Rx but for some reason ch 4 glitches like hell. I switched all servos around on different channels to see if it was the servo (it was not) and even tryed them on a FM hitec micro 555 DC 5 ch and all was solved, so it was neither servos or Tx. Would the hitec 555 be adequate for a 1.20 size plane? Another thing I noticed was the 555 says its "ultra narrow band" maybe some one could tell me what this means. Thanks . I do not plan on using this on my 1.20 size ultmate bipe cause I want to put it in my .15 size pitts bipe.