jdwiflyrc

My Feedback: (31)

How or why is it possible for a 72mhz am tx to operate a 72mhz fm d/c rx.

I donot think that this should be possible, someone with more sense please help me understand why?

Thanks, John

strato911

I agree, it shouldn't work. However, if the two are operated in close proximity it has been known to work. Not reliable enough to fly with though. The range is typically only a dozen feet before glitching starts.

theox

I wonder if it's like getting hit by interference,only with the interference

being of the exact frame rate,pulse,duration,reset,(however it works)

that the Rx is set up for. Put it on the same channel for even more

effect. Does AM put out more splatter tham FM?

jdwiflyrc

My Feedback: (31)

Hi Dennis, I range tested it to 100 ft on the ground with no problems and with the correct tx as a back up I flew the plane as if it was made to fly with that combination. Smooth with no glitching and plenty of range in the air.

I thought that amplitude modulation and frequency modulation would not mix. and I am looking for an electrical answer as to what part of the transmitting and receiving theory will allow this to happen.

Anyone that can help me with the answer please do so.

Thanks and keep em fly'in, John

Rich-RCU

I have seen it several times the other way around FM TX and AM RX. It is called slope detection and works well when the receiver is slightly tuned off from the signal center. If you consider FM generates an infinite number of sidebands and the sidebands actually increase in amplitude as the main Carrier energy is being redistributed into those sidebands. There are a lot of other considerations.

I have not looked at the R/C receivers in over 15 years but have read (on the internet, so it must be correct) that some of the R/C receivers do not have AGC (Automatic Gain Control) and some do not perform the customary peak clipping of the signal in the demodulator stage. Some FM discriminators (demodulators) do not adequately filter out AM components.

I would be suspect of trying to mix FM and AM intentionally. You may be losing some desirable RX characteristics.

Rich

strato911

Although I have some electronic training, it isn't specialized in RF transmission. All my schooling says it shouldn't work, but you have proven my teachers wrong. I look forward to reading an explanation for this anomaly .

Could you please provide details of your setup? What make & model of TX? What make & model Rx? What channel are you using? Are you 120% certain it is an AM transmitter?

jdwiflyrc

My Feedback: (31)

Hello All,
The tx is a Futaba FP-T5UA clearly labeled AM on the front of the case with a FP-TP-AM module with the silver and black circle with CH 26 and 72.310mhz on the label. The tx has a gold sticker without the date written on it as they came from the factory with.
The rx is a Expert D.B.M & x-tal filter Dual Conversion FM Mdl #HP-7RM72F on CH 26 AND 72.310 MHZ made for the J style plugs.
I normally fly this plane with a HITEC Prism 7x with the Spectra module set on ch 26.
It is still a mystery to me that this works.
I did fly as the only plane in the air for safety reasons with the above mentioned tx as backup and everyone at our field were warned to look out if we had a uncontrollable problem which we did not.

Keep em fly'in, John

Dave Bowles

My Feedback: (7)

If I have understood correctly, most people equate the AM and FM R/C signals with our regular audio AM and FM signals for our car and homes. But my understanding is the FM and AM signal for R/C is not the same as that used for Stereos, The industry has labeled them this way because AM for public broadcast is a single wave and AM for R/C is a single wave , FM for broadcast has two waves (stereo) and FM for R/C has two waves, and that is the main reason the R/C has been given the AM or FM designation, The actual signal of a R/C AM and FM is not that different and nothing like the broadcast AM and FM. It has always been known that a AM Can interfere with an FM of the same frequency and FM with AM similar to FM and FM/PCM. The fact is AM radios are just as reliable as FM if properly tuned , back before narrow band it was not always the case because of inaccurate tuning .

Dave Bowles

My Feedback: (7)

If I have understood correctly, most people equate the AM and FM R/C signals with our regular audio AM and FM signals for our car and homes. But my understanding is the FM and AM signal for R/C is not the same as that used for Stereos, The industry has labeled them this way because AM for public broadcast is a single wave and AM for R/C is a single wave , FM for broadcast has two waves (stereo) and FM for R/C has two waves, and that is the main reason the R/C has been given the AM or FM designation, The actual signal of a R/C AM and FM is not that different and nothing like the broadcast AM and FM. It has always been known that a AM Can interfere with an FM of the same frequency and FM with AM similar to FM and FM/PCM. The fact is AM radios are just as reliable as FM if properly tuned , back before narrow band it was not always the case because of inaccurate tuning .

Dave Bowles

My Feedback: (7)

If I have understood correctly, most people equate the AM and FM R/C signals with our regular audio AM and FM signals for our car and homes. But my understanding is the FM and AM signal for R/C is not the same as that used for Stereos, The industry has labeled them this way because AM for public broadcast is a single wave and AM for R/C is a single wave , FM for broadcast has two waves (stereo) and FM for R/C has two waves, and that is the main reason the R/C has been given the AM or FM designation, The actual signal of a R/C AM and FM is not that different and nothing like the broadcast AM and FM. It has always been known that a AM Can interfere with an FM of the same frequency and FM with AM similar to FM and FM/PCM. The fact is AM radios are just as reliable as FM if properly tuned , back before narrow band it was not always the case because of inaccurate tuning .

Rich-RCU

The Expert brand radios were made by Hitec as well as the Aristo-Craft radios. I think your receiver may be the same as the Aristo-Craft/Hitec Challenger 720.

As for a more detailed answer as to why your AM receiver is working with FM is that it may be the type of demodulator/discriminator used in your Rx. Some types convert an FM signal to AM then demodulate using conventional AM circuitry. Your AM signal is just passing through the FM to AM conversion circuitry. This site gives additional explanations:

http://murray.newcastle.edu.au/users...#Descriminator

http://murray.newcastle.edu.au/users...ecev/FMRec.htm

Rich

strato911

That's the odd thing here Rich. It's an AM Tx working with an FM Rx.

Rich-RCU

Originally posted by Rich
As for a more detailed answer as to why your AM receiver is working with FM...

I knew that. I reversed AM and FM to see if anybody was reading what I posted (grin).

I'm getting my discriminators and demodulators confused....

Rich

jdwiflyrc

My Feedback: (31)

Hi Rich,

If I understand this correctly the FM signal enters the FM rx and is converted to an AM signal and then works as though it is an am rx.
Then my original am signal is passing through the fm to am conversion circuitry and that makes it work as an am receiver.

DUH --- did I say that?

Keep em fly'in, John

P.S. A friend called Radio South and asked if it would work and they told him that it absolutely would not work!!!!

KB9STD

My Feedback: (1)

I've seen and heard of this before(although this is the first time I've heard of someone actually FLYING with it). I Have wondered about how it works, and have a personal theory (I'm just guessing). First you have to understand the "AM" and "FM" systems.Neither of them is accurately named. An "AM" RC transmitter is actually an ON/OFF switched modulation (much like Morse code), While the "FM" is really "Frequency shift keyed".The "AM" system turns the signal OFF on each pulse of the data stream,and ON in between pulses.The receiver doesn't see a signal during each pulse.In the "FM" system the transmitter frequency is shifted (either from above the channel center to below, or visa versa) during each pulse.The transmitted signal "shifts" out of the passband of the receiver, So again the receiver is not seeing a signal during each pulse.If either the "AM" transmitter or the "FM" receiver is "off frequency" by about the same amount as the offset in an "FM" system, the switching off of the signal will produce the same effect as shifting out of the passband of the receiver (the receiver NOT getting a signal for the duration of the pulse,then the signal returning until the start of the next pulse).The receiver CAN"T know why the signal disappears,only that it forms a "Valid" pulse train at the decoder.
Let me say again,that I am only guessing! It seems to make sense to me. I am eager to hear other theories, and or comments about my thoughts.

adam_one

Further thoughts:

According to what I've learned (so far) a FM receiver's demodulator consists in a tuned circuit whose response characteristic has a steep edge in the nominal frequency.
This circuit produces an output at one frequency and a different output at slightly different frequency.
Some demodulators are of the phase discriminator type and operate by comparing the phase of the incoming signal to the response it produces in a tuned circuit along with a quadrature detector.

FM receivers exhibit a capture effect whereby when an interfering signal enters the receiver it beats with the true signal producing an amplitude modulated IF signal. That's why the demodulator/discriminator is always preceded by a Limiter stage to ensure that the FM signal is at constant amplitude.
The FM receiver demodulator/discriminator is therefore designed to ignore amplitude variations.
However, the AM transmitter may produce a kind of phase shift every time it switches the RF output on /off , which may "deceive" the FM receiver discriminator since the carrier frequency is the same.

Dave Bowles

My Feedback: (7)

Heres where I read the info about AM and FM.

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


AM vs. FM

PPM and PCM define the manner in which servo position is encoded into an electrical signal. If we could connect the flyer to the plane with a long wire this is all we would need. You think this is a joke? Many anti-tank missiles do exactly that: R/C advertises the position of the launcher and invites a return favor.

AM (Amplitude Modulation) and FM (Frequency Modulation) specify the manner in which PPM or PCM are impressed on the bass, or carrier, radio wave i.e., how it is "modulated." Many modelers equate AM to interference prone AM stations and FM to crisp, clear FM broadcast stations. BS, and I don't mean a university degree. R/C AM is NOT AM and FM is NOT FM!

True Amplitude Modulation implies that servo position is proportional to amplitude analogous to music loudness in broadcast AM, so full left might be maximum carrier strength, neutral mid-strength and full right minimum strength.

In R/C "AM" there are only two states -- FULL signal and NO signal, nothing in between. R/C AM is more PM. Seriously, a more descriptive term would be Pulse Modulation, but that looks like PPM sans a P. How about AM being BM -- Binary Modulation.

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.

The Futaba, Hitec and Tower Hobbies camp are NEGATIVE shift (carrier - 5 Khz)= ON. The Airtronics/ JR camp are POSITIVE shift (carrier + 5 kHz)= ON. Carrier only is OFF for both. For example an Airtronics PPM frame, with all servos at Neutral, would start with 72.075 MHz (ON) for 1.5 m then 72.070 MHz (OFF) for 0.5 ms repeated for 2 - 9 servos and wraps up with 2 ms at 72.070 (OFF) synch.

From this we can see that R/C "AM" is not fundamentally inferior to R/C "FM" - it just uses one frequency instead of FM's two. Indeed a properly tuned AM radio is theoretically superior to its FM counterpart because it can only be glitched on ONE frequency whereas a glitch on either FM frequency will knock the FM receiver out.

Unfortunately AM gets a bad rap because the older AM radios were not very selectively tuned (pre 1991), there being fewer channels those days so channel separation was not the 20 Khz of today. So an older, mistuned, AM transmitter may have "side splatter i.e., emit an image on a neighboring channel. Also older AM receivers may pick up adjacent channels. Post 1991 (yellow sticker) AM transmitter/receivers are at least as good, if not better than many FM counterparts. (I use several "garage sale" bargain AM radios regularly at Torrey Pines, Poway and elsewhere and have yet to detect an interference glitch, or cause one. Likewise for the up-to-date AM radios I have seen.) The only possible advantage of FM is that one of the 2 frequencies (carrier and carrier +/- 5 Khz) HAS TO BE PRESENT, otherwise transmission is lost; whereas in AM no carrier just means OFF. It does not appear that any present day PPM radios make use of this (e.g., for fail safe servo positioning).

Bottom line: Most AM transmitter/receivers are compatible.
Futaba, Hitec and Tower Hobbies (Futaba knock-off) FM are compatible (negative shift). Airtronics and JR FM are compatible (positive shift).
If you want to save cash and weight consider the Hitec 2 servo channel AM (Focus 2). Also the Futaba 2DR 2 channel - $50 from Tower Hobbies with transmitter, receiver, 2 servos, battery box and switch (the 2 stick mode 1 transmitter is incompatible with standard, single stick, mode 2 and therefore almost useless).
Current model AM receivers (usually 2 servo channel) work excellently with 4 channel AM transmitters (e.g., Futaba Attack 4 and Conquest AM) widely available at swap-meets and garage sales typically for $10-$15. If you are going to use them at the slope or places with other flyers, it would be neighborly to make sure they have a yellow AMA sticker or, better yet, have them tuned by an R/C radio shop.
Several high-end FM computer transmitters support negative AND positive shift interchangeably (e.g., Futaba 8UAP, Airtronics Stylus and Hitec Prism 7X). These can control models with Futaba/Hitec and Airtronics/JR FM receivers equally well. Swap-meeters take note: the Hitec Prism 7X is a boon because for little more than most 6 channel radios it can operate almost every Futaba/Airtronics/JR/etc. FM PPM receiver bargain (that works). I love mine.


Ian Hirschsohn

Rich-RCU

Yes, that is correct depending on the type of demodulator or discriminator they are using. For example a disadvantage of the FOSTER-SEELEY discriminator is that it will allow AM signals to pass through unless certain precautions are taken.

http://www.tpub.com/neets/book12/51c.htm

And we really don't know what your receiver is using. I wonder if the receiver was designed that way intentionally and some were labeled AM and some FM?

Rich

Rich-RCU

Dave, I don't want to get into a long discussion on the above article. But here are a few points I would like to make about it. The article has some good points. The article also has some bad points. The article has been changed several times over the years as a result of several hundred posts on the rec.models.rc.air newsgroup, so I don't know how accurate it is today but the last three times I have read it over the years it lead to more confusion than education. I think the author has good intentions. That is just my opinion.

Rich

Dave Bowles

My Feedback: (7)

I don't disagree with you and why I used that little "IF" in my first post because I don't know if everything in the artical is fact , but it offers some simple explinations that I though were at least interesting. I have yet to see and or read (untill now) of a cross of AM and FM operating one another reliably enough to fly.

jdwiflyrc

My Feedback: (31)

hello Dave,

It was a surprise to me that it would even work at close range but intrested me enough to go farther with it.


When I got a good range test and I had a backup tx and someone to operate it I decided to try to fly it.

Surprise, Surprise it worked fine and the flight was for about 5 min. with us switching back and forth between the am and fm tx with more flight time on the am tx.

I plan to try this with other brands of rx's and will report back to this thread on the results.

Keep em fly'in, John

Phil Cole

RC receivers use integrated IF strips, typically with a quadrature detector. These are just as sensitive to AM as FM unless preceeded by a good limiter. Even if you have the limiter, it can't do anything if there is no signal. So the FM receiver sees a signal pulsing on and off.

The AM tx has to have just the right frequency for this to work. The frequency has to correspond to the "pulse on" frequency from an FM transmitter. Changing to an FM receiver with the opposite shift (e.g. JR instead of Futaba) is the thing most likely to stop this working.

adam_one

More thoughts:

One of the best frequency demodulators is considered to be the Phase-Locked Loop (PLL).
It consists in a phase detector, a voltage controlled oscillator (VCO) and a low-pass filter.
The phase detector compares the frequency of the incoming signal with the VCO frequency and produces an output that is filtered by the low-pass filter resulting in an error signal that controls the VCO frequency.
The PLL is synchronised or locked when the input and VCO frequencies are equal.
The correction signal produced and fed into the VCO is proportional to the frequency deviation, thereby a demodulated signal is obtained.
Thanks to the feedback principle and filtering action, noise and interference rejection is good, also providing a highly linear output that faithfully reproduces the modulating signal.

jdwiflyrc

My Feedback: (31)

Hello everyone,

A little more info on the AM TX is that I have the RX that came with it and it is a Futaba FP-117H AM RX and still works with the TX.

I have been accused of putting a FM circuit board in a AM case to fool everyone and I assure you that is NOT TRUE.

I am still going to check the AM TX with other brand FM RX's and will let everyone know of the results.

Keep em fly'in, John

jdwiflyrc

My Feedback: (31)

Hello all

I tried the AM FP-T5UA tx with a FM FP-R127DF rx and got no response at all not even a glitch.

As soon as I can get another brand rx and xtal on 26 I will let you know how that works.

Keep em fly'in, John