RC Transmitter tuning proccedure

[PLMS]

Hi guy's
Further to the thread on receiver tuning I've started this thread on transmitter tuning or more correctly, determining if the unit is 'on frequency' ? Making sure it's in band is also a worthwhile thing.

OK, I've run my Hitec Flash5 over my Spec An and saved some traces as JPG's below. One shows the carrier with PPM ON, the other shows the carrier with PPM OFF. The TRAINER switch BTW works as a PPM OFF switch if no other slave radio is attached :-)

(There is NO PROBLEM with my transmitter that I'm aware of)

The traces show that my transmitter carrier is about 1.5khz high with PPM OFF, and the other trace shows PPM ON and the main carrier is now about 1.5khz low with modulation clearly visible for about 5khz either side of center frequency (36.120Mhz).

OK, is it on frequency more or less ? (I have a fair idea, but I'd like comments from people who KNOW how it's determined).
cheers
Martin

[dkselw]

Just out of interest, how accurate is your spectrum analyser?
The ones I have used tended to drift around a fair bit (fairly old ones though). The only way I could get an accurate frequency measurement was to use frequency generator with a small antenna to act as a marker.

[Mr.RC-CAM]

Here is how it is generically done to a 72Mhz pos shift Tx:

Force the modulation input to ground (logic low). Observe the frequency counter. Adjust the high side trimmer to 1.8 KHz higher than the channel frequency. Then set the modulation input to VCC (logic high) and low side trimmer for an output frequency that is exactly 1.8 KHz lower than channel frequency.

The sidebands need to be reviewed on a spectrum analyer. They must be greater than -50dB below the carrier. Especially review 90Mhz and 144Mhz.

A 35/36Mhz Tx's specs would be different. But regardless of Operating Freq, measurements with PPM turned on are not too useful.

RC-CAM

[PLMS]

Just out of interest, how accurate is your spectrum analyser?
The ones I have used tended to drift around a fair bit (fairly old ones though). The only way I could get an accurate frequency measurement was to use frequency generator with a small antenna to act as a marker.

It's very good, the unit is a barely year old Rohde & Schwarz FHS3 and it's in Cal. I checked it against our frequency counter and it's within 50hz of it. Good enough for this sort of work.
Martin

[PLMS]

Here is how it is generically done to a 72Mhz pos shift Tx:

Force the modulation input to ground (logic low). Observe the frequency counter. Adjust the high side trimmer to 1.8 KHz higher than the channel frequency. Then set the modulation input to VCC (logic high) and low side trimmer for an output frequency that is exactly 1.8 KHz lower than channel frequency.

The sidebands need to be reviewed on a spectrum analyer. They must be greater than -50dB below the carrier. Especially review 90Mhz and 144Mhz.

A 35/36Mhz Tx's specs would be different. But regardless of Operating Freq, measurements with PPM turned on are not too useful.

RC-CAM

From what you say it sounds like you balance the carriers either side of center, with the modulation input tied high and then low. This may be possible via the Trainer socket and the PPM input without opening the radio, might have to see if that works.
My guess is my radio is within about 200Hz anyway from looking at the balance between the carriers and the modulation envalope as a whole.
As the TX seems to work fine with two out of three of my RX's I'll go work on the RX now.
Thanks

[Mr.RC-CAM]

This may be possible via the Trainer socket and the PPM input without opening the radio, might have to see if that works.
Maybe, but that depends on the Tx's design.

RC-CAM

[vintage1]

Not sure what the next channel over is, but in UK its 10Khz IIRC. So -1.8 to +1.8Khz is a good swing on the actual transmitted frequency.

Sidebands ought to be about 40dB down at +- 5Khz.

which yours almost are. leastways you appear to be running into the noise floor at around +-5Khz.

Tuning up the coils would consist in peaking up the maoin blip, but checking that the passband edges are clear of trash..

[gadgetguy]

I want to throw in a curve ball here; Several people have mentioned leaving the iF stages alone due to factory settings, etc, and probably for good reason. But the reason this thread was started, and the reason we are tuning a receiver is because it is out of tune. A 100 mph impact with the earth from a crash will require a complete check of the system, especially if we are to tust it again with an expensive airplane. Receivers rarely drift by themselves from my experience with 72 mhz systems, maybe in Europe the other frequencies are prone to more problems? But I assume the average person is retuning likely due a a mishap, and I think myself and many others would like to see the whole process covered. I am an electronics student and highly respect your talents, and I think it would be extremely benificial to all of us to walk through a tuneup of say a Futaba FM system, Hitec, etc. I dont know about other countries, but all I see is Airtronics, JR and Futaba in Texas, and I'd sure like to see a complete walk-through of a tuneup, and it would save us all alot of surfing through fragmented information. I now hand it to the experts.....Andy

[vintage1]

Well, mostly the shaping is dne by fixed frequency filters, and if they get cracked its replacement time.

However, this is a transmitter tuning thread, so go and say that again in the receiver tuning thread :D

[gadgetguy]

OK Vintage1, but maybe I am a crazy American and maybe I crashed my transmitter! And it needs tuning :)

http://www.geocities.com/andymcnew/tojo.jpg