FM receiver alignment

[resurgence2012]

Hi

I bought an Fm receiver kit recently. It has two ICs. The first is a TA8127N which is the front end of the receiver and includes the stereo decoder.

I assembled the kit and it works but I am not sure how to align it. There are two coils and two transformers.

The instructions that came with the kit are in Thai.

Is it possible to align a receiver without using an RF signal generator? I do have an oscilloscope that goes to 100 MHz

Is it possible to build a very simple RF signal generator if one is needed?

Thanks

Peter

 

[Audioguru]

Good luck aligning the Oriental kit radio.

We have no idea (you did not post its schematic and parts list) if the kit uses the circuit in the datasheet of the TA8127 or if it uses a different circuit.
The datasheet from Toshiba provides no details but the datasheet from UTC has details about the coils and transformers.

 

[resurgence2012]

Good luck aligning the Oriental kit radio.

We have no idea (you did not post its schematic and parts list) if the kit uses the circuit in the datasheet of the TA8127 or if it uses a different circuit.
The datasheet from Toshiba provides no details but the datasheet from UTC has details about the coils and transformers.

yes - sorry. I guess I was just asking in first place whether in general terms it can be done without a RF generator.

The circuit is the same as the application circuit in the data sheet. I looked at the UTC data sheet but did not really see any extra information. There was another data sheet which was actually about a full retail product that uses the chip but it only said how to align the tuner using an RF generator.

One small thing (or not so small) is that the very first coil in the data sheet specifies 2.5 turns. The coil that can came with the kit is actually 4 turns.

I am guessing that I need to start with the IF. I was wondering whether I might be able to get a 10.7 MHz signal from another fm radio that I know is properly aligned. It's a wild guess!! And I am still not quite sure where that should be applied/or measured. Please excuse my lack of knowledge. Thanks.

 

[Audioguru]

A simple radio like this can be aligned for maximum sensitivity. But the automatic gain control on the AM receiver covers up any level changes and an FM radio has the same output level regardless of the signal strength. Then maximum sensitivity is difficult to sense.

If you tap into the IF of another radio then the capacitance of the tap will change the tuning of that circuit.

I built an FM radio kit a long time ago. It came with a simple field strength meter that was used to peak the tuned circuits during its alignment.

 

[resurgence2012]

Thanks Audioguru

I take the point about the capacitance of a tap affecting the tuning of that circuit.

I am ignoring the AM section of the radio and didn't actually bother to put in all the parts that were solely involved I the AM side.

The approach that I have been taking is to try to find a station - any station - then fiddle with each of the variables in turn to try to get the clearest signal. Then I tuned around until I could find another station that was very feint and repeated the process. Gradually it appeared that I was picking up more and more stations. However there was tremendous variation between the volume of each station which seems to contradict what you were saying. There was also a lot of distortion regardless of what I did.

 

[Audioguru]

You could use an antenna length that is so short that a station is received with plenty of noise. Then adjust the coils for the lowest amount of noise.
adjust the "quad" coil at pin 12 for the lowest distortion on loud sounds.

The IC you are using makes a very simple radio. Its sensitivity is poor. Since it has only one IF transformer then its selectivity is also poor. when aligned properly then it has fairly high distortion.

A real FM radio is extremely sensitive and all stations produce the same loudness. Its good selectivity can pickup a weak distant station beside a strong local station. It has low distortion.

 

[resurgence2012]

Thanks Audioguru

That's very interesting and valuable information.

If I did want to try to build a really god quality FM receiver are there any chips or circuits that you would recomen?

I have spent quite a bit of time looking at Aalan Yate's pulse counting FM
circuit. It appealed to me because it dosn't have any coils to align but somehow works with direct capacitor coupling between the IF stages. I don't know how that works. Why do superhets have transformers usually between the IF stages?


As I said before I really know and understand very little but this topic of FM radios for some reason fascinates me.

I did build a super regen a couple of weeks ago and was surprised by the sensitivity and even quality out of the sound at times.

I will try out your suggestions and let you know how I get on.

Cheers and thanks again

 

[Audioguru]

There are a few excellent FM stereo radio ICs but they are made for, used for and bought by equipment manufacturers, not hobbyists.
High quality car radios also use them.

Selectivity is when a radio selects one station but has no interference from stations on nearby frequencies.

A superhet radio converts the very high frequency FM band down to 10.7MHz where ordinary parts can amplify well and where ordinary IF transformers and Quartz crystal filter IF modules can work. The IF is a fixed frequency but the input of the radio and its local oscillator are tuned to the station you want. The IF provides a radio with good selectivity because it has more than one IF transformer and/or quartz crystal filters.

 

[resurgence2012]

A superhet radio converts the very high frequency FM band down to 10.7MHz where ordinary parts can amplify well and where ordinary IF transformers and Quartz crystal filter IF modtuned to the station you want. The IF provides a radio with good selectivity because it has more than one IF transformer and/or quartz crystal filters.[/QUOTE]

Thanks that's really clear. The thing I don't quite understand yet is why a proper Fm receiever needs transformers between the IF stages where as a single chip like the TDA7000
can get by without them.

I basically got the radio working as cle arly as I can. Then for comparison I through together on a breadboard a circuit using a TDA7000 chip. I was very surprised because it worked very well. With nothing more than the hook up wires acting as an arial it could pick up and play with an acceptable quality local stations with none of the 'gross' distortion that the other receiver is producing.

This makes me wonder whether the tuning of the IF and other coils could and others on the other set is more critical than I had imagined. Or whether there is something about the layout of the other circuit that is not good. In the other circuit the RF coil is snuck very tightly next to the variable capacitor, so I am wondering whether it might work better if there is a couple of cm (or at least one) between the two. I will try this and see what happens.

My apartment has pretty poor reception. But I don't think that matters now because I have the TDA 7000 set up right in the same spot and also my decent Sony PLL as a benchmark.

I got hold of a TEA5767 at the weekend which I want to try and set up with my Arduino.

Cheers and thanks

 

[Audioguru]

The TEA5767 does not have very good stereo separation and has fairly high distortion. I didn't see a recommended pcb layout for it.