Radios with the same integrated circuit but with different FM selectivity

[ursebastian7]

Hi everyone. I comment. I have 2 radios, a Sony icf-s14 and a Sony icf-s10mk2. Both use the same integrated circuit (cxa1019s), however they have different selectivity in FM. The icf-s14 is the one that assigns the most bandwidth to the stations and the icf-s10mk2 is the most selective. The problem with this is that, in the most selective radio, it is difficult to select the stations a bit, since, when allocating little bandwidth, it is difficult to escape from the adjacent channel, and the consensus must be handled very accurately. However, in icf-s14 this does not happen, I can get away from interference without problems and use more bandwidth. My question is: is there anything I can do to make the Sony icf-s10mk2 allocate more bandwidth to the stations? From already thank you very much

 

[Nigel Goodwin]

I would imagine it's down to the specific filters used in the design, and perhaps even the tuning of the front end (if any).

However, it sounds like you've confused the two radios, the most selective 'should' be better at rejecting adjacent stations.

 

[ursebastian7]

I would imagine it's down to the specific filters used in the design, and perhaps even the tuning of the front end (if any).

However, it sounds like you've confused the two radios, the most selective 'should' be better at rejecting adjacent stations.

And what am I supposed to do? What do you mean by front tuning?

 

[Les Jones]

I agree with Nigel except that I don't think the tuning of the RF amplifier stage (If it has one.) before the mixer would be sharp enough to effect the bandwidth. It's main purpose is to filter out the image frequency which is twice the IF frequency away from the received frequency. If the radio is fairly recent design it will probably use SAW filters rather than IF transformers. From your description of the problem I think it is how effective the AFC (Automatic frequency control.) is on each radio. The AFC is attempting to correct drift of the local oscillator by controlling the DC voltage to a varicap diode in the oscillator. This voltage will be derived from the discriminator. When you try to tune away from the frequency being received the AFC tries to hold it on that frequency. Try Googling "superhetrodyne receiver" for a more complete explanation.

Les.

 

[Nigel Goodwin]

I agree with Nigel except that I don't think the tuning of the RF amplifier stage (If it has one.) before the mixer would be sharp enough to effect the bandwidth.

Neither did I, which was why I said 'perhaps' - as it could be a difference between the two radios, one with a tuned frontend, and one without.

I never even considered IF transformers, although I thought ceramic filters rather than SAW - I've no idea if SAW's were used in later radios?, but ceramic filters replaced IF transformers a great many years ago. However, for practical purposes, Ceramic filters and SAW are the same thing anyway.

SAW did replace transformers in TV IF circuits, due to their complex nature, but radio IF's (and the audio IF in those TV's) used ceramic filters as they did everything required, and are probably cheaper

As far as TV's go, I modified a LOT of grey imports (with 5.5MHz audio IF instead of 6MHz) over the years - in fact I modified them both ways (imports and exports). Initially it was only the audio ceramic filters you had to change, plus the notch filter on the video - but with the advent of SAW filters for the main IF you had to replace that as well, as it was sharp enough to reduce the audio carrier.

 

[Ylli]

From your description of the problem I think it is how effective the AFC (Automatic frequency control.) is on each radio. The AFC is attempting to correct drift of the local oscillator by controlling the DC voltage to a varicap diode in the oscillator. This voltage will be derived from the discriminator. When you try to tune away from the frequency being received the AFC tries to hold it on that frequency.

That was my first thought too, but looking at the chip, nothing is accessible.

 

[unclejed613]

do the radios have an AFC switch on them? this would keep them centered on the station's signal instead of

I agree with Nigel except that I don't think the tuning of the RF amplifier stage (If it has one.) before the mixer would be sharp enough to effect the bandwidth.

the RF section passband is pretty wide on FM receivers (a couple hundred KHZ maybe), and the RF input stage probably consists of a transistor and a tank circuit (on some real cheap ones, it's just a tank circuit just before the mixer). one section of the tuning cap will be the cap on that tank circuit. if there's a second RF stage, the passband might be 100-200khz. after the mixer, you now have a 10.7Mhz IF, and most modern radios (even super cheap ones) use ceramic filters to achieve the proper passband characteristic. the selectivity comes from the 10.7Mhz crystal filter, not the RF stages

 

[ursebastian7]

do the radios have an AFC switch on them? this would keep them centered on the station's signal instead of

the RF section passband is pretty wide on FM receivers (a couple hundred KHZ maybe), and the RF input stage probably consists of a transistor and a tank circuit (on some real cheap ones, it's just a tank circuit just before the mixer). one section of the tuning cap will be the cap on that tank circuit. if there's a second RF stage, the passband might be 100-200khz. after the mixer, you now have a 10.7Mhz IF, and most modern radios (even super cheap ones) use ceramic filters to achieve the proper passband characteristic. the selectivity comes from the 10.7Mhz crystal filter, not the RF stages

Please if you explain to me if there is anything to do to expand the bandwidth of the sony icf-s10mk2 I would be very grateful. Thank you all for the answers but I don't understand much of what you are talking about.

 

[Nigel Goodwin]

Please if you explain to me if there is anything to do to expand the bandwidth of the sony icf-s10mk2 I would be very grateful. Thank you all for the answers but I don't understand much of what you are talking about.

For a start obtain circuit diagrams for both receivers - and why do you imagine one is narrower bandwidth, and why do you think that would cause the 'problem'?, it's more likely to improve it. Simple answer?, replace the ceramic filter with the same one used in the other radio, assuming they are different?.

 

[Les Jones]

You would need to find the schematic of both receivers on the web or trace them out yourself if you can't find them. I have had a look at the data sheet on the CXA1019S and can't see how the IC can effect the bandwidth or how strong the AFC effect is. I found a picture of this radio online and it looks quite small so you would need to be good at working on circuit boards that probably used surface mount components.

Les.

 

[ursebastian7]

For a start obtain circuit diagrams for both receivers - and why do you imagine one is narrower bandwidth, and why do you think that would cause the 'problem'?, it's more likely to improve it. Simple answer?, replace the ceramic filter with the same one used in the other radio, assuming they are different?.

The icf-s14 (radio that allocates more bandwidth to the stations) uses TDK and blue IF filters. Instead, the icf-s10mk2 model uses MURATA brand filters, yellow.

Service manual icf-s14:

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Service manual icf-s10mk2:

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[Ylli]

You could *try* adding a jumper between the chip pins 7 & 9. Do it on the one where the strong signals are preventing you from locking onto the weaker signals. This should disable the AFC.

 

[ursebastian7]

You could *try* adding a jumper between the chip pins 7 & 9. Do it on the one where the strong signals are preventing you from locking onto the weaker signals. This should disable the AFC.

Sorry for my ignorance, but I don't understand why you insist so much on deactivating the AFC, if both circuits are the same, shouldn't it work the same way?

 

[Nigel Goodwin]

The icf-s14 (radio that allocates more bandwidth to the stations) uses TDK and blue IF filters. Instead, the icf-s10mk2 model uses MURATA brand filters, yellow.

Service manual icf-s14:

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elektrotanya.com

Service manual icf-s10mk2:

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Electronics forum for electronics experts to repair defective appliance.

elektrotanya.com

Makes a lot more sense with the schematics - so change the filters for the same type as the one you prefer.

 

[Nigel Goodwin]

Sorry for my ignorance, but I don't understand why you insist so much on deactivating the AFC, if both circuits are the same, shouldn't it work the same way?

It's something that's easy to do, and costs nothing - so worth a try to see if it helps.

 

[unclejed613]

if you have access to an FM signal generator and a oscope you can measure the actual selectivity of the receiver. you would use an RF demodulator probe (consists of a germanium diode voltage doubler and a BNC cable) connected at the input of the radio's FM demodulator, and the sweep generator connected to the antenna input of the radio. the scope is used in XY mode with the modulating audio (preferably triangle wave audio... you can use sine wave modulation if you are adjusting the RF and IF, but for measuring, you want a linear sweep) driving the X axis, and the output of the probe driving the Y, axis. with the sig gen set to 200khz deviation (if the sig gen doesn't have a deviation meter, you can use an RTL-SDR to measure it), and the y axis gain adjusted to take up the whole 10 divisions across, you now have a way of measuring the bandwidth. you could also skip the scope and sig-gen and just use an RTL SDR to measure with, but use a capacitor to isolate the RTL's antenna input, as there are often DC levels present in the radio.

 

[Nigel Goodwin]

You're going to mention 'wobbulator' next aren't you?

 

[Ylli]

[This should have posted yesterday but obviously I forgot to click on the "Post Reply" button. ]

AFC tends to lock a receiver on a strong signal and hold it there even as you tune slightly off of frequency. So it is quite possible the AFC is holding your receiver on the stronger signal as you tune over the weaker one.

There is a slight difference between the two receivers in the AFC implementation. One couples the AFC tuning diode to the FM oscillator tank with a 1 pF cap, and the other couples it to the tank with a 2 pF cap. Could be enough to make the difference you are seeing.

 

[unclejed613]

You're going to mention 'wobbulator' next aren't you?

you mean those old, clunky, mechanical things with the plates of a tuning cap driven by a cam, that passed as an RF sweep generator in the 1940s?

for the OP to measure the overall selectivity of the radio, an RTL-SDR would be the simplest way to do it, as it doesn't require much more than background noise to provide a test signal.

 

[Nigel Goodwin]

you mean those old, clunky, mechanical things with the plates of a tuning cap driven by a cam, that passed as an RF sweep generator in the 1940s?

I was thinking more if the electronic versions

for the OP to measure the overall selectivity of the radio, an RTL-SDR would be the simplest way to do it, as it doesn't require much more than background noise to provide a test signal.

As the OP is asking about two tiny little low-spec portable radios I doubt he has the equipment or knowledge to carry our such tests - and it would be easier and cheaper to just buy a better radio. Mind you, the time this thread is taking FM radio could be shut down before it finishes!

The suggestion in post #12 is simple and zero cost, but he doesn't seem interested in trying it?.