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Old 23rd Mar 2019, 5:55 am   #1
Jolly 7
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Default Function of diodes in superhet radio circuit ?

Can anyone please advise what is the function of the IN4148 diodes in this radio circuit.
They are marked D1, D2 and D3. If D1 and D2 have some purpose in providing bias to the transistors, could resistors have not been used instead ?
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Old 23rd Mar 2019, 7:25 am   #2
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Default Re: Function of diodes in superhet radio circuit ?

Resistors could have been used, but the diodes have a few advantages.

Their voltage drop decreases with temperature, at about -2mV per degree C. This acts to compensate the temperature sensitivity of the base-emitter junction of the transistor being biased and so the quiescent current the transistor is set to will be more stable over temperature.

The voltage across the bias diodes will be less dependent on battery voltage than if only resistors had been used. This means that the quiescent currents are less dependent on battery voltage.

So the presence of the diodes is an indication of better design. Accountants would have gone for resistors every time.

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Old 23rd Mar 2019, 8:32 am   #3
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Default Re: Function of diodes in superhet radio circuit ?

The supply voltage is only 3 volts and it would seem that D1,D2 across the supply rail after resistor R12 form a stabiliser. The forward bias required for a silicon diode is nominally 0.7V. Two in series would require 1.4V so I think that D1,D2 are to insure that the voltage presented to the mixer/oscillator and IF transistors will never exceed 1.4 volts. One may wonder why a zener diode wasn't used.....very low voltage zener diodes are not available for technical reasons so the use of two normal diodes in series makes perfect sense. Quite a novel design and probably works very well.
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Old 23rd Mar 2019, 10:52 am   #4
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Default Re: Function of diodes in superhet radio circuit ?

There used to be parts like BZY88C1V4 and -C0V7 which were at the bottom end of the BZY88 range of zener diodes but they were just one or two forward biassed silicon diodes inside the package.
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Old 23rd Mar 2019, 3:00 pm   #5
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Default Re: Function of diodes in superhet radio circuit ?

Quote:
Originally Posted by Jolly 7 View Post
If D1 and D2 have some purpose in providing bias to the transistors, could resistors have not been used instead ?
No they don't provide bias. They are used as a stabiliser...see my post #3.

As stated in post #2, the diode D3 is a much better way of stabilising the output stage quiescent current.
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Old 23rd Mar 2019, 3:41 pm   #6
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Default Re: Function of diodes in superhet radio circuit ?

Actually, they do provide bias- the bases of the front end transistors are fed from the diode stabilised supply via simple series resistors (except where the AGC gets in on the act). They do also provide the collector supplies.
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Old 23rd Mar 2019, 6:06 pm   #7
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Default Re: Function of diodes in superhet radio circuit ?

Quote:
Originally Posted by Sideband View Post
Quite a novel design and probably works very well.
Yes it works very well but requires a lot of careful component checking, as this is the HX108-2 kit radio which often comes with some dodgy parts. Also the audio output stages need additional resistors to prevent possible transistor damage, as advised by an engineer friend. Once the changes are made, it should work fine.
When I built my first kit radio 35 years ago, it only had one diode, an OA79.
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Old 23rd Mar 2019, 6:18 pm   #8
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Default Re: Function of diodes in superhet radio circuit ?

Thanks everyone for the responses.

Here is another 3V superhet design that I haven't had much luck with. I got the oscillator stage to work and all the IF cans are also responsive to slug tuning as evident from the audio output, but when I join the the oscillator, mixer and IF stages together, it simply will not receive any stations. The only change I made was replacing the IN918 diodes with IN4148s.
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Old 23rd Mar 2019, 11:01 pm   #9
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Default Re: Function of diodes in superhet radio circuit ?

Shouldn't make any difference - the 1N914 and the 1N4148 are virtually the same. (Beware of writing IN rather than 1N, search engines may not find them!)
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Old 24th Mar 2019, 1:31 am   #10
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Question Re: Function of diodes in superhet radio circuit ?

Quote:
Originally Posted by Jolly 7 View Post
Here is another 3V superhet design that I haven't had much luck with. I got the oscillator stage to work and all the IF cans are also responsive to slug tuning as evident from the audio output.
Two thoughts spring to mind, upon reading that. What you've written is a bit vague - so please let me enquire further . . .

1. What did you do to assure yourself that the oscillator stage was working as required? Did you use a 'scope or a freq. counter to check that the osc. would tune to the appropriate frequencies?
2. Checking the response of the IF amplifier by 'noting the audio output' is not really adequate - that's if I understand correctly what you've written. Did you use a signal generator on the correct frequency and align the cores of the IFTs accordingly?

Just trying to help.

Al.

Last edited by Skywave; 24th Mar 2019 at 1:32 am. Reason: Re-write the quote.
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Old 24th Mar 2019, 2:42 am   #11
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Default Re: Function of diodes in superhet radio circuit ?

Two thoughts spring to mind, upon reading that. What you've written is a bit vague - so please let me enquire further . . .

1. What did you do to assure yourself that the oscillator stage was working as required? Did you use a 'scope or a freq. counter to check that the osc. would tune to the appropriate frequencies?
2. Checking the response of the IF amplifier by 'noting the audio output' is not really adequate - that's if I understand correctly what you've written. Did you use a signal generator on the correct frequency and align the cores of the IFTs accordingly?

Just trying to help.

Al.[/QUOTE]

Thanks Al. Most of my radio experiments are in building blocks, making it easier for me to troubleshoot if the radio is not working as it should. Hence I built the oscillator section as a standalone unit on one board and the rest of the radio on another. I don't have an oscilloscope but do have a multimeter that reads frequencies. Therefore I could verify by reading a frequency of 2.6 MHz at the collector of the oscillator transistor that it is oscillating. I have no idea if an ideal oscillation frequency exists for MW but maybe you can advise ?

In answer to your second question, I have a DIY signal generator set at 455 kHz with audio tone. I get a strong tone through the speakers just by touching the can of the 1st IFT with the probe. It is possible to peak it by adjusting the slugs of all three IFTs, yellow, white and black respectively.

Hope this helps you understand a bit more about my post. Suggestions for correcting/ improving the circuit will be much appreciated.
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Old 24th Mar 2019, 9:20 am   #12
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Default Re: Function of diodes in superhet radio circuit ?

Ref #9, when I did a search for IN4148 it came up as a Zener on a number of sites, why would this be?
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Old 24th Mar 2019, 9:37 am   #13
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Default Re: Function of diodes in superhet radio circuit ?

Could someone explain why the radio schematic in post No.8 has two diodes in series in the detector stage.

I have never seen that before.
It may make sense in the AGC circuit, but seems like a disadvantage in the detector circuit.
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Old 24th Mar 2019, 9:39 am   #14
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Default Re: Function of diodes in superhet radio circuit ?

Quote:
Originally Posted by Jolly 7 View Post

I don't have an oscilloscope but do have a multimeter that reads frequencies. Therefore I could verify by reading a frequency of 2.6 MHz at the collector of the oscillator transistor that it is oscillating. I have no idea if an ideal oscillation frequency exists for MW but maybe you can advise ?

In answer to your second question, I have a DIY signal generator set at 455 kHz with audio tone. I get a strong tone through the speakers just by touching the can of the 1st IFT with the probe. It is possible to peak it by adjusting the slugs of all three IFTs, yellow, white and black respectively.

Hope this helps you understand a bit more about my post. Suggestions for correcting/ improving the circuit will be much appreciated.
....2.6Mhz is wrong for a 455khz I.F when tuning MW....basic superhet operation.....are you aware of this? Without going in to all the details of how it operates (there is loads of stuff on the internet about it) basically the oscillator is designed to run at a frequency higher than the wanted (tuned) frequency by the difference of the I.F which in your case is (presumably) 455Khz. So at the highest frequency of MW (let's call it 1500 metres which is 1.2Mhz or 1,200Khz) = 1200 + 455 = 1655 Khz which is 1.66Mhz. At the other end of MW (Roughly 540Khz) 540 + 455 = 955Khz so your oscillator should never be running higher than around 1.655Mhz or lower than about 955Khz for MW tuning.

The purpose of the mixer stage is to combine the oscillator frequency with the wanted (tuned) frequency, mix the two together (hence mixer) and produce the difference at the first I.F which should be 455Khz (if that is indeed the I.F that you are using). It's not going to do that if your oscillator is running at 2.6Mhz....at least not for MW.

So for MW operation you need to sort out the oscillator frequency. Your meter may be giving you false readings in the sense that it could be altering the oscillator frequency by the simple act of connecting it. The oscillator is VERY sensitive to external capacitance and it's likely that the length of your meter leads will affect the reading. Try fitting a 100K resistor in series with one of the meter leads and see if that gives a better reading.
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Last edited by Sideband; 24th Mar 2019 at 10:01 am.
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Old 24th Mar 2019, 9:40 am   #15
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Default Re: Function of diodes in superhet radio circuit ?

Quote:
Originally Posted by John10b View Post
Ref #9, when I did a search for IN4148 it came up as a Zener on a number of sites, why would this be?
Possibly because it should be 1N4148 not IN4148.
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Old 24th Mar 2019, 9:55 am   #16
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Default Re: Function of diodes in superhet radio circuit ?

With regards to the circuit in post #8, looking into the emitter of the mixer transistor, the input impedance there is roughly the impedance in the base circuit divided by the transistor's hfe, and that coupling coil in the base circuit has a low Z at the frequencies the oscillator operates on anyway. So likely when you couple the osc to the mixer it kills the osc or heavily loads it. You need to step down the osc with an impedance matching transformer, or maybe add a series resistor with the coupling cap, or better still interpose a buffer stage between the osc and the mixer.
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Old 24th Mar 2019, 10:23 am   #17
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Default Re: Function of diodes in superhet radio circuit ?

Quote:
Originally Posted by Sideband View Post
The purpose of the mixer stage is to combine the oscillator frequency with the wanted (tuned) frequency, mix the two together (hence mixer) and produce the difference at the first I.F which should be 455Khz (if that is indeed the I.F that you are using). It's not going to do that if your oscillator is running at 2.6Mhz....at least not for MW.
Ran out of edit time so just to add a little more....

With your oscillator running at 2.6Mhz your actual received (tuned) frequency would be 2.6Mhz (2600Khz) - 455Khz (the I.F) = 2145Khz (2.145Mhz) which is way out of the MW band and is low shortwave....probably why you are not receiving anything as this is outside the MW tuning coil range.

Hope I've not confused you too much....it can be confusing getting your head around sum and difference frequencies but if you want to understand how radio works well.....!!
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Old 24th Mar 2019, 12:15 pm   #18
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Default Re: Function of diodes in superhet radio circuit ?

Sideband thanks for that you are correct, I did wonder as the circuit doesn’t show them as zeners.
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Old 24th Mar 2019, 12:16 pm   #19
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Sideband: thank you for those two recent posts above (#14 & #17) which are not only supplementary to my earlier post (#10) but have also saved me the time and trouble of writing what you have written - and, moreover, what needed to be written.
Hopefully, the OP will find what you have stated to be usefully instructive and helpful.

Al.
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Old 24th Mar 2019, 12:53 pm   #20
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Default Re: Function of diodes in superhet radio circuit ?

Hi Al.

Can you correct some errors on my post 14? I've incorrectly said highest frequency of MW was 1500 metres when it should be (approx) 200 metres or 1500Khz. I was obviously thinking frequency rather than metres!


So at the highest frequency of MW (let's call it 200 metres which is 1.5Mhz or 1500Khz) = 1500 + 455 = 1955 Khz which is 1.95Mhz.

so your oscillator should never be running higher than around 1.95Mhz or lower than about 955Khz for MW tuning.

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