How to use a Si diode as AM detector

[MrAl]

Hi,

Resistance eats power so avoid resistance when possible.

The best way to bias the diode i with a voltage source, but it's a little hard to keep the voltage at the right value when the diode temperature varies. Too much voltage forward biases the diode, and not enough doesnt do anything. Getting it right means more sensitivity...getting it wrong means less sensitivity.

 

[Willen]

Once I was guided like that way- If we used coil+caps (tank) direct to antenna and earth, it will be overloaded so has extremely high bandwidth. So if used main coil and caps for antenna to earth and on same core if we make few turns with another tuning capacitor and take it for radio (may be amplifier needed), this can give us better selectivity.

I got double loud audio from 1N5819 than 25 pcs of Ge diode each.

 

[audioguru]

So a regenerative receiver should be the way forward for more selectivity.

No.
A regenerative receiver has poor selectivity except the bandpass at its tuned peak is very narrow which cuts high audio frequencies, then further down the slopes of its single LC tuned circuit the bandpass is too wide causing stations to overlap each other. Also a regen receiver is overloaded by strong local stations.

 

[BkraM]

So what would you advise as a next step?

 

[MrAl]

Hi,

Buy a small radio? <he he> Might be the simplest choice really unless you want to learn more about radio.

 

[audioguru]

A small FM radio is sold for only $1.00 at The Dollar Store. Its headphones are worth more than its cost and its button cells battery is also free.
Its performance is horrible because it uses a Chinese copy of the Philips TDA7088 IC that is a Mickey Mouse superhet.

 

[BkraM]

I would expect there are some improvements to be made to this simple circuit, and still keep it within hobby limitations?

 

[Miles Prower]

JimB,

Thanks for your reply. I'll have a look at your suggested videos.

I figured adding a resistor would reduce the amplitude of the reception, but would narrow down the bandwidth.
Requiring more amplification, but giving a better selectivity.

https://www.mathworks.nl/help/control/ug/analyzing-the-response-of-an-rlc-circuit.html

What am i missing here?

Thanks,

Mark

If you go to that Math Works web site, that parallel tuned RLC circuit consists of one inductor and one capacitor. That's fine if you're tuning across a very narrow band. The AM BCB band is a good deal wider.

The Q-factor of a parallel RLC is:

Q= Rp / wL

For good selectivity, you want the parallel resistance large, and the reactance small. If you use just the one L and a variable C, what happens? At the bottom of the BCB, you have maximum Q. There, you might have sideband cutting that loses the audio highs. At the top of the BCB, wL has increased, and you might not be able to separate stations. (Looks like you had that problem already.)

It was because of that that the superhet design was originated. If all the received frequencies are translated to one frequency (the IF) you have consistent selectivity, and you can distribute the overall gain over radio and audio frequencies. It's trivially easy to design an RF amp that operates at one fixed frequency. Not so easy to design one that operates over a wide band,

If you look up some early crystal radio schemos, you will frequently find multi-tapped tuning coils. The tap-offs allow for dialing in the required inductance while keeping the variable capacitor near full capacitance. You can select for wider bandwidth for improved audio, or narrower bandwidth for greater selectivity (at the cost of good audio performance.)

So a regenerative receiver should be the way forward for more selectivity.
Thanks for the tips, i'll give it a go.

Absolutely! That's one of its advantages over a purely passive receiver like a crystal set. However, a regen that operates over the BCB is going to require a lot of patience to operate. It will have excellent selectivity, but the audio quality is going to suffer, due to sideband cutting that lose the audio highs.

I'm trying to build an AM radio. All circuits i have found use an Ge diode as detector as it has a low voltage barrier. I don't have access to a Ge diode only to a Si. I'm wondering if it is possible to adjust the circuit so a Si diode wil do the trick. Would an amplification stage of the RF signal with a BJT after the tank circuit, prior to the demodulation stage work?

Yes, it would. However, remember that BJTs have a rather Lo-Z input. You'll have to tap off the tuning coil so's not to compromise the selectivity.

Is there another trick i could use to make it work?

You could go for a TRF design that uses the Si diode as an audio frequency mixer, and follow up with lots of audio frequency amplification. However, these are no longer strcitly passive detectors.

 

[MrAl]

Hi,

The Q of the circuit depends highly on the series resistance of the inductor. The Q for that is:
Q=w*L/Rs

where L is the inductance and Rs is the series resistance.

The higher the Q the better the selectivity (and the harder it is to tune and keep tuned). Because most of these inductors will be air core and hand wound using the same gauge wire and because Rs goes up linearly with N (the number of turns) and the inductance itself goes up as the square of N, that means a larger inductance will lead to higher selectivity in most cases. Thicker wire for the inductor also leads to higher Q.

A decent bandpass filter is made with a parallel capacitor on the input, a series inductor, and another parallel cap on the output. If each C is the same and the inductance L, then the center frequency is approximately:
Fc=1/(2*pi*sqrt(L*C))

 

[BkraM]

Hi guys,

I've been looking at this regenerative circuit from Charles Kitchen and found i don't really get what i'm looking at.
( http://www.mtmscientific.com/swradio.html )

1. I see two capactitors in parralel, a 33uF and a 0.01 uF. why not use just the 33 uF? what functionallity/filtering is added by this small capacitor?

2. I would have expected the transistor (emitter on 1000 pF) having the filtered signal (after the tank circuit) on it's base and feeding it back into the tank circuit.
But as i now see it the signal on the base is quite already filtered by 3 caps (0.01 and 33 uF and the 1000pF), damped by 3 diodes and then guided though a 100K resistor. I would expect not much being left of the signal from the tank circuit.
The Collector is also connected to the antenna, where i would have expected a Voltage source.

I can only conclude I'm quite lost here. The circuit does work, so i'm clearly not getting it.
Can someone explain it a bit more, or maybe point me in a direction.

 

[Willen]

My quick assumption-

- 0.01uF (10nF) is a good filter for resonant frequency of AM like few hundred KHz to few MHz. And 33uF is a filter for line noise hum and it is also a reservoir when battery has high internal resistance.

- Collector of a 1st transistor (left) is not only connected to antenna. It is also connected to positive of VCC through 150k and through L4 and L8.

 

[BkraM]

Cant the Capaitors in parallel can be added to form a equivalent replacement of 33.01uF? Probably not, but i don't see why not in this case.

Does the fact that the collector is also attached to the antenna make any difference? Why not connect it to vcc directly?

Thanks,

 

[JimB]

1. I see two capactitors in parralel, a 33uF and a 0.01 uF. why not use just the 33 uF? what functionallity/filtering is added by this small capacitor?

In a perfect world, the 33uF capacitor would be the only one needed.
However, in this world there are no pure components. Every capacitor also has a little bit of inductance and resistance.
At audio frequencies, for the 33uF cap the capacitance predominates, the overall impedance is low and it adequately decouples audio frequencies from the line.

At radio frequencies, the inductance is predominant and what was a nice capacitor starts to behave like an inductor and does not provide the decoupling needed.
To ovecome this the lower value capacitor (o.01uF) is used. Due its construction it has much less inductance and so works correctly as a decoupling capacitor.
In circuits intended for VHF ans UHF, the decoupling capacitors will be of lower capacitance, and yet work quite correctly where a 0.01uF or a 33uF whould be no use at all.

2. I would have expected the transistor (emitter on 1000 pF) having the filtered signal (after the tank circuit) on it's base and feeding it back into the tank circuit.
But as i now see it the signal on the base is quite already filtered by 3 caps (0.01 and 33 uF and the 1000pF), damped by 3 diodes and then guided though a 100K resistor. I would expect not much being left of the signal from the tank circuit.
The Collector is also connected to the antenna, where i would have expected a Voltage source.

Sorry but you are very on these points.

The first transistor is confiured as an oscillator. In normal operation it is not actually oscillating but very nearly.
This near oscillation has two usefull effects, it dramatically improves the selectivity of the tuned circuit (the two coils and the 365pF variable capacitor), and, it provides gain for the RF signal.

As far as the antenna goes, think of it being connected to the top of the coil rather than to the collector of the transistor.

The three diodes are being used as a simple voltage stabiliser to provide (about) 2 volts supply for the first transistor.
The two capacitors 33uF and 0.01uF mentioned earlier, provide decoupling for the collector supply for the transistor.
The supply connects to the collector through the coil.
The 100k resistor and 1000pF capacitor provide the bias supply to turn the transistor on.


Have a quick look at my modified version of the circuit with a few components deleted.
It becomes a simple crystal radio and an audio amplifier.

JimB

 

[RadioRon]

JimB, i couldn't help notice that your schematic does not provide any DC paths to either side of the detector diode. Will it not ultimately charge up the 0.01uF coupling cap following the diode and stop detecting?

 

[Nigel Goodwin]

JimB, i couldn't help notice that your schematic does not provide any DC paths to either side of the detector diode. Will it not ultimately charge up the 0.01uF coupling cap following the diode and stop detecting?

I don't think it would detect at all in the first place, it needs the 0.01uF in series with coil shorting out, and a load resistor (with a capacitor across it) on the output of the diode - however, the supposed 'audio amplifier' is really rubbish as well (but nothing to do with JimB). The original article linked to also probably doesn't work, with no load resistor there either - it's a really naff design all round, and they sell a hugely expensive kit for it!

 

[JimB]

JimB, i couldn't help notice that your schematic does not provide any DC paths to either side of the detector diode. Will it not ultimately charge up the 0.01uF coupling cap following the diode and stop detecting?

RadioRon, you are quite right.
The circuit as I cut it, is rather lacking a DC path for the diode.
I was not proposing my modification as a viable radio, but simply cutting out the regenrative transistor stage to illustrate the development from a simple crystal set.

It could be argued that there is no DC path for the diode in the original circuit, but they manage to "get away with it" due to the connection to the transistor collector and it works well enough, even if it is a bit iffy.

Nigel, I must admit that I thought the kit was a bit on the expensive side, and the audio amplifier was a bit on the marginal side.

The object of my post was to give the OP an understanding of what the RF stage is doing and why the anomaly of the two capacitors of very different values in parallel.

JimB

 

[Willen]

The collector is also attached to the antenna make any difference? Why not connect it to vcc directly?

Connecting the collector directly to Vcc, the signal (oscillation) will be attenuated.

 

[BkraM]

Hi,

I just fail to understand the regen basics of this circuit.

As i understand the goal of the (bottom left) transistor is to amplify the RF signal in the tank circuit.
I would assume that the RF signal is presented at the base of this transistor in order to amplify it. This doesn't appear to be the case.
How is the RF signal than amplified? Willen mentions that the collector of the transistor needs to be connected to the antenna to allow for (near)oscillation.

Probably important is the splitting of the coil, i also can't figure out what that actually does in this circuit.
Hoe does this help the transistor to amplify the signal?

Thanks,

 

[audioguru]

A transistor amplifies voltages when it is connected two ways:
1) When it is "common emitter" with the base as the input and the collector as the output which you are thinking about.
2) When it is "common base" with the emitter as the input and the collector as the output like this regen circuit that you did not know about.

You posted a link instead of posting the schematic but I do not want to find it in this long thread then open it again.

 

[BkraM]

Thanks audioguru,

I did not know that yet, brings me a lot further in understanding the circuit!


For (possible future) reference :