Voltage produced from a radio receiver

[MitchElectronics]

Hi all,

So I have started to construct an AM receiver and noticed that the average peak to peak voltage from the LC tank circuit was around +- 50mV. What I want to know is what is the average voltage generated by an LC tank circuit? I ask this because I noticed that for a diode rectifier circuit to function properly it needs to be forward biased. So even if you use a shotky diode it would still have a forward voltage of ~0.1V which is still to high from what my LC tank circuit is producing...

Any ideas?

All the best,
Robin

 

[betwixt]

If you are referring to the input LC circuit, 50mV is extremely large! Most radio receivers work well with tens of uV.

The forward voltage drop in the diode specification is measured at a relativey high current and when the diode is fully conducting. It is the voltage where the I/V curve has straightened out and the diode behaves more like a constant voltage device. For radio receiver detectors you don't want that to happen, instead you want the diode to conduct on the non-linear part of it's curve which is in the sub-Vf region. They actually start to conduct from almost zero voltage.

Brian.

 

[MitchElectronics]

Wow! Learn something new every day!
Thanks alot! So why is it then that when I probe the other side of the rectifying diode I do not see any waveforms that resemble what I pick up on the tank circuit?

 

[ljcox]

When I first started electronics (in about 1957) I built a crystal set.

It used a Gemanium diode (an OA71 if I remember correctly) to rectify the RF from the Tank Circuit. It worked quite well.

The nearest station was about 40 km away.

Len

 

[jiripolivka]

If you make a crystal radio for AM frequency range, then depending on your antenna and distance to various transmitters, amplitude on the LC circuit at resonance can range from uV to > 10 V. If you add an amplifier before that LC circuit, the amplitude may be even larger.
This is why all AM and also FM receivers must have AGC, a circuit that reduces amplifier gain to avoid distortion.
In some receivers, detectors are DC-biased but for AM detection it is generally not necessary as the gain before the detector is high, so when the LC circuit is tuned away from an AM signal, noise appears at output.

 

[betwixt]

Robin, you will have to post your schematic to answer your waveform question but in most circuits, there will be RF on one side of the diode and a filter to remove RF (leaving only the audio) on the other. So one side might see a sine wave varying in amplitude, the other side would see the signal causing the amplitude change, in other words the recovered modulation.

It might be different in your application, how the diode is used will vary from one design to another, only a schematic will show how it is implemented.

Brian.

 

[MitchElectronics]

betwixt, thanks for the response!
My question is not design based, just wanted to know what typical voltages are produced by LC tank circuits for radio receivers. I could post all my schematics and designs to get help but I prefer not to be told the answer, I like to figure it out myself (learn so much more).

All the best!
Robin

 

[FvM]

You stated in post #1 "I have started to construct an AM receiver".

Unfortunately you didn't clarify the kind of AM receiver, so the question remains mostly unclear. Where do you see 50 mV? Are You talking about an amplifierless "detector" receiver?

 

[MitchElectronics]

FvM, Your right! I apologise...

Its a really simple one, here is an example:
http://www.circuitdiagram.org/images/simple-am-receiver-diagram.gif

I built the whole thing with no success so I have gon back and just have the LC part of that circuit with the diode. My areial is an incredibly long piece of wire that is about 3~4 meters. The ground is connected to my power supply and the oscilloscope which are both connected to the mains.

 

[betwixt]

That is basically a crystal set with a single stage audio amplifier.

The antenna is far too short to be effective, you really need one much longer (ideally about 300 metres!) but connected to a tap in the coil near the ground end to match it's impedance better.

Your question about how much signal to expect is difficult to answer because all the voltage you observe comes from the signal received over the air. If you are very close to a transmitter (there are several in the IOW area!) and you are tuned to it, the voltage can be quite high, maybe several hundred mV but for the vast majority of signals you hear on MW, the voltages are more likely to be < 10mV.

There are thousands of weaker signals from around the World which you will not be able to pick up for two reasons, 1. they need significant amplification before the detector diode to have sufficient amplitude and 2. the single tuned LC circuit is unable to distingiuish them from each other, for that you need greater selectivity. These require a more complex circuit.

Brian.

 

[MitchElectronics]

Nice response!

Will work on an amp stage first!

 

[Audioguru]

You need an EARTH ground and high impedance earphone (2000 ohms) for that extremely simple crystal radio. Did you find a very old OA91 germanium diode somewhere?

 

[MitchElectronics]

AUDIOGURU!

Soooo long since I have seen you! A sight for sore eyes

Well its a germanium diode but I cant remember the type. I have dropped that circuit now and currently making an LC tank connected to a BJT darlington amplifier. So far, just noise but thats better than before!

The strange thing is that sometimes I pick up an unusual continuous pulse.

I will let you guys know if I get anywhere!
Many thanks,
Robin

 

[jiripolivka]

I guess we can discuss this simple matter for a long time. I recommend you to go and try to make a crystal set and see how it works.
OA91 is a point-contact Ge-diode, you can find 1N60, 1N34 and other types in a discarded transistor radio. Instead, you can try any good transistor and use its e-b diode as a detector. Not the power types, though.
If you do not want to extend an outdoor long-wire antenna, try the ferrite-rod with the coil from a discarded AM radio (including the variable capacitor), or make a larger frame antenna, say 20-30 turns on a wooden frame, 3 x 3 feet square. This works too for a local AM station.

- - - Updated - - -

Instead of an old 2 kOhm headphones, find or make a piezo headphone, with > 100 kOhm impedance! A watch piezo element will work fine as a speaker if inserted in a small plastic box. Only frequency response is poor.

 

[vinodstanur]

This circuit remindes me the good old days. I still remember, I did it in a different way, I amplified the RF signal using a single stage transistor amplifier and then rectified it using the ge-diode and again amplified it using an audio amplifier. I used a very low value capacitor (around 30pF) to couple the parallel LC resonant circuit to the common emitter bjt amplifier (bc547) input. I was able to receive even far stations but mostly at NIGHT. My nearby station is 50 km away and I was able to receive it properly and was comparable to my portable radio. (only for 50km station, not for far away stations).

 

[Audioguru]

AUDIOGURU!

Soooo long since I have seen you! A sight for sore eyes

I was away for only one week in Mexico where my wife and I celebrated Christmas and our son got married. The Mariachi band was fabulous with 11 players.

I have dropped that circuit now and currently making an LC tank connected to a BJT darlington amplifier.

A darlington transistor has the same low (or no) amplification as an ordinary transistor when it drives low impedance headphones.

A piezo earphone cannot be driven with the transistor circuit because piezo does not conduct. The transistor needs a resistor (about 2.2k ohms) from its collector to +3V as its load then the collector can drive a piezo.

Your cell phone transmits pulses (I am here, I am here) that maybe the radio is picking up.

 

[betwixt]

As it is difficult to guess the actual inductance and therefore resonant frequency of your LC circuit, can you post a picture of it. Those of us with enough experience will be able to make a good guess at what is wrong. Almost certainly it is working but not letting you tune to any recognizable signal.

Rules of amplifiers in this kind of radio:

1. the amplifier input impedance must be as high as possible.
2. the amplifier input impedance must be as high as possible.

What you should strive to achieve is an LC circuit that resonates at the radio stations frequency and is as free as possible from any loading on it. Sadly, all amplifiers (and the antenna) load the LC circuit to some extent and that reduces the sensitivity and increases the bandwidth. A narrower bandwidth will make it easier to isolate stations from each other. In your locality there are several high power stations which you may have difficulty separating.

Brian.