TheRadioBoard

I still can't get it into my mind's eye, the detection process in a tube or transistor circuit.

ie. What (magic) suddenly turns a stage from an RF amp, into a detector?

In a regen, the one device does three jobs at once (go figure).
Is it as simple as having an input capacitor (or not) on the grid input?
Or AF filtering on the output?

I have read a lot of descriptions of the process but I just can't "get" it.
Need a very simple explanation (hopefully not including sidebands, modulation envelopes and the like. etc.)

.............................

I also would like to know!

SW

In an AM detector the signal is multiplied by itself in a non-linear device. You need a load that responds to audio frequencies. Use a high-inductance choke or just a resistor, with an rf choke to block what you don't want.

Sorry but that's as simple as I can make it.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

I think it's non-linearity, i.e., square-law detection.

For a regen, let's consider a BJT detector. My understanding is that device gain always varies with input current due to inherent non-linearity of the device. The variation may be small, but it is there, even if the device is biased to operate in its "linear" region. So higher input current leads to higher gain and higher output current, and lower input current leads to lower gain and lower output current. This distorts the waveform; positive signal peaks are amplified more than the negative signal peaks. (Note that "positive" and "negative" refer to the original incoming AC signal voltage, whereas the actual voltage at the transistor input is shifted upwards by the device bias voltage.) Now if we pass in a modulated RF sine wave into such a detector, the positive peaks of the signal will get much larger while the negative peaks will get only slightly larger. If we repeat this process over and over as in regeneration, the positive peaks (and the modulated differences between the positive peaks) get bigger and bigger while the negative peaks get comparatively flatter and flatter. The result is that we effectively have chopped off the bottom half of the incoming AC waveform. Smooth out the resulting waveform (filtering out the high-frequency components) and the original modulation, riding on the peaks of the carrier, is recovered, i.e., detection has occurred. (This is only possible because the bottom half of the waveform has been eliminated so that the negative peaks don't pull down the positive peaks for an average output voltage of zero.)

For an RF amp vs. an RF detector I think the story is the same: it depends on whether the device is biased to operate in its linear or non-linear (square law) region. Negative incoming signal voltage, in a detector, tends - I think - to pull the device into cut off so that little or no amplification occurs, whereas positive incoming signal voltage pushes the device bias up into its amplifying region so amplification occurs. That's how I visualize it - if I got it wrong, somebody please correct me!

There's a more thorough mathematical explanation about how the presence of squared terms in a device's input-to-output function guarantees the recoverability of the original modulation, but I'm not sure I grasp that yet.

Looking forward to any more insight or discussion.

I guess I should have included "or mention of non-linear devices".

It seems that on one hand, the device "amplifies" an analogue (RF) signal (ie. modulated sine wave "in" and "out") but in the other case, it must (of necessity?) "rectify" (demodulate) an RF signal to produce pulsed AF across the RF choke.

So is the RF choke the key?

........................

This may be somewhat redundant to what has already been said:

A practical answer to is the use of "improper" bias in order to asymmetrically clip the waveform and the addition of a filter to remove the RF portion of the clipped waveform.

I can get that bit (WRT a regen).

Looks like it's tied up with different biasing points on the "graph".

Tube detection seemed to be tied up with the gridleak (timing) circuit in old sets (only on the detector tube in TRF's) but the explanations, as I remember them, went over my head. (Something about charge and discharge of the gridleak cap. - value affects audio bandwidth.)

.................................................

So:

RF amp
1. biased to linear portion "of graph".

Detector
1. Biased to non-linear section of graph
2. Removal of RF component (filter).

................

I don't know how tube detectors work, but this fellow seems to: http://www.zen22142.zen.co.uk/Circuits/rf/marr.pdf

There's an explanation there about how the grid leak works in a tube regen. I don't have enough of an intuitive understanding of tubes to easily follow the explanation, but perhaps it might be decipherable to those of you who know more about tubes.

Ramon tells all - thanks, I think I have a better understanding now (at least for tubes).

Grid leak turns grid/cathode into a diode for AF detection which (AF) is also amplified by the tube in the "normal" way. Leak values (RC time constant) are important.

Lack of grid leak allows for normal RF or AF amplification.

....................................

It appears that with solid state devices, the BE or GS "junctions" demodulate a (modulated) signal regardless, and you just filter off the audio, for further amplification if necessary, at which ever RF stage (point) it suits?

....................................................

Hi there golfguru,

When you make it a requirement like that its impossible!

To be a detector "requires" a non linear device. Even if that non-linear device is JUST a single diode.

Also, to adequately explain detection "requires" talking about sidebands.

Detection is NOT just rectification!

ok?

73
kb0lxy

uhhm not quite. Grid and cathode is already a diode(even without grid leak components). Since it is a diode it is also a non-linear device and can detect!

I just thought of something that may help.

There is no magic. There is no difference! Its all a matter of sensitivity!

If the "diode" of the amplifying device is sensitive enough(as a detector),compared to the level of the incoming AM, then it WILL detect it.
You may or may not be able to tell that there is detection going on.

If for example I connect an RF amp to an audio amp and speaker, I might not hear anything. Then if I bring an AM transmitter In close (like a 4 Watt CB) I will hear the transmission, because the sensitivity of the amplifier(as a detector) is enough to detect this stronger signal.

This actually happens sometimes!

The grid leak components that you talked about earlier set the bias on the tube and therefore set the "sensitivity" of the tube(as a detector).
This lets the tube amplify a weaker signal than it can detect.

Does this help at all?

73
kb0lxy

Hi Hamer,
I guess I should have phrased the question "how detection is achieved within the different topologies".
I am happy accepting that a diode demodulates - how it does it, can stay in the land of "smoke and mirrors".

Understanding where and when the demodulation occurs in the relative circuits was my main aim.

............................