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stanislav_maslovski
modified 8 years ago

Reflex AM Detector

4
16
136
01:58:02
In this circuit, the RF AM signal is amplified by the transistor, detected, and applied back to the input of the transistor. Then, the same transistor amplifies the detected signal. Hence, one transistor has a function of two in this circuit. Even more: this circuit has a built-in automatic gain control: for signals with higher amplitude the gain is lower.
published 9 years ago
hurz
9 years ago
Interesting, an extra 12.7pF cap parallel to the coil would better suppress the carrier signal. Keep it up!!!
stanislav_maslovski
9 years ago
Thanks! Yes, adding a cap will help to filter the carrier better, but it will also narrower the low-frequency bandwidth of the detector.
hurz
9 years ago
No, i m not talking about the lowpass cap. Sure increase it just lowers the corner freq. I mean a bandstop tank made with the existing 2mH plus parallel extra 12.7,pF! This would almost completely block the carrier.
hurz
9 years ago
Here you can see the modification of your circuit http://everycircuit.com/circuit/5032795027013632
stanislav_maslovski
9 years ago
I got what you meant, hurz. What I mean is that, theoretically, replacing an inductive load by a high-Q LC tank makes an RF amplifier more narrowband. But as I now checked in the simulator there is still pretty much bandwidth in this circuit :) Capacitor added!
hurz
9 years ago
It just blocks the carrier and does not make the AM bandwidth smaller. Its still 100kHz. The only thing one could argue, the cap is quite small and the inductor big, so the parasitic capacitance of this coil might already be bigger then 12pF. However this can also be changed by adapting the lowpass impedance. Now the sinewave looks perfect ;-)
stanislav_maslovski
9 years ago
Hurz, the LC tank is the load of the RF amplifier. If this tank has (external) Q factor such that Q > 5, then its bandwidth will not be enough to fit the whole AM signal bandwidth (200 kHz in this case). This is what I am talking about.
hurz
9 years ago
stanislav_maslovski, the lowpass -3dB corner is already at about 100kHz. So having a notch at 1MHz wont change anything. Anyway Q will be much higher than 5! The notch freq is 10 times higher then the AM bandwidth!!!
stanislav_maslovski
9 years ago
Ok, let me try to explain better. This transistor in this circuit is both RF and LF amplifier. At RF the load of this amp is the LC tank (and the input of the detector) , and at LF it is the low-pass filter. As you say correctly, the cutoff frequency of this low-pass is around 100 kHz. Adding the capacitor you suggested does not change this cutoff frequency. However, adding this capacitor changes the load impedance at RF, thus it affects amplification at RF. In other words, it affects the amplitude of the RF signal at the point BEFORE the low-pass (at the collector). Do you get it now?
stanislav_maslovski
9 years ago
Now more on Q. In this circuit, the characteristic impedance of the LC tank is about 12.5 kOhm. This tank is shunted by the input impedance of the detector which is relatively low (let us say 10-20 kOhm), and because of that the EXTERNAL (i.e. loaded) Q of this LC tank is low, and its bandwidth is large enough to fit the 200 kHz AM signal band.
hurz
9 years ago
To make it short, the cap improves the demodulator output a lot! Now, were exactly do you see a problem?
stanislav_maslovski
9 years ago
It improves suppression of the carrier, yes, but depending on the other parameters of the circuit, it can reduce the amplitude of the detected LF signal, and also distort the spectrum of such signal (in a situation with many frequency components in this signal). I do not get why you do not get it :)
stanislav_maslovski
9 years ago
The mentioned frequency distortions will appear when the bandwidth of the LC tank 2*df = f_res / Q is smaller than twice the modulating signal bandwidth.
hurz
9 years ago
Keep cool, i like what you are publishing on EC. There are not many circuits i pay attention when users are going public with it. But your name is always promissing good circuits ;-) check this and let us continue in discussion http://everycircuit.com/circuit/4557394358042624
stanislav_maslovski
9 years ago
I checked your circuit. It surely demonstrates what you want to show, but as I said above myself, in THIS particular circuit, with THESE particular parameters of all the elements, there is no problem of adding the cap: The external Q of the LC tank in this case is below unity, and the respective resonance in transistor's load impedance is very wide around 1 MHz. But imagine another situation: one may think that 12 pF cap is too small in practice for 2 mH coil (as you mentioned yourself!), and thus one may want to split the 2 mH coil in two parts, with the smaller part shunted by a cap to get the same notch effect at 1 MHz. Then, there can appear a problem which is shown and explained here: http://everycircuit.com/circuit/6430306658680832
hurz
9 years ago
I wont split the coil in two parts!

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