There are many problems with slope detection, poor interference rejection, poor selectivity, poor adjacent channel rejection, they are less sensitive because you are tuned to the signal fringe and their linearity depends on the passband shape. As the signal moves closer or further away from the tuning point it amplitude seems larger or smaller and this is what is recovered as AM. It relies on the tuning point being half way down the edge of the bandwidth slope so the center point is only half signal level. In a slope detector, tuning to the carrier may result in little or no audio at all. This means you can filter the carrier to limit it's bandwidth and the optimum tuning point is central to the carrier where the signal is strongest. In a real FM demodulator, the carrier frequency equates to the 'zero' of the recovered audio and shifting the frequency above or below center results in an increase or decrease in the voltage output from the demodulator. It's not a true FM demodulator although it can recover some audio. They can receive FM signals of course but they work equally well, if not better, when receiving AM and they do not take full advantage of the properties of true FM receivers. I'm not critisizing the quality of slope detectors or super-regens, but to it's wrong to confuse the way the designs shown here work. The FET design in post #35 is better in most respects but is still AM. It is an AM receiver and the audio recovered from it is from slope demodulation. Granted, there will be a degree of injection locking as there is in most self-quenching super-regenerative receivers but that would tend to reduce the audio even further. Furthermore the change in oscillator current due to a small frequency shift compared to the 100MHz or so it's designed to work at would only recover an incredibly tiny audio signal, especially when developed across only 10K of resistance. It IS posible to lock the oscillator to the incoming signal if there is a control method for doings so (PLL style), but there is none. I have to side with Audioguru on the way this functions. Although you might be able to hear the radio traffic, you could be blocking it for essential personnel. If being used near an airport or emergency services PLEASE heed the warning about the interference they can cause. I have built super-regen receivers working up to almost 1GHz with success so higher frequencies than the FM broadcast band are certainly possible. I would go with the pre-amp if at all possible.Īs pointed out earlier these are AM receivers not FM but they will convert frequencies shifting in and out of their passband into variable amplitudes (slope demodulation) so FM can be resolved. Adding the pre-amplifier stage will not only make the receiver more sensitive, it will block some of the interference and also reduce the effects that nearby objects have on altering the tuning. The nature of super-regenerative receivers dictates they must transmit a certain amount of interference and it will not be a 'clean' signal, it could spread quite widely into nearby frequencies. The complete schematic of this superheterodyne shortwave receiver is available at google drive.Be careful using these receivers in areas where they might cause interference to other users. To receive other stations which including VoA, Radio France, Deutsche Welle, ABC we use our 2 × 12 feet T antenna.įor initial IF transformer tuning we recommended to use a quite large antenna and some powerful station. With small wire antenna, this receiver is capable to receive most of the stations which including CRI, BBC World service, All India Radio, etc. In our testing, we use a TDA2050 audio power amplifier with this receiver. This receiver is designed to work with 9V regulated power supply and it requires an external AF amplifier to drive the headphone or speaker. For precision tuning, connect small trimmer capacitor with parallel to variable capacitor’s gangs or use mechanical wheel assembly with the variable capacitor. For tuning use Polyvaricon or air variable capacitor. This receiver can be constructing on PCB, breadboard or commonly available veroboard or stripboard. Prototype version of two transistor superheterodyne shortwave receiver. With quite good external antenna this receiver performs similarly to the commercially available shortwave receiver and its audio quality is at a very satisfactory level. This is two transistors and two IF transformer based superheterodyne receiver which is designed to receive 13m to 41m bands.
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