The 'S' Meter
An 'S' meter is a RF signal strength indicator used in communication receivers. Signal strength is an inevitable quality factor the status of which is shared during official/amateur radio communications. Usually it is the AGC voltage from the IF stage of the receiver that is used as reference input for an S meter.
In certain rigid Receiver designs an additional preamplifier that is supposed to bring the input RF voltage to the level of 'S' meter reading also are found. In C-11/1 the circuit diagram of an 'S ' meter is shown.
Those who have explored communication receiver technicalities might have come across words like Blanker, Squelch, Notch etc. In full-fledged communication receivers there is special arrangement for Noise Blanking. When a noise interferes in a Receiver front end, usually it appears in pulses with frequent breaks. Noise Blanker circuit cuts off the functioning of the receiver during those fractions of a second in which noise pulses appear.
If squelch controls are practically irrelevant in HF ranges, it is unavoidable in VHF and higher ranges. With squelch open, the receiver functions only if the incoming signals are above a specified strength level. A squelch circuit consumes only very negligible energy.
The purpose of Notch filters is to avoid the notches formed in the front end of a receiver because of signals from unexpected stations for varying reasons. Fig: C-11/2 shows how a Notch Filter functions. In fig. C-11/2, the working parameters of a Notch Filer is shown.
If a coaxial cable at quarter wavelength of a frequency is open at one end the other end will be closed to that frequency and vice versa is the theory behind Notch Filters.
In addition to all these, overload protector modules also are found in the IF stages of some receivers. This is interstage degenerative feed back using diodes. If there are similar feed back biasing arrangements in the IF stage, we need to consider the take over time also. 'Take over time' is the time required for developing a bias voltage from the rectified signal of the frequency that passes through the IF stage. During CW between every 'Dit's AGC is cut off and take over happens at the next ''Dit'. This intermittent activity may appear as a sort of pulling in the receiver. In such case it is better to switch off the AGC during CW reception.
In the IF stage of an FM Receiver, there will a Discriminator stage and a Limiter circuit also. The purpose of the limiter circuit is to remove the effects of Noise Distortion signal that changes the amplitude of the cycles. A limiter circuit helps to maintain a signal at even amplitude. The purpose of the discriminator is detecting the modulated intelligence (sound) from the RF. Those Discriminators which do not require Limiter circuits are called Ratio Detectors. Generally, the sensitivity of Ratio Detectors are comparatively low.
To be frank, none of these strict design parameters are necessary to access an AM Radio Frequency signal. A Tuned Radio Frequency Receiver is very simple. I did not miss that experiences too. IN my early days I enjoyed listening the near by station with a receiver which had only just 3 components - a 5 meters long wire, a headphone and a RF diode. See a Headphone Receiver in fig. C-11/3. Diode polarity is not significant in this circuit. Just keep the headphone close to the ear - no power required, no rigid control arrangements too.
It is with an eye on enthusiastic home brewers that one more simple Receiver circuit is given here.
The Super Reflex Receiver detailed in C-11/3 (A) is enough to cover a wide range of Radio signals between 5 MHz and 21 MHz. To assemble this Super Reflex Receiver, a custom Printed Circuit (PC) board or a bread board is not necessary. This can be done with 2 leg strips having ten connection tags each fixed parallel at a distance of one inch. To avoid microphone related problems fix the strips lose on rubber pads, instead of fixing them strongly on solid surfaces. To reach best results kindly do not compromise on components quality. To access 21 MHz, perhaps you may need the S-1 coil shown in C-11/3B. S-1 coil is to be wound on a .75" dia. plastic tube. One part of the coil is to be wound over the six match sticks glued around the plastic tube at equal distance. It is to be noted that both the coils are to wound on the same direction. Even though BC 169C is recommended at Q1 2N 3900 also is found to be giving equal performance. Best if the NTC trimmer is variable from 1 PF to 7.5 PF. Set the 100K control (connected in series to a capacitor and diode) in the middle of its tuning range and tune this NTC trimmer. If required, the value of the 100PF capacitor connected in series to the Antenna also can be reduced a little.
To avoid capacitive coupling between both the coils wound on either sides of the plastic tube wind a few turns in between them and ground one end of winding. Fig. C-11/3 shows how the coils are wound.
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