Chapter 9 - Communication Receivers

  

    On our way of upgrading Receivers for higher performance in quality and operation Tuned Radio Frequency (TRF) Receivers were replaced by Direct Conversion Receivers and later by Super Heterodyne Receivers. Receivers are still evolving. It is not easy to demodulate all the Radio signals that are different in modulation modes, frequency, bandwidth, strength and stability at equal quality and extract from it a standard hi-fi audio out put. It is interesting to learn that some ultra modern receivers need special coaching classes to operate them.

Lat us call that part of the Radio Receiver which receives a Radio signal 'Front end'. We know that those Radio signals which brushes the front of a receiver precisely belongs to three categories, Ground waves (LF),  Sky Waves (HF) Direct Waves (VHF), depending upon the frequency range in which they are. Such a classification because they are different in propagation features too. 

These signals come to the receiver together with all distortions they could gather from the atmosphere. At HF and lower ranges the External Noise Figure (ENF) will be higher than the Internal Noise Figure that develops from within the receiver. The first part of a front end certainly is the antenna. An antenna perfectly matching with the front end impedance of the tuned circuit there will definitely excel very much in picking even very weak signals belonging to the range to which it is tuned for, along with rejecting out of band signals to a great extend.

It is this rejection - selection process that we call selectivity. Until the modulated intelligence is extracted and produced in the out put all modules in a Radio concentrates on selectivity principles. 


In exclusive communication receivers meter bands like 120, 80, 40 20 etc. (those allotted to Amateur Radio Operation)  are isolated and let through exclusive circuits for them to improve selectivity. Noted receivers may have more than one resonant networks at its' front ends. To compromise with the differences in bandwidths at various bands, preselector controls are used. If preamplifiers are used in the front end tuned networks are necessary at the input and out put areas. Every time when commercial circuits are copied, the components used should also be strictly of the same make and quality. If there are additional RF stages, at all conditions the RF stage gain of the circuitry should be set a little higher than the Noise Figure (NF) of it. Connecting Preamplifier stage performance with  AGC (Automatic Gain Control) considerably reduces the Dynamic range of any receiver. Dynamic Range suggests the ability of a Receiver to convert all types of signals varying in power to a more or less standard strength in the output.

Image signals, harmonics and atmospheric noise factors are major disturbances that hurt the quality of signals seriously. One effective way to avoid these issues considerably is converting the modulated signal into a different Intermediate Frequency (IF) having no division multiplication connection with either the fundamental frequency or the oscillator signal frequency. There are no definite rules with regard to what the IF should be. Even though at lower frequencies the selectivity and stage gain will be higher oscillator stability will be adversely affected proportionally. Even at the standard IF of 455 KHz, while receiving  14 MHz and above image signal possibility is higher. One technique to avoid them is using one more IF stage with a different frequency. Adding more no. of IF stages however adds to Inter Modulation Distortion (IMD) and fall in Dynamic Range. Frankly speaking, a single conversion receiver with a well tuned single IF stage is found better.

While designing a RF preamplifier at the Receiver front end, selectivity, stage gain, stability, dynamic range ..... all these factors are to be seriously considered. In fig: C-9/1, four distinct types of Pre amplifiers are shown. 

In Chapter five, we had already given the circuit of a high gain RF amplifier stage using bipolar transistor 2N2222A (Fig: C-5/3).
In C-9/1C, see a variable resistor itself connected in series to the antenna which proposes to control the stage gain. The purpose of the diodes connected at T1 secondary in C-9/1C is to ground off high voltage Noise Factors right at the front end itself.
In C-9/1D stage gain is controlled using a variable resistor in series to the ground at the transistor emitter. This certainly will affect the transistor operation factors and also raise NF of the stage.

In Receivers proposed to receive signals from multiple bands, it is common that an extra mixer is used to convert the RF signals at the front end itself. 

Fig. C-9/2 is the block diagram of a Model Communication Receiver. 

Here, the Master oscillator is intended only for a single frequency range. The arrangement is such that the signal coming through the RF amplifier will be such that at Mixer two the frequency of that will be exactly what that produces the desired IF at Mixer two output. Even though only two Meter bands are shown in the diagram more bands can be added with more no. of corresponding X'tals also simultaneously switched. 

All the disciplines and design rules applicable for a good VFO is relevant here for he oscillator stages used at the mixers. If the master oscillator is well shielded it will avoid stray radiations that affect other stages in its' vicinity.  If the frequency variation done in the master oscillator is limited to a maximum of 15 KHz, VXO (Variable Crystal Oscillator) are the best choice. Even though the frequency variation without losing stage stability is limited in VXOs, it is to be noted that it proportionally increases with the oscillator frequency of the Crystal.  In C-9/3,  a 7 MHz VXO circuit with a tuning range of 15 KHz is given. 

In C-9/3, first bring the inductance of L1 to 20 μH and further set the tuning range by adjusting the 100 PF capacitor connected between L1 and ground. If the 7 MHz VXO output is let through a frequency doubler, at 14 MHz the tuning width also will be doubled to 30 KHz.  That is, if a VXO having 12.5 MHz tuning width at 160 meters is multiplied twice more using two more Crystal Oscillator stages to bring the output to 7 MHz, even a full 100 MHz tuning width is possible. It is mainly because of this favour that VXOs are used in VHF receivers. Whatever be the oscillator stage, it is always best to let the output signal through a bandpass filter. If the oscillator signal is not clean the NF factor in the receiver will go higher and the selectivity lower.  

The Oscillator signal in C-9/2 reaches the main Mixer (Mixer ll) after an initial conversion in the front end. In some receivers, even the RF received at the front end reaches the main mixer (Mixer ll) only after a conversion.  In similar cases another conversion is not necessary at  Master Oscillator. 

In C-9/4, the circuit diagram of a 144 MHz - 14 MHz convertor receiver is shown. 

Before extracting the audio intelligence (demodulation) from the signal coming through the tuned IF stages, a lot of critical treatments are essential to the received RF signal. Most important of them are setting the tuning width of the signal and maintaining the selectivity factors. The efficiency of any IF stage is dependent on the strength of the input signal, the peculiarities of the AGC (Automatic Gain Control) circuit and finally the  quality factors of the active components used. If the number of tuned circuits in an IF stage is increased, it will definitely add to 'selectivity factor. At the same time it will adversely affect signal fidelity and causes side band cuttings too. In 455 KHz IF amplifier circuits two active stages are recommended for optimum performance. 

At the same time, remember that a mechanical, crystal or ceramic filter is enough to avoid unnecessary mixer products, image signals and harmonics from IF stages along with assuring necessary selectivity and bandwidth. In commercial receivers, different different types of filters are used both at different different bands and modes like CW and SSB. The same result can be obtained using Ladder Filters using more than one crystal (fig: C-9/5A).  
    

In fig. C-9/5A 6 easily available 4.433618 MHz Crystals are used to make a filter of 2.26 KHz bandwidth. 455 KHz ceramic filters for use in ordinary receiver IF stages also are available. 

Fig. C-9/5B shows how a 455 KHz ceramic filter is used in a 455 KHz IF stage. The internal structure  of a Crystal filter is shown in fig. C-9/5C. Since the In - Out sides are identical in it, connecting leads are one and the same for both In and Out. 

Those using SSB transmitters can easily use the filter in the SSB exciter as Receiver IF Filter, provided the receiver IF and the filter rating is one and the same.

 In C-9/5D a 9 mHz Crystal used in a SSB exciter is shown being used simultaneously as a Receiver IF Filter. It is diode switching that is used here. 

When Positive voltage reaches at the anode point of D1 through R1, D1 gets forward biased and the signal from the balanced modulator comes at the input of the Crystal Filter. At that time D2 will not be forward biased Here, neither the Receiver IF signal disturbs the Crystal Filter nor the signal from the balanced modulator hits at the Receiver IF through D2. Instead of applying positive voltage at D1 if D2 is charged through R2, the performance is reversed. If the output of the post filter amplifier is fed into the primary of a 9 MHz tuned transformer primary with two secondaries one secondary can be connected to transmitter mixer and the other receiver IF stage. The stage which is activated only will be working. 

Chapter 8                                                         Chapter 10