Friday, 15 April 2016

PIXER- An empirical H.F. Superhet Receiver.-I

In my last posts I discussed an IF amplifier and a simple direct conversion receiver. Why not try building an empirical H.F. Station Superhet using these building blocks? It can make an excellent basis for both fun and learning.

Receiver basics:  It is imperative that certain criteria must be met in the design of even a basic receiver even of a simplest form. These include sufficient gain to provide reasonable sensitivity and selectivity.

The first requirement is to provide reasonable gain. The signal levels from the antenna are quite low, while reasonable power to drive a speaker is usually required. If a signal of 0.1uV is assumed to be received by a receiver with 50 ohm antenna impedance, the power available to the receiver will be 2 X 10^-16 watts. If we would have to produce a discernible signal at the output of the receiver by driving an 8 ohm speaker at 10 X 10^-4 watts or say one milliwatt , a gain of  (10 X 10^-4 )/( 2 X 10^-16 ) is thus desirable. This translates to 5 X 10^-12 or nearly 127 dB of the gain required. This is a typical figure for many receivers. Since signals having lesser power can be copied easily by sensitive speakers or headphones and/or full volume reception is usually not desirable, a little less gain is thus suffice our needs. The absolute minimum figure thus comes down to around 100 dB, you can say.

The second requirement is to process the input signal to produce a tangible output signal. This process is called demodulation or detection. This process differs for the signals carrying information in different forms called radio modes. And the third requirement of any receiver is to separate the desired signal from the crowd of un-wanted signals. This quality is referred as selectivity of the receiver which will be achieved using a home made x-tal filter. Shown below is the block diagram of a common superhet receiver:

The superhet receiver can really be seen in two parts: From IF amplifier onwards, it is a fixed frequency receiver, a receiver pre-tuned and optimized for the reception of a signal on the IF frequency. The RF amplifier and mixer/oscillator receive signals from the antenna and then convert them to the frequency of this optimum receiver - to the IF frequency. It is in the RF amplifier and mixer/oscillator sections of the receiver where the actual operator adjustment and tuning for the selection or choice of received signal takes place.

The choice of Intermediate Frequency:  There are two conflicts with the choice of the IF Frequency. A low intermediate frequency brings the advantage of higher stage gain and higher selectivity using high-Q tuned circuits. Sharp pass-bands are possible for narrow-band working for CW and SSB reception. A high intermediate frequency brings the advantage of a lower image response. The image frequency problem can be seen in this example.

Consider a receiver for 10 MHz using an IF frequency of 100 kHz. The local oscillator will be on either 10.1 MHz - i.e. 100 kHz higher than the required input signal - or on 9.9 MHz. We will consider the 10.1 MHz case - but the principles are the same for the case where the oscillator is LOWER in frequency than the wanted signal frequency. . Because of the way that mixers work, a signal at 10.2 MHz will also be received. This is known as the "image" frequency.

The image rejection of a superhet receiver can be improved by having more tuned circuits set to the required input frequency, such as more tuned circuits in the RF amplifier ahead of the mixer. This brings practical construction difficulties. Another solution is to choose a high IF frequency so that the required received frequency and the image frequency are well separated in frequency.

Choosing an IF of 2 MHz for the 10 MHz receiver would put the local oscillator at 12 MHz, the image frequency then being at 14 MHz. When receiving a signal at 10 MHz, it is easier to reject a signal at 14 MHz (the image in the 2 MHz IF case) than at 10.2 MHz (the image in the 100kHz IF case).

(Note that the Image Frequency is TWICE the IF Frequency removed from the WANTED signal frequency - on the same side of the wanted frequency as the oscillator).

PIXER H.F. Superhet Receiver:  The design of our empirical receiver is largely based upon the IF strip using home brewed mixer ICs, from my old posts of 9th and 24th March,2016. This IF strip is designed around a handful of "off the shelf" category inexpensive components and represents a quite respectable gain of about 68db on 4.43 MHz along with good AGC characteristics. I have chosen this frequency for my IF since color burst  X-tals for this frequency are easily available and are quite cheap. The complete circuit of IF amplifier is given below:

A product detector has to follow the IF amplifier. I decided to use the simple DC receiver described in my last post as the product detector. Minor modifications were needed and this work was not difficult as all the construction was done, using dead bug technique. The input slug tuned inductor was a commercial unit out of an old equipment. It has nine turns with a tap at five turns from ground and has a value of 4.7 uH as measured. A four turn link near ground end is wired as secondary. The tail transistor T3 is wired as a colpitts oscillator and an external BFO injection is thus not required. This added some simplicity to the final product detector shown below:
As you can see, Q4 has been removed as the product detector along with AF amplifier, an LM386N, provides about 30db gain which is more than enough. I used the LM386N in its low gain configuration i.e. I did not use a capacitor between its pin 1 and pin 8. As per my experience this basic configuration is really stable for this little device. It becomes unstable and noisy when we try to strain it by juicing up more gain out of it.

This completes our AF and IF stages. In the next post we will have a look at front end design of our receiver.

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