Sunday, 24 April 2016

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

In the last post I described the IF and product detector of an empirical superhet receiver developed around home made ICs as described in my post of 9th February (parts shown in dotted squares). In this post I am going to describe the front end design of this receiver.

Initially, I decided to wire the front end using another home brewed mixer IC. But out of an amateur's true spirit I decided to do experiment a bit more. After a receiver's sensitivity, the next requirement is its ability to discern weak signals in the presence of strong signals in its pass-band. This is known as dynamic range of the receiver.

There are several types of dynamic range. The first one, and probably the easiest to understand-"AGC range"-concerns whether a receiver is capable of maintaining a constant audio output level over a large input-signal amplitude range. The traditional school of thought requires AGC action to commence at about 3┬ÁV, leading to a condition where signals that produce an excellent signal-to-noise ratio may show absolutely no S-meter indication-a most undesirable effect. The reason for this is inappropriate receiver gain distribution-generally, a lack of gain at the IF. Maintaining constant audio output must involve gain control at the receiver's IF, and possibly even at its input.

IMD Dynamic Range:  The output of a linear stage tracks the input signal decibel by decibel, with every 1-dB change in its input signal corresponding to an identical 1-dB output change. This is the stage's first-order response. Because no device is perfectly linear, however, two or more signals applied to it intermodulate to some degree, generating sum and difference frequencies. These intermodulation distortion (IMD) products occur at frequencies and amplitudes that depend on the order of the IMD response as follows:

•Second-order IMD products change 2 dB for every decibel of input-signal change, and appear at frequencies that result from the simple addition and subtraction of input-signal frequencies. For example, assuming that its input bandwidth is sufficient to pass them, an amplifier subjected to signals at 6 and 8 MHz will produce second-order IMD products at 2 MHz (8 - 6) and 14 MHz (8 + 6).

•Third-order IMD products change 3 dB for every decibel of input-signal change, and appear at frequencies corresponding to the sums and differences of twice one signal's frequency plus or minus the frequency of another. Assuming that its input bandwidth is sufficient to pass them, an amplifier subjected to signals at 14.02 MHz (f1) and 14.04 MHz (f2) produces third-order IMD products at 14.00 (2f1 - f2), 14.06 (2f2 - f1), 42.08 (2f1 + f2) and 42.10 (2f2 + f1) MHz. The subtractive products (the 14.00 and 14.06-MHz products in this example) are close to the desired signal and can cause significant interference. This is why our receivers' third-order IMD performance is so important. It can be seen that the IMD order determines how rapidly IMD products change level per unit change of input level. Nth-order IMD products therefore change by n dB for every decibel of input-level change. IMD products at orders higher than three can and do occur in communication systems, but the second- and third-order products are most important in receiver front ends.

Intercept Point: The second type of dynamic range concerns the receiver's intercept point, sometimes simply referred to as input intercept. Intercept point is typically measured by applying two or three closely spaced signals to the antenna input, tuning the receiver to count the number of resulting spurious responses, and measuring their level relative to the input signal.

Because a device's IMD products increase more rapidly than its desired output as the input level rises, it might seem that steadily increasing the level of multiple signals applied to an amplifier would eventually result in equal desired-signal and IMD levels at the amplifier output. Real devices are incapable of doing this, however. At some point, every device overloads, and changes in its output level no longer equally track changes at its input. The device is then said to be operating in compression. Pushing the process to its limit ultimately leads to saturation, at which point input-signal increases no longer increase the output level.

The power level at which a device's second-order IMD products equal its first-order output (a point that must be extrapolated because the device is in compression by this point) is its second-order intercept point. Likewise, its third-order intercept point is the power level at which third-order responses equal the desired signal. The following figure represents these relationships:

A linear stage's output tracks its input decibel by decibel on a 1:1 slope-its first-order response. Second-order intermodulation distortion (IMD) products produced by two equal-level input signals ("tones") rise on a 2:1 slope-2 dB for every 1 dB of input increase. Third-order IMD products likewise increase 3 dB for every 1 dB of increase in two equal tones. For each IMD order n, there is a corresponding intercept point IPn at which the stage's first-order and nth order products are equal in amplitude. The first order output of real amplifiers and mixers falls off (the device overloads and goes into compression) before IMD products can intercept it, but intercept point is nonetheless a useful, valid concept for comparing radio system performance. The higher an amplifier or mixer's intercept point, the stronger the input signals it can handle without overloading. The input and output powers shown are for purposes of example; every receiver exhibits its own particular IMD profile.

Input filtering can improve second-order intercept point; device non-linearities determine the third, fifth and higher-odd number intercept points. In pre-amplifiers, third-order intercept point is directly related to dc input power; in mixers, to the local-oscillator power applied.

Intercept point can be confusing because it can be specified in terms of input or output power. Intercept point should be referred to device output because that's where the trouble occurs, but input intercept is commonly given. Therefore, if an amplifier or a mixer has a particular intercept point-let's say +30 dBm at 10 dB gain-and then its gain is increased by an additional 10 dB, its dynamic range decreases by the amount of the gain.

Thus the first requirement for a receiver's front-end to have a good dynamic range, is a good mixer. My choice thus zeroed on the simple diode ring mixer which already has gained popularity among amateur fraternity. Double balanced mixers are a form of what is termed a "reversing switch mixer." Reversing switch mixers operate by using electronic switches in a bridge formation to reverse the input RF signal under the action of the local oscillator used as a square wave switching signal. They normally offer significant advantages over analogue mixers for radio communications and general RF design applications as they are able to offer better levels of dynamic range and noise. In view of this fact, they are normally used in high performance applications where noise and dynamic range are of importance - e.g. in the front end of a radio receiver or spectrum analyzer.

Although there are comparatively few components in a double balanced mixer, their individual performance is crucial to the performance of the RF mixer as a whole. Normally Schottky barrier diodes are used for the diode ring. They offer a low on resistance and they also have a good high frequency response. Ordinary signal diodes may be used for low performance applications, although the cost difference is small. It is found that the diode forward voltage drop for the diodes determines the optimum local oscillator drive level. RF mixers requiring to handle a high RF input level will need a correspondingly high LO input level. As a rule of thumb the LO signal level should be a minimum of 20dB higher than either the RF or IF signals. This ensures that the LO signal rather than the RF or IF signals switch the RF mixer, and this is a key element in reducing intermodulation distortion, IMD, and also maximising the dynamic range.

To increase the required drive level, it is possible to place multiple diodes in each leg. The most common LO drive level for a double balanced mixer is probably +7dBm. However they can be obtained with a variety of drive levels. Values of 0, +3, +7, +10, +13, +17, +23, and +27 dBm are normally used.

I decided to use a home brew mixer using common inexpensive diodes in the front-end, the schematic is as under:

The input signals is passed through a double tuned band pass circuit made around slug tuned, self wound inductors. I included a low pass circuit owing to ring mixers ability to respond to strong, harmonic signals. The given values are chosen for forty meter band, but they can easily be scaled for other frequency bands, too. Or even a multi band operation is also possible using suitable band switching. The home made mixer uses four matched diodes and two RF transformers wound on pig-nose balun cores. Transformers have thirteen trifilars turns. I used inexpensive diodes in the mixer but they behaved extremly well. Any 2.5MHz VFO capable of delivering reasonable power can be used. Amateur literature is already full of several circuits. For a good input intercept to be maintained it is important to properly terminate all mixer ports. IF port of the mixer is terminated in a post mixer amplifier that terminates the mixer output to an appropriate impedance, required to maintain a good IP3. The IF signal is then routed to a crystal filter through a post mixer amplifier, as shown below:

The post mixer amplifier uses noiseless inductive feedback. I used 2N3866 as it was available, but 2N4427, BFW16A etc. will seem to work equally well. Keep the transistor leads as short as possible and try to use ferrite beads in the collector lead to avoid spurs. I used a variable bandwidth X-Tal filter whose bandwidth can be controlled with R12. Cheap color burst X-Tals of 4.43 MHz are used for the x-tal filter. On twenty meters and above a low noise amplifier ahead of mixer is recommended to achive optimal noise figure.

The overall performance is amazingly good and despite designed around common off the shelf type inexpensive components it performs really well, far better than many commercial receivers.

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