Inside Receivers

The Universal Rules for Good Receiver Design

The most important factor in the operation of a wireless system is the overall quality of the receiver design. The better the receiver design, the better the overall performance of the entire system. There will always be manufacturers in the marketplace making wild claims about specific "proprietary" circuitry, however there are recognized standards for good RF design as published in the American Amateur Radio League handbook. These rules are not unique to any one manufacturer, but are a basis for a quality design as recognized by unbiased source. These principles are designed to help the user make an informed decision on the true virtues of one product over another, and better educate the user to screen-out "marketing advantages" from actual real world performance standards. There are no industry standards for wireless microphone measurements, so it can often be difficult to determine the real specifications of a given product. Some universal "standards" still apply however, with the key ingredients in a good receiver design being:

• High Sensitivity

  High sensitivity coupled with a wide dynamic range and good linearity to allow the receiver to cope with both weak and very strong signals that can appear together at the input. It should be able to do this without a reduction of signal-to-noise ratio, while keeping intermodulation and hum and noise to a minimum. Generally, the more sensitive the receiver is, the greater the effective operating range of the system. Sensitivity is measured in microvolts which is expressed by the symbol (mV). A sensitivity of .5 mV is considered excellent.

• Good Selectivity

  To allow the receiver to "discriminate" between possible competing signals on adjacent or near adjacent frequencies.

• Good Spurious Rejection

  Freedom from signals which appear to be transmitting on specific frequencies when in fact it is not the case. Spurious responses can also be a breakthrough of signals and harmonics of the receiver's internal oscillator. If the oscillator is not properly shielded inside the receiver, it can have a detrimental effect on performance.

• Frequency Stability

  So that short term fluctuations in frequency will not cause drifting, noise, etc. The most effective way to have good frequency stability is to use a quartz crystal, although other ways such as frequency synthesizers, and analog phase-locked loops can also be effective.

• Signal-to-Noise Ratio

  Signal-to-noise ratio is the primary measurement of audio quality as measured by signal on/signal off, and taking measurements of the difference. Generally speaking, the higher the ratio the better, however it's very easy to hide the true noise figures with this number to make it appear better than it actually is. One common standardized measurement technique is to use an "A weighting" filter network. This filter is based on a DIN standard and allows the user a better perspective for comparison. One problem with A weighted tests however is that the filter removes some of the noise in the process, and can actually improve the S/N figure upwards of 6 dB over the actual usable reading.

• Sturdy Construction

  The receiver should be made of quality materials inside and out, and should have proper external shielding. Internal "fencing" (copper shielded circuits) are helpful to keep noise down inside the receiver. Construction should also include accessibility to the internal circuitry for service, frequency changes and adjustments that time and use may deem necessary.

• Quality Antenna

  Since this is the first part of the receiver "system" it often times is the single most important part of the receiver. Whether using ¬ wave whips, coil loaded dipoles, Yagis, or log periodic type antennas, the proper antenna should have a good impedance match to the receiver and should be the proper length for overall efficiency.

• Whip Being the Complement

  Using a ¬ wave whip without a ground plane is actually worse than no antenna because without the impedance match that the ground plane provides, the antenna feed-coax pickups extraneous RF in the outer shield, and this is in turn fed directly to the receiver. Any sources of RFI such as computers, motors, dimmers, etc. can have a very adverse affect on an improperly installed antenna system.

Inside Receivers - RF Filtering

RF Filtering

In any quality radio, a series of filters are used in the RF input section or initial “front end” of the circuitry to restrict the bandwidth of the received signal, and reject other undesired signals close in frequency to the desired signal. There are a number of common filter designs used for this process including ceramics, hand wound coils, tuned inductors and cavity filters (helical resonators). These filters essentially separate the desired signal from the other hash present near the bandwidth of the receiver. Each of these designs have their advantages and disadvantages depending on the application and the desired effect. It is extremely important to have adequate filtering in this stage to keep the receiver quiet, and keep the signal free from interference and noise. There are several filters used in wireless systems today, and they include:

• Cavity Filters (helical resonators)

  Cavity filters are essentially a copper can or mechanical filter that allow certain frequencies to pass while rejecting others by their physical size. They are often used in receivers because of their narrow response characteristics and linearity. Most helical resonators used by wireless microphone manufacturers are really not true cavity filters, as a true cavity filter at VHF frequencies would be the size of a three pound coffee can. A filter this size would make the size of the receiver totally unacceptable. The most commonly used cavity filters (helical resonators) are a fraction of the size required for true effectiveness.

• Ceramic Filters

  With inexpensive receiver designs, ceramic filters are an effective off-the-shelf alternative, although they tend to have a non-adjustable bandwidth, and are more difficult to use in demanding applications. Typical uses include hearing assistance receivers, and economical wireless microphone receivers.

• LC Inductor Capacitor (tuned circuit)

  LC type filters are essentially hand-tuned copper coils which tends to make them considerably more expensive than the ceramics and the quasi-helical filters. LC's can offer tighter RF filtering performance without the audio distortion of other filter types. Selectivity of the LC circuit can be increased when multiple stages are ganged or cascaded. This often raises the price of the product, and consequently they are usually found only in the more expensive receiver designs

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Superheterodyne Receivers - Single Conversion IF

The most common type of receiver used in wireless systems today is called a Superheterodyne or superhet. In this type of design, the carrier frequency is mixed with a high power local oscillator, and the resulting combination produces a sum and a difference of the two signals. The sum is generally higher than the original carrier frequency, so it is rejected, whereas the difference is a fraction of the original, usually 10.7 MHz. This fraction, or intermediate frequency (IF) is amplified, and filtered by either ceramic, LC, or crystal filters to separate the desired signal from the harmonics created in the mixing process. By converting the original frequency to a lower frequency such as 10.7 MHz (a common IF) it becomes practical to build more effective amplifiers and filtering networks. After the IF stage, the signal is sent to an FM demodulator where the original audio signal is detected for further processing prior to output to the sound system.

Double Conversion IF

Another common technique in IF design is to double convert the IF with two separate oscillators operating at different frequencies. In a double conversion IF, two oscillators and mixers are used to process and filter the signal. Double conversion generally has better selectivity than single conversion IF's due to the steeper filter slopes of using two filter paths rather than one. The disadvantage if not designed properly, is that the dual conversion process gives rise to unwanted harmonic spurs which increase spurious responses and may add distortion to the audio signal.

IF Filtering

The IF section is the most important signal processing section of the receiver. Proper filtering in the IF is critical because a poor design in this area will create problems like interference from out of band signals, and audio problems such as phase distortion, group delay, and intermodulation distortion. Many types of filters can be used in the IF section. Each filter type has advantages as well as disadvantages. The choice of filtering used in a given design largely depends on the performance criteria, and the intended market cost of the product. These filters are used in the IF section of the receiver for screening wanted from unwanted signals from being amplified in the receiver. They include:

• Ceramic Filters

  As with ceramics used for RF filtering, ceramic filters can be an effective off-the-shelf alternative, although they tend to have a non-adjustable bandwidth, and can be more difficult to use in demanding applications.

• LC Inductor Capacitor (tuned circuit)

  LC type filters are essentially hand-tuned copper coils which tends to make them considerably more expensive than the ceramics and the quasi-helical filters. LC's can offer tighter RF filtering performance without the audio distortion of other filter types. Selectivity of the LC circuit can be increased when multiple stages are ganged or cascaded. This often raises the price of the product, and consequently they are usually found only in the more expensive receiver designs.

• Crystal (quartz crystal)

  An effective (and expensive) filter is accomplished with the use of Crystal lattice type filters where a series of quartz crystals are used to filter the incoming signal. Crystal filters are generally too narrow for use in wireless microphone receivers because their high Q can result in group delay distortion. Group delay distortion is a phenomenon where the filter does not pass all frequencies in the audio band at the same rate. The result is a condition where higher and lower frequencies run through the filter at different rates resulting in audio distortion. Due to the drawbacks of crystal filters, they are typically used on in communications grade products where multiple harmonics must be eliminated in the receiver.

IF - General

All of the filters described above are considered accepted circuits for use in wireless microphones. As with any filtering process, good design techniques, coupled with proper application and good parts selection can give excellent results, likewise good parts coupled with poor design techniques will usually yield negative results. In either single conversion or double conversion receivers, very careful consideration must be used when designing the IF section to maintain the audio quality of the system.

Inside Receivers - Squelch Circuits

Squelch Circuits

As we discussed earlier, a good receiver design begins with the RF & IF filtering, but another important part of the receiver circuitry is the squelch or RF detection circuitry. This circuitry is the “gate” that allows RF signals to enter the receiver front-end, and thus literally turns the audio on and off. Simple squelch circuits have a detector that opens a gate as soon as it detects a preset level of RF energy within the bandpass (frequency range) of the receiver.

Amplitude Squelch - Gate Squelch

Gate squelch circuits are commonly used in many wireless receivers. In a gate squelch, a circuit monitors the RF level, and either opens the squelch gate, or keeps it closed depending on the signal strength of the incoming RF energy. The obvious problem with the gate squelch is that any RF information including distortion, harmonics, noise etc. are indistinguishable from the real transmitter, and the extraneous RF opens the squelch gate just as well as the intended transmitter. The “squelch level” adjustment on these systems allows the sensitivity of the gate to be increased or decreased, but this also increases or decreases the usable operating range of the system as well.

True Noise Squelch

In a true noise squelch, high frequency noise of the receiver (noise present in non-coherent RF hash) is monitored and applied to an audio gate. If no coherent carrier signal is received, the high frequency noise keeps the audio gate closed, and no audio is heard. Unlike an amplitude squelch, if more extraneous noise is received, the gate is simply kept closed. If a coherent carrier signal is received (from a real transmitter) and the level exceeds the preset internal level, the high frequency noise in the receiver disappears, and the audio gate opens allowing normal audio output. The major advantage to this system is that it can actually determine the difference between a coherent transmission, and plain RF hash, plus there is no external user adjustable squelch pot to act as a range reducer. This type of squelch allows for any coherent transmission to open the gate regardless or whether it’s the right transmission but a competing transmission will adversely affect all systems regardless of the squelch design. In the event of interference, a frequency change would be the best solution.

Tone Coding

A more effective method is to use “tone coding” where a subaudible or supersonic tone rides with the carrier frequency, and is detected by a circuit in the receiver. This type of squelch circuit is preferred over gate squelches because the tone detector must “hear” the tone prior to opening the audio, keeping similar frequency transmitters without the inaudible tone out of the system. In recent years tone-coded squelch circuitry has been added to wireless microphone vernacular with the idea that it would somehow make the system “interference proof,” or less immune to other RF problems. Actually tone-coded squelch is a very old idea that has its origins from the two-way radio market, where it was used to selectively activate a single user among a number of users all sharing the same frequency. The CTCSS (tone-coded squelch) method allowed numerous users to share a single repeater frequency, and receive a transmission only when their particular “tone” was sent along with the radio transmission. It eliminated the extra traffic on their frequency that they did not want to hear, however once the channel was under use, they could not attempt to key their transmitter, or major problems would occur.

The main advantage of using a tone squelch is that the receiver will not open until after the transmitter has stabilized (1/10th of a second after power-up) which makes it possible to use a single switch on the transmitter, rather than separate mute and power switches. In wireless microphones, the circuitry can be used to keep a receiver quiet only when the transmitter is off. Once the transmitter is turned on, the tone opens the squelch on the receiver, and the receiver is susceptible to any other errant transmissions on the same frequency, even when they come from a transmitter that is not tone-coded. It is important to remember that the squelch is the final circuit point prior to the audio section, and the circuits that come before the squelch section are far more important in determining overall system performance. The bottom line is this; tone-coded squelch is no panacea. Premium wireless microphone performance is more a result of superior receiver designs, and effective diversity circuits, rather than these basic squelch schemes.

Newer systems are now appearing with a tone defeat switch, so that the installer can check to see if there is any RF clutter present on the channel prior to actual installation, and eventual user problems. Even with a tone-coded squelch, you never want to use a frequency that has potential interference problems, because a tone squelch cannot overcome interference. The only time a tone squelch is useful is when the receiver is left on, and the transmitter is turned off. In this application, the receiver will not “break squelch” for an errant transmission. However, once the transmitter is turned on, any errant transmission will create problems in the audio, tone squelch or not.

CONTINUE: WIRELESS MICROPHONES

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