Extra Class question of the day: Filter types and applications

Different types of filters have different characteristics. For example, a Chebyshev filter is a filter type described as having ripple in the passband and a sharp cutoff. (E7C05)On the other hand, the distinguishing features of an elliptical filter are extremely sharp cutoff with one or more notches in the stop band. (E7C06)

Filters have both amplitude and phase-response characteristics. In some applications, both are important. Digital modes, for example, are most affected by non-linear phase response in a receiver IF filter. (E7C14)

The Chebyshev filter was named for Pafnuty Chebyshev, whose mathematical work led to the development of these filters. Sometimes filters are named for their circuit topoology. Pi is the common name for a filter network which is equivalent to two L networks connected back-to-back with the inductors in series and the capacitors in shunt at the input and output. (E7C11) When you look at the circuit diagram for a filter of this type, you’ll see that it looks like the Greek letter pi.

Often, you’ll choose a filter type for a particular application. For example, to attenuate an interfering carrier signal while receiving an SSB transmission, you would use a notch filter. (E7C07)

Today, many of these filters are implemented using digital signal processing. The kind of digital signal processing audio filter might be used to remove unwanted noise from a received SSB signal is an adaptive filter. (E7C08) The type of digital signal processing filter might be used to generate an SSB signal is a Hilbert-transform filter. (E7C09)

Some filters are used almost exclusively in a particular application. A cavity filter, for example, would be the best choice for use in a 2 meter repeater duplexer. (E7C10)

Extra Class question of the day: Receiver performance characteristics

In the past, sensitivity was one of the most important receiver performance specifications. Today, instead of sensitivity, we speak of a receiver’s minimum discernible signal, or MDS. The MDS of a receiver is the minimum discernible signal. (E4C07) This is the weakest signal that a receiver will detect.

One parameter that affects receiver sensitivity is the noise figure. The noise figure of a receiver is the ratio in dB of the noise generated by the receiver compared to the theoretical minimum noise. (E4C04) Lowering the noise figure of a receiver would improve weak signal sensitivity. (E4C08)

A related specification is the noise floor. When we say that the noise floor of a receiver has a value of -174 dBm/Hz, it is referring to the theoretical noise at the input of a perfect receiver at room temperature. (E4C05) If a CW receiver with the AGC off has an equivalent input noise power density of -174 dBm/Hz, the level of an unmodulated carrier input to this receiver would have to be -148 dBm to yield an audio output SNR of 0 dB in a 400 Hz noise bandwidth. (E4C06)

A receiver’s selectivity is the result of a lot of things, including the filters a receiver has. 300 Hz is a desirable amount of selectivity for an amateur RTTY HF receiver. (E4C10)2.4 kHz is a desirable amount of selectivity for an amateur SSB phone receiver.(E4C11)

In addition to a 300 Hz filter and a 2.4 kHz filter, high-end receivers also have filters called roofing filters. A narrow-band roofing filter affects receiver performance because it improves dynamic range by attenuating strong signals near the receive frequency. (E4C13)

Back in the day, when superheterodyne receivers had intermediate frequencies, or IFs, in the 400 – 500 kHz range, image rejection was a problem. If there was a strong signal present on a frequency about two times the IF away from the frequency your receiver was tuned to, you might hear that signal. Accordingly, 15.210 MHz is a frequency on which a station might be transmitting if is generating a spurious image signal in a receiver tuned to 14.300 MHz and which uses a 455 kHz IF frequency. (E4C14)

One solution to this problem is to select an IF higher in frequency. One good reason for selecting a high frequency for the design of the IF in a conventional HF or VHF communications receiver is that it is easier for front-end circuitry to eliminate image responses. (E4C09) A front-end filter or pre-selector of a receiver can also be effective in eliminating image signal interference. (E4C02)

Another way to get rid of image signals is to use a narrow IF filter. An undesirable effect of using too wide a filter bandwidth in the IF section of a receiver is that undesired signals may be heard. (E4C12)

Because most modern transceivers use digital techniques to generate a local oscillator signal to tune a receiver, synthesizer phase noise might be a problem. An effect of excessive phase noise in the local oscillator section of a receiver is that it can cause strong signals on nearby frequencies to interfere with reception of weak signals. (E4C01)

Finally, here are two miscellaneous questions on receiver performance characteristics. Atmospheric noise is the primary source of noise that can be heard from an HF receiver with an antenna connected. (E4C15) Capture effect is the term for the blocking of one FM phone signal by another, stronger FM phone signal. (E4C03)

Extra Class question of the day: system noise; electrical appliance noise; line noise; locating noise sources; DSP noise reduction; noise blankers

Noise is often a real problem for radio amateurs. Fortunately, by understanding how noise is generated and how to reduce or eliminate it, noise can be tamed.

Atmospheric noise is naturally-occurring noise. Thunderstorms are a major cause of atmospheric static. (E4E06) There’s not much you can do to eliminate, but you can often use a receiver’s noise blanker to help you copy signals better. Signals which appear across a wide bandwidth (like atmospheric noise) are the types of signals that a receiver noise blanker might be able to remove from desired signals. (E4E03) Ignition noise is one type of receiver noise that can often be reduced by use of a receiver noise blanker. (E4E01)

One undesirable effect that can occur when using an IF noise blanker is that nearby signals may appear to be excessively wide even if they meet emission standards. (E4E09)

Many modern receivers now use digital signal processing (DSP) filters to eliminate noise. All of these choices are correct when talking about types of receiver noise can often be reduced with a DSP noise filter (E4E02):

  • Broadband white noise
  • Ignition noise
  • Power line noise

One disadvantage of using some types of automatic DSP notch-filters when attempting to copy CW signals is that the DSP filter can remove the desired signal at the same time as it removes interfering signals. (E4E12)

While filters can be very effective at reducing noise, it is often better to figure out what is  generating the noise and taking steps to reduce or eliminate the amount of noise generated in the first place. For example, one way you can determine if line noise interference is being generated within your home is by turning off the AC power line main circuit breaker and listening on a battery operated radio. (E4E07) If by doing this you determine that an electric motor is a problem, noise from an electric motor can be suppressed by installing a brute-force AC-line filter in series with the motor leads. (E4E05)

All of these choices are correct when it comes to the cause of a loud roaring or buzzing AC line interference that comes and goes at intervals (E4E13):

  • Arcing contacts in a thermostatically controlled device
  • A defective doorbell or doorbell transformer inside a nearby residence
  • A malfunctioning illuminated advertising display

Sometimes your own equipment may be the cause of received noise. A common-mode signal at the frequency of the radio transmitter is sometimes picked up by electrical wiring near a radio antenna. (E4E08)

The main source of noise in an automobile is the alternator. Conducted and radiated noise caused by an automobile alternator be suppressed by connecting the radio’s power leads directly to the battery and by installing coaxial capacitors in line with the alternator leads. (E4E04)

Personal computer and other digital devices can also generate noise. One type of electrical interference that might be caused by the operation of a nearby personal computer is the appearance of unstable modulated or unmodulated signals at specific frequencies. (E4E14) All of these choices are correct when talking about common characteristics of interference caused by a touch controlled electrical device (with an internal microprocessor) (E4E10):

  • The interfering signal sounds like AC hum on an AM receiver or a carrier modulated by 60 Hz hum on a SSB or CW receiver
  • The interfering signal may drift slowly across the HF spectrum
  • The interfering signal can be several kHz in width and usually repeats at regular intervals across a HF band

Noise can even be generated by the most unlikely things. For example, it is mostly likely that nearby corroded metal joints are mixing and re-radiating the broadcast signals if you are hearing combinations of local AM broadcast signals within one or more of the MF or HF ham bands. (E4E11)

Roofing Filters

Here are a couple of good explanations of roofing filters from the Elecraft mailing list:

  • Don, W3PFR, writes:

    The ultimate selectivity of the K3 is determined by the DSP.
    Now think about the input to the DSP – if one restricts the number of signals entering the DSP (actually the ADC front end for the DSP), the DSP will not have to deal with as much ‘garbage’. The more narrow the roofing filter, the less ‘garbage’ will be present at the DSP front end.

    Consider also that the DSP can handle an S-9 +20 signal input from an unwanted signal – above that level it will be overloaded. In situations where signals greater than that level are encountered within the passband of the roofing filter, the Hardware AGC will limit the signal level presented to the DSP. Even though the DSP may be cranked down to hear only the desired signal, the signal gets through to the DSP front end and will reduce the gain of the receiver because the Hardware AGC is being activated by the strongest signal within the passband of the roofing filter – the result is AGC “pumping” as a result of a signal that one may not even hear. Using a more narrow roofing filter can eliminate this unwanted signal and eliminate the AGC pumping.

    Think of the roofing filter as a roof on a house – the wide filter allows the DSP (and hardware AGC) to respond to all the rain falling on the roof. To reduce the amount of rain falling on the roof, a more narrow roofing filter is required.

    If you are interested in contesting and/or heavy DXing, you are liable to encounter nearby unwanted strong signals within the passband of the roofing filter which will cause the hardware AGC ‘pumping’, and you should invest in narrow filters that will eliminate that effect. For those whose operation is more casual and do not wish to dig for weak signals in the presence of stronger nearby signals, a wide roofing filter will do the job nicely.

  • What “Roofing Filter” means to Elecraft. This is an explanation on the Elecraft website.