Setting receiver gain controls
Modern receivers that include digital signal processing technology contain two forms of AGC:
- ADC-protection AGC used to prevent errors in the digital conversion of the analog IF signal. The analog-to-digital conversion (ADC) stage of the receiver has a maximum level, above which it fails to accurately convert signals. For ordinary fixed-point arithmetic commonly used in HF receivers (as this is written in 2010), the dynamic range (dB) is approximately 6.02 × (number of bits). For a 16-bit ADC, about 96 dB of range is available. If the receiver has been designed to put the ADC's minimum discernible signal at -135 dBm, then the ADC will generate errors on signals above -44 dBm, or S9+28 dB. These errors, if permitted, would show up as significant distortion at the receiver output. To prevent them, the ADC-protection AGC reduces the signal levels. The operator has no control over this ADC-protection AGC.
- operator-controlled AGC used to reduce the audio output range of the receiver.
All AGC technologies introduce distortion in the receiver output. Distortions make it more difficult to understand signals and make long-term use more fatiguing. With careful selection among design choices, and proper choice of settings by the operator, these distortions can be reduced somewhat -- but some distortion will remain. In addition, many AGC implementations and operator settings cause weaker signals and stronger signals to have more similar apparent volume levels at the receiver output. These similarities make it more difficult for the operator to separate multiple signals and to concentrate on the desired one.
When to use operator-controlled AGC
Operator-controlled AGC may be appropriate when:
- only one signal is present in the receiver bandwidth.
- the operator has no interest in hearing weak signals while stronger signals are present; e.g., casual listening.
- the operator is listening in a room with a high level of ambient distractions (other operators, fan noise from other equipment, etc). In this case, the operator will adjust the radio so that weaker signals and band noise can be heard above the local distractions... and rely on operator-controlled AGC to reduce the power of strong signals. More information about listening environment is found below.
In other situations, AGC is unnecessary. The operator can avoid the distortion and extra concentration effort by switching the operator-controlled AGC off. To use this approach effectively, one needs to set gain levels set so that the loudest signal the receiver delivers to the headphones is a level that operator can tolerate, and so that the weakest signals (typically, the underlying band noise) are audible.
Setting controls for operation without AGC
Many operators run their gain controls way too high. Here is one approach to setting up gain levels correctly. It involves four sets of adjustments.
Adjustments for a specific set of headphones: The first two settings are done once for a specific headphone set. If the operator changes to a different headphone mode, these parameters may need to be reset. Each model of headphone has a different sensitivity to the signals provided by the receiver. For a given receiver output level, one model may sound very loud while a different model may be quite soft. When a manufacturer does not provide good quality control, even individual headphones of the same model may exhibit significant variations in sensitivity between individual headsets.
- Protective audio limiter level (if available).
- AF gain level.
Adjustments for the particular band and model: These adjustments will differ from band to band, and may differ slightly between antennas on the same band. Once these settings have been selected, they can be written down and reused in the future. The operator will not need to change them unless band noise levels change markedly; e.g., lots of continuous QRN, or a change from day to night noise levels on 80/160m.
- Set coarse adjustment of front end gain: Choosing the correct pre-amp/attenuator level for the band.
- Set fine adjustment of front end gain: choosing the correct RF gain control level.
Set the protective audio limiter level
Not every receiver includes a protective audio limiter. When one is present (e.g., Elecraft K3), the goal is to set the receiver's maximum possible audio output to a level that does not injure the operator's hearing. Again, this level will be different for different operators, and for different headphones. Here is one method to make this setting:
- Apply a signal strong enough to max out the receiver's internal ADC-protection AGC. This can be provided by a signal generator set to -5 dBm: strong enough to trigger the internal ADC-protection AGC, but not strong enough to damage the most sensitive amplifier stages. In some transceivers the CW monitor signal level can also be turned up to the maximum level and used for the purposes of this adjustment.
- Adjust the AF limiter so that the signal is very strong and just barely tolerable. Note: Since the limiter is operating, the tone will be quite distorted (close to a square wave).
If a protective audio limiter is not present within the receiver, hearing protection can still be accomplished:
- Insert an attenuator between the receiver output and the speaker or headphones used for listening.
- Apply a signal strong enough to max out the receiver's internal ADC-protection AGC, as described above.
- Adjust the attenuator so that the signal appears very strong and just barely tolerable.
A variable attenuator can be replaced by a fixed attenuator (for this combination of operator, receiver, and headphones/speaker) once this level has been determined.
Set the AF gain level
The goal is to set the AF gain level just high enough that you can hear the receiver's AF amplifier noise floor.
- Turn off AGC, pre-amp and attenuator.
- Disconnect the antenna (preferable) or switch to a port with no antenna. Note: Most receivers "leak" signals between their antenna ports, since the port-to-port isolation often is as low as -40 dB on the higher frequency bands. Disconnecting all antennas eliminates weak, leaked signals during these adjustments.
- Set to CW mode, and to the narrowest typical listening bandwidth; e.g., 400 Hz.
- Set RF gain to minimum.
- Set AF gain to minimum. Note: If one hears hiss or noise when both the RF and AF gain is at a minimum, the headphones are too sensitive. Insert an audio attenuator in the headphone leads. (I run 15-20 dB attenuation on my in-ear monitors. -- K3NA)
- Now increase AF gain until one just barely hears the receiver's internal noise floor. This level usually is quite high; a fully clockwise position is quite OK. If one reaches full clockwise with AF gain, and do not yet hear any noise, it is OK to leave this control at full clockwise. But, with sensitive headphones, one may reach a level where the AF amp noise floor can be detected at a point below full AF gain.
Do not change the AF gain in future. This is the gain level appropriate for these headphones.
Set coarse adjustment of front end gain
The goal is to provide enough gain so that the band noise from the antenna is just above the receiver noise floor, which in turn is just above the AF amp noise floor (if detectable).
- Without connecting an antenna, turn on the pre-amp.
- Advance RF gain until one can just start to hear the receiver noise floor (just above the audio noise floor, if that is audible). At this point there is enough gain in both the RF and AF stages to hear the internal noise of the receiver. This is the maximum useful gain setting. One never benefits from using more gain than these levels.
- Connect an antenna. Antenna noise should be heard above the receiver noise floor. Tune to a empty frequency, even if it is just outside the band edge.
- Switch off the pre-amp. If the operator can still hear the antenna noise above the receiver noise floor, continue to next step. If not, the pre-amp is needed with this antenna on this band; go to the fine adjustment of front end gain section. Typically one will need the pre-amp on the highest frequency bands.
- Add attenuation. (For a receiver with multiple attenuation levels, increase the attenuation step-by-step.) When the operator no longer hears band noise above the receiver noise floor, too much attenuation has been added. Remove/reduce the attenuation until the band noise is heard just above the receiver noise floor. This is the correct adjustment on this band for this antenna. Go to the fine adjustment section.
Note the setting of the pre-amp and attenuator settings. This is the setting to be used for this band/antenna combination in future, unless band noise level changes markedly. One never needs more gain than provided by this setting; extra gain just chews up the dynamic range of the operator's hearing and of the receiver's analog-to-digital converter.
Set fine adjustment of front end gain
In the previous steps, the attenuator/preamp provided a coarse adjustment (e.g., in 10 dB steps for the Elecraft K3) of front end gain. The goal now is to improve this a bit by tweaking the RF gain control.
If all attenuation was applied in the previous steps (typical on bands with higher noise levels, such as 160-40m at night), a substantial further front end gain reduction may be needed; this will be done now using the RF gain control.
- Reduce the RF gain control until the band noise is just above the receiver noise floor. This is the final setting for this band/antenna combination under these band noise conditions.
Other factors affecting listening
As an example, the Elecraft K3 has an internal range of about 80-90 dB for its DSP A/D and D/A converters. When listening in a quiet environment, the above adjustments allow the operator to enjoy almost the full instantaneous dynamic range of the K3 before signals hit the painful level in his ears. Unfortunately, many operators fail to design their listening environment with the same care as they use in picking out a radio transceiver, ultimately wasting much of the benefit of today's excellent radios.
Ambient room noise
Human hearing has about 100 dB range -- but this can be easily compromised when listening in a noisy environment! If the ambient room noise chews up 60 dB of hearing range, then the operator is left with just 30 dB to play with between "just audible above the ambient noise of the room" and "ouch!". Since signal strengths on the band easily exceed this range, the operator must either (a) ride the RF control when encountering loud signals or (b) accept the distortion and compromises inherent in any AGC system.
A library-like room has an ambient noise level about 40 dB above the threshold of human hearing. This ambient room noise can be reduced by:
- Using headphones that encapsulate the entire ear and rest only on the skull around the ear (pinnae). Encapsulating ear muffs can reduce the ambient noise level by 15-25 dB, depending on the quality of the ear muff and the frequency of interest.
- Using in-ear monitors. In-ear monitors reduce the radio room's ambient noise level by about 25 dB and have excellent audio performance. (Switching to in-ear monitors from the typical headset used by operators today often causes people to think they have bought a new, superior receiver!) Custom-fit in-ear monitors are especially comfortable for long wear during contests. Examples:
- Using in-ear monitors together with hearing protection ear muffs. These muffs are widely available for purchase over the Internet at prices beginning around US$18. Choose a model based on comfort, encapsulation, and measured sound reduction. The combination of in-ear monitors and external ear muffs provides about 40 db reduction in ambient room noise. In a quiet room and with the radio gain adjustments outlined above, this combination means the operator will never say "ouch!" when tuning in a loud signal (or hearing a big static crash) with the receiver AGC off.
Active devices between receiver audio output and operator ears
External active devices often introduce additional impairments to the operator's listening environment. Examples:
- Audio amplifiers: Amplifiers have a specific range of signal inputs across which their output is a very close (but not perfect) linear amplification. If the signals from the receiver fall outside the amplifier's range (either above or below), further distortion will be added to the output. To avoid this problem, an external audio amplifier's input range must be matched to the output signal range of the receiver; e.g., an 80-90 dB range peaking at 1.5 Vrms.
- Devices using digital signal processing: Noise-canceling headphones and external DSP filter systems add three potential sources of distortion: the A/D conversion stage, the signal processing stage, and the D/A conversion stage. Of these, the A/D stage is the most likely to cause problems. When the receiver output level exceeds the encoding ability of the A/D converter, distortion is generated by. This distortion arises from the inability of the converter to encode the signal peaks, or from the internal AGC system of the device used to reduce gain in front of the converter.