Setting receiver gain controls

From Contesting

Contents 1 Introduction to setting gain controls for operation without AGC 2 Setting AF gain 3 Coarse adjustment of front end gain 4 Fine adjustment of front end gain 5 Other factors affecting listening 5.1 Ambient room noise 5.2 Active devices between receiver audio output and operator ears

Introduction to setting gain controls for operation without AGC

Modern MF/HF receivers with integral DSP systems can often be operated without AGC. 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.

Most people run their gain controls way too high. Here is one approach to setting up gain levels correctly. It involves three sets of adjustments:

1. Setting the AF gain level. This is done once for a specific headphone set, and is good for any band/antenna.
2. Coarse adjustment of front end gain: Choosing the correct pre-amp/attenuator level for the band.
3. Fine adjustment of front end gain: choosing the correct RF gain control level.

Stages 2-3 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.

Setting AF gain

The goal is to set the AF gain level just high enough that you can hear the internal receiver noise floor from the later stages of the receiver.

1. Turn off AGC, pre-amp and attenuator.
2. For the Elecraft K3 receiver, one should also disable the K3's internal audio clipper or set it at the highest possible level.
3. 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.
4. Set to CW mode, and to the narrowest typical listening bandwidth; e.g., 400 Hz.
5. Set RF gain to minimum.
6. 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)
7. 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.

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).

1. Without connecting an antenna, turn on the pre-amp.
2. 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.
3. 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.
4. 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.
5. Add attenuation. (For a receiver with multiple attenuation levels, add increase the attenuation step-by-step.)
   • If the operator no longer hears band noise above the receiver noise floor, too 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.
   • Otherwise, leave the attenuator in and go to the fine adjustment section. You will need the attenuator for this antenna on this band.

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.

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.

1. 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:
  • Westone universal-fit in-ear monitors and Westone custom-fit in-ear monitors
  • Ultimate Ears SuperFi and Ultimate Ears TripleFi.
• 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 V_rms.
• 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.

— originally contributed by Eric K3NA 12:45, 15 August 2009 (UTC)