Receiver Control

This page covers the CAT commands that control receiver parameters on the K3/K3S. These commands let you adjust everything from gain and filtering to noise reduction and signal metering, giving software full control over the receive signal path.

For complete command syntax and all parameter details, see the K3/K3S/KX3/KX2 CAT Command Reference.

Commands Used

Command	Description	GET	SET	$ (VFO B)
AG	AF gain	Yes	Yes	Yes
RG	RF gain	Yes	Yes	Yes
PA	Preamp	Yes	Yes	Yes
RA	Attenuator	Yes	Yes	Yes
GT	AGC speed	Yes	Yes	No
BW	DSP bandwidth	Yes	Yes	Yes
FW	Filter bandwidth + number	Yes	Yes	Yes
IS	IF shift	Yes	Yes	No
NB	Noise blanker	Yes	Yes	Yes
NL	NB level	Yes	Yes	Yes
NR	Noise reduction	Yes	Yes	No
SQ	Squelch	Yes	Yes	Yes
SM	S-meter	Yes	No	Yes

1. Signal Path Overview

The following diagram shows a simplified view of the receiver signal path and where each CAT command acts. Use it as a mental model for understanding how the commands relate to each other.

Note

This is a simplified conceptual diagram. The actual DSP signal chain order within the K3/K3S may differ from what is shown here. The purpose is to illustrate which stage of the receiver each command controls.

2. AF and RF Gain

AF Gain (AG)

The AG command controls audio output level (volume). The parameter range is 000 to 255.

AG;              → AG127;          Query AF gain (currently 127)
AG200;                              Set AF gain to 200
AG$;             → AG$080;         Query sub receiver AF gain
AG$120;                             Set sub receiver AF gain to 120

RF Gain (RG)

The RG command controls RF gain. The parameter range is 000 to 250, where 250 is maximum gain.

RG;              → RG250;          Query RF gain (currently maximum)
RG200;                              Set RF gain to 200
RG$;             → RG$250;         Query sub receiver RF gain

Tip

RF gain at 250 is maximum. Decreasing the value reduces gain, which can be useful for managing strong-signal environments where AGC alone is not sufficient. Reducing RF gain lowers the noise floor and can improve readability of strong signals.

3. Preamp and Attenuator

Preamp (PA)

The PA command toggles the receiver preamplifier, which adds approximately 10-15 dB of gain.

PA;              → PA0;            Query preamp state (currently off)
PA1;                                Turn preamp on
PA0;                                Turn preamp off
PA$1;                               Turn sub receiver preamp on

Attenuator (RA)

The RA command toggles the receiver attenuator, which reduces signal level by approximately 10 dB.

RA;              → RA00;           Query attenuator state (currently off)
RA01;                               Turn attenuator on
RA00;                               Turn attenuator off
RA$01;                              Turn sub receiver attenuator on

Note

Preamp and attenuator are independent controls. You can have both off, one or the other on, or both on simultaneously (though having both on is rarely useful). On crowded bands with nearby strong signals, engaging the attenuator can reduce intermodulation distortion and improve reception of weaker stations.

4. AGC

The GT command sets the AGC (Automatic Gain Control) speed. AGC controls how quickly the receiver adjusts its internal gain in response to signal level changes.

Value	Speed
GT000;	AGC off
GT002;	Fast
GT004;	Slow

GT;              → GT004;          Query AGC speed (currently slow)
GT002;                              Set AGC to fast
GT000;                              Turn AGC off

Tip

Use slow AGC (GT004;) for SSB and AM, where signal levels vary gradually. Use fast AGC (GT002;) for CW, where you need the receiver to recover quickly between characters. Turning AGC off (GT000;) is useful for digital modes or when you want manual gain control via RG.

5. IF Filters

The K3/K3S offers two ways to control receive bandwidth: DSP-based filtering via BW and crystal filter selection via FW.

DSP Bandwidth (BW)

The BW command sets the DSP filter bandwidth in Hz. The parameter is a 4-digit value.

BW;              → BW2800;         Query bandwidth (currently 2800 Hz)
BW2400;                             Set bandwidth to 2400 Hz
BW0500;                             Set bandwidth to 500 Hz
BW$1800;                            Set sub receiver bandwidth to 1800 Hz

The allowable range is BW0050; (50 Hz) to BW9999; (limited by the installed crystal filter width).

Filter Bandwidth and Number (FW)

The FW command provides more control by specifying both a bandwidth and a crystal filter slot number. The format is 5 digits for bandwidth in 10 Hz units followed by 2 digits for the filter number.

FW;              → FW0280001;      Query filter (2800 Hz on filter 1)
FW0240002;                          Set 2400 Hz on filter 2
FW$;             → FW$0280001;     Query sub receiver filter

The filter number (01-08) selects which crystal filter slot is used. Each slot can hold a different roofing filter.

Direct Crystal Filter Selection (XF)

The XF command selects a crystal filter slot directly without changing the bandwidth setting.

XF01;                               Select crystal filter slot 1
XF02;                               Select crystal filter slot 2

Valid values are XF01; through XF08;.

Note

When you set a bandwidth with BW or FW, the radio snaps to the nearest available value supported by the currently selected crystal filter. The response will reflect the actual bandwidth the radio applied, which may differ slightly from the requested value.

6. IF Shift

The IS command shifts the IF passband up or down in frequency, measured in Hz. This moves the filter passband relative to the received signal without changing the tuned frequency.

IS;              → IS 0000;        Query IF shift (currently centered)
IS+0500;                            Shift passband up 500 Hz
IS-0300;                            Shift passband down 300 Hz
IS 0000;                            Return to center (no shift)

The range is IS-9999; to IS+9999;. Positive values shift the passband up in frequency, negative values shift it down.

Tip

IF shift is useful for avoiding nearby interference without retuning. If a station is interfering on one side of your passband, shift the passband away from the interference. This is often more effective than narrowing the filter bandwidth because it preserves audio fidelity.

7. Noise Blanker

The noise blanker reduces impulse noise (ignition noise, power line interference, etc.).

NB On/Off

NB;              → NB0;            Query noise blanker state (off)
NB1;                                Turn noise blanker on
NB0;                                Turn noise blanker off
NB$1;                               Turn sub receiver noise blanker on

NB Level (NL)

The NL command sets the noise blanker threshold. The range is 000 to 021.

NL;              → NL010;          Query NB level (currently 10)
NL015;                              Set NB level to 15
NL$008;                             Set sub receiver NB level to 8

Higher values make the blanker more aggressive, which removes more noise but may also clip strong signals. Start with a moderate value and increase until impulse noise is reduced without degrading desired signals.

8. Noise Reduction

The NR command toggles DSP-based noise reduction, which estimates and subtracts broadband noise from the received audio.

NR;              → NR0;            Query noise reduction state (off)
NR1;                                Turn noise reduction on
NR0;                                Turn noise reduction off

Note

Noise reduction works best on steady signals like CW carriers or SSB speech. It can introduce artifacts on rapidly changing signals. On weak CW signals, combining NR1; with a narrow BW setting is often very effective.

9. Squelch

The SQ command sets the squelch threshold. When the received signal drops below this level, audio is muted. The range is 000 to 029.

SQ;              → SQ000;          Query squelch level (open / no squelch)
SQ010;                              Set squelch to level 10
SQ029;                              Set squelch to maximum
SQ$005;                             Set sub receiver squelch to 5

A value of SQ000; disables squelch (audio is always open). Increasing the value raises the threshold, requiring a stronger signal to open the squelch.

10. S-Meter Reading

The SM command reads the current signal strength. This is a GET-only command (no SET).

Standard S-Meter

SM;              → SM0012;         Read main receiver S-meter
SM$;             → SM$0008;        Read sub receiver S-meter

The returned value ranges from 0000 to approximately 0021, mapping to S0 through S9+60 dB.

Value	Approximate Reading
0000	S0
0009	S9
0015	S9+20 dB
0021	S9+60 dB

High-Resolution S-Meter (SMH)

The SMH command returns a 5-digit high-resolution S-meter value, available on the K3.

SMH;             → SMH00115;       High-resolution S-meter reading

Tip

The S-meter commands are useful for building signal-level monitoring tools, auto-fading systems, band scanning utilities, and propagation logging applications. Poll SM; periodically to track signal strength over time.

Practical Example: Optimize for Weak Signals

The following sequence configures the receiver for pulling out weak signals in a quiet band segment:

PA1;         Preamp on
RA00;        Attenuator off
RG250;       RF gain maximum
BW0500;      Narrow 500 Hz filter
NR1;         Noise reduction on
NB1;         Noise blanker on
GT004;       Slow AGC

This combination maximizes front-end gain (PA1, RG250), narrows the receive bandwidth to reduce noise, and engages both DSP noise reduction and the noise blanker. Slow AGC prevents the receiver from overreacting to brief noise bursts.

Note

For strong-signal environments (contests, crowded bands), you would typically do the opposite: turn the preamp off (PA0;), engage the attenuator (RA01;), reduce RF gain, and use a wider filter to maintain audio quality while relying on AGC to manage levels.