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Chapter 6

Chapter 6   K3 Transmitter

6.1    Introduction

Chapter 3 discussed the basic operation of the various K3 transmitter controls (mic gain, compression, etc.). This chapter will investigate how the transmitter is designed and operates and look at the KPA3 and KAT3.

6.2    Transmitter Block Diagram, the 10,000' View



Figure 61. K3 transmitter block diagram.

 

Figure 6-1 shows the K3 transmitter block diagram. You have seen some of these blocks being used in the receiver. To understand the transmitter, it is useful to segment the block diagram into four elements to understand how the transmitter works.

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Digital Signal Processing

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Second IF

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First IF

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The RF Stages

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6.3    Transmitter Schematic Tour, the 1,000' View

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Digital Signal Processing

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Transmit Second IF

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RF Board

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6.4    Automatic Level Control – ALC

Automatic level control, or ALC, is the transmitter's equivalent to the receiver's AGC. ALC is a feedback system to keep the transmitter output within certain operating parameters, such a maximum output power, best linear operating point, etc. The traditional way to do this is to sense the output power, perhaps grid or plate current in tube amplifiers or source or drain current in solid-state amplifiers, rectify it, and to feed back a DC voltage to control the gain of an IF amplifier. Indeed, many transmitters and linear amplifiers used this mechanism to control the output power of the final amplifier because, unlike the K3, they had no "power" control. This scheme has disadvantages. It can cause intermodulation distortion products in the transmitter's final output. If it is fast enough to compensate for quickly occurring overdrive conditions, such as some radio's keying power spike, it will act to create even more non-linearities and IMD.

The Elecraft designers have included ALC in the K3 but have implemented it differently. Instead of creating an ALC that actually sets and controls the output power level, the K3 ALC acts as a fine control to reduce or eliminate situations where the transmitter can create IMD products. It is not an output power control. Power control is achieved separately from the ALC. As you can see in Figure 6-1, the forward and reverse power measured by the SWR bridge, and the KPA3 current measured by a sensor in the PA, are digitized by the front panel microcontroller and then fed to the digital signal processing. The DSP uses this information to calculate the correct digitally generated signal to provide the optimum output and reduce signal distortions, splatter, key clicks, etc. A significant factor contributing to the success of this approach is that ALC effects are applied in the digital signal processing before the first IF crystal roofing filter. The inherently non-linear IF amplifier gain changing of other approaches is avoided. Further, the onset of ALC control is "soft" rather than abrupt to reduce the harsh effects of clipping produced by some ALC designs.

With this in mind, let's review the Elecraft recommendations for setting the K3 ALC. Chapter 8 will discuss in more detail the use of external ALC when using an amplifier with the K3.

1.     Remember to enter test mode so you do not transmit while testing and setting up audio levels.

2.     Hold METER to select the compression (CMP) and ALC display.

3.     Adjust gains and ALC with compression set to zero.

4.     The first four bars of the ALC display act as a VU (Volume Unit) meter. They do not indicate ALC activation.

5.     When you are adjusting ALC for SSB, set the microphone gain for five to seven bars, with the seventh just flickering.

6.     When adjusting a sound card's output in the DATA A modes, set its level so that the ALC shows four bars with the fifth just flickering.

 

6.5    Transmit Antenna Selection

Figure 6‑2 shows a K3 with KAT3 ATU and KXV3 or KXV3A receiver antenna interface. You saw in Chapter 5 that the Main receiver can be connected through the KXV3(A) to an external receive antenna. However, as Figure 6‑2 shows, the transmitter is always switched through the Low Power Amplifier (LPA), the 100 watt power amp if it is installed, and the low pass filter to one of the antennas connected to the ATU. It is important to remember this if you ever connect a receive-only antenna with a preamp to one of the ATU's antenna jacks. If you do, you might accidently transmit into that antenna and destroy the receive antenna's preamplifier.

 

Figure 62. K3 with KAT3 ATU and KXV3A receive antenna interface.


6.6    The KPA3 100 Watt Power Amplifier

Figure 6‑3 shows the KPA3 100 watt power amplifier. Relay K1 is the high power bypass relay on the KPAIO3 board and is switched to the PWR > 12 W position when the PWR  control calls for more than 12 watts. For less than 12 watts the 100 watt amplifier is switched out of the circuit. The TR Switch is a solid-state switch using diodes as described in Chapter 5.

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Figure 63.  KPA3 100 W amplifier.


6.7    The KAT3 Antenna Tuner Unit

The KAT3 antenna tuner is shown in Figure 6‑4. It does a remarkable job of tuning from 160 meters to six meters over at least a 10:1 SWR range. Relay K18 can select ANT1 or ANT 2 by tapping ANT. If a Sub receiver is connected to the ATU (CONFIG:KRX3=Atu) the Sub receiver may use whichever antenna the Main receiver and transmitter are not using. Although not shown in Figure 6‑4, each antenna input is protected from overvoltage by a surge protector.

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Figure 64. KAT3 ATU.

 

6.7.1    Adjusting the ATU

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Reading the L and C ATU Values

1.     Select LC SEt in the CONFIG menu.

2.     Tap MENU to exit.

3.     Tap ATU TUNE; VFO A shows the capacitance (CA indicates the capacitance on the antenna side, Ct on the transmitter side). VFO B shows the inductance.

4.     Tap MENU to exit.

Manually Setting L and C Values

1.     Select LC SEt in the CONFIG menu.

2.     Tap MENU to exit.

3.     Tap ATU TUNE; VFO A shows the capacitance (CA indicates the capacitance on the antenna side, Ct on the transmitter side). VFO B shows the inductance.

4.     Tune VFO A to change the capacitance; VFO B to change the inductance. Tap ANT to toggle between CA and Ct.

5.       Tapping CLR clears the stored LC data for the present band.


6.8    Other Transmitter Configuration Menu Items

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