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Chapter 5

Chapter 5:  K3 Receiver

5.1    Introduction

Chapter 3 discussed the basic operation of the K3 receiver and the operation of the filter passband controls, filter presets, and RIT. This chapter will investigate more of the receiver's controls and learn some more about how Elecraft designed the receiver.

5.2    Receiver Filtering Overview

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5.3    Receiver Block Diagram – the 10,000' View



Figure 52. K3 receiver block diagram.

 

The RF Stages

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First IF

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Digital Signal Processing

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5.4    Receiver Schematic Tour – the 1,000' View

Download the K3 schematics from the Elecraft website[1]. The version we refer to here is June 2010. Open the schematic for viewing with Adobe Acrobat reader and we will follow the received signal's path and view the schematic details. To make the story somewhat simpler, we will assume a K3 with a K3 ATU but no 100-watt power amplifier, no KXV3 Transverter Interface, no KRX3 Sub receiver, and no Digital Voice Recorder.

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KAT3

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RF Board

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Diode Switching

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The Diode TR Switch

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RF Board Continued

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Audio Output

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5.5    Automatic Gain Control – AGC

An AGC circuit detects a received signal's strength and then changes the overall system gain to make different signal levels sound much the same. It can also limit strong signals so they don't hurt our ears or exceed hardware limitations. Overwhelming the receiver with too-strong signals can cause noise, thumps, pops and distortion - all very bad. During casual operation, say in a round table of friends, this is easier to listen to because the stronger signals sound like the weaker signals. In another scenario, say when trying to pull a signal out of a pileup, a good operator may not want AGC to make the strong signals sound about as loud as the weaker ones. To help you deal with these very different scenarios, the K3's well-designed AGC system will allow you to choose how the AGC works in various operating situations.

The K3 has a variety of user configurable AGC controls, but before learning about these, let us first consider how the AGC operates (1) as a function of time – that is, how fast or slow it activates and deactivates, and (2) as a function of signal strength – that is, how different signal levels are converted to audio levels.

AGC Time Behavior

The behavior of the AGC circuit as a function of time includes attack rate, hold time, and decay rate.

AGC Attack Rate:

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AGC Hold Time:

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AGC Decay Rate:

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AGC-Fast and AGC-Slow:

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AGC as a Function of Signal Strength

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Intermodulation Distortion and AGC Pumping

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5.5.1    The K3's Hardware AGC (HAGC)

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5.5.2    DSP AGC

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AGC Decay and Hold Effect on Received Audio

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AGC SLP

Figure 5‑14 shows the effect of changing the SLP parameter from SLP=0 (steepest slope) to SLP=15 (flat) with THR=8.

If SLP=0, we get the largest change in audio level for an input signal change. You may wish to choose this for contest operating. When SLP=15, the curve is virtually flat – all signals will give about the same audio. You may wish to do this for casual operating and rag chewing or when static crashes are annoying you. The AGC slope is the same for slow and fast AGC.

 


Figure 514.  K3 AGC and SLP.

 

AGC THR

Figure 5‑15 shows how the threshold or onset of AGC changes as we change THR from 8 to 2. You may wish to choose a higher THR value if, for example, you are trying to pull weak signals out of the noise on 160 meters so the weak signal is not attenuated by the AGC's attempt to be helpful. The AGC threshold is the same for slow and fast AGC.

 


 

Figure 515.  K3 AGC and THR

 

 

5.5.3    Hints for Setting your AGC

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5.6    Audio Effects – AFX

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5.7    Receiver Audio Equalizing

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5.8    VFO B Alternate Display

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AFV and dBV Measurements

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5.9    The K3 with the KAT3 ATU

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5.10The KRX3 Sub Receiver

The K3 Sub receiver is virtually the same as the Main receiver with only small circuit changes. It has independent signal level controls (RF Gain, AF Gain, preamp, attenuator, etc.) and independent digital signal processing (filter selection, AGC, bandwidth, noise blanking, noise reduction etc.). It is a true second receiver and with it you can listen to two separate frequencies simultaneously in the K3. For example, if you are chasing a DX station operating split, you can listen to the DX on one receiver and tune through the pileup on the other to find the station the DX is working. You can even listen on two separate bands at the same time, although there are some restrictions on which receiver is listening to which band.

 

5.10.1 Decisions, Decisions

When you include the Sub receiver in your K3, you must make two major decisions. The first is to define the complement of filters and the second is to decide what antenna configuration to make.

Sub Receiver Filters

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Sub Receiver Antenna Choices

When you install your Sub receiver you must chose an antenna configuration and then physically connect a TMP cable inside the K3 from the Sub receiver's J92 antenna input to one of two places. First, you can connect the receiver to the K3's ATU, which allows the Sub receiver to either share the same antenna the Main receiver is using (ANT1, ANT 2, or RX ANT IN if a KXV3 is installed). You may also choose to have the Sub receiver use whichever antenna the Main receiver is NOT using (ANT 2 if Main is using ANT1, or vice versa). You must set CONFIG:KRX3 Ant=Atu for this option. This may be your best choice for all around operating with a variety of antennas. Section 5.10.3 shows antenna selections when you make this choice.

The second choice is to connect the receiver to a separate auxiliary RF BNC antenna connector. In this case, the Sub receiver can share an antenna with the Main or it can use a different antenna connected to the AUX RF BNC. Set CONFIG:KRX3 Ant=bnc if you choose this option. This may be your best choice for diversity receiver operation with specialized receive antennas. Section 5.10.4 gives diagrams showing antenna choices for this configuration.

 

 

5.10.2 Basic Operation of the Sub Receiver

The Sub receiver can be used totally separately from the Main receiver, say when you are chasing DX in split mode. Also, VFO A and VFO B can be linked so that VFO B tracks VFO A (regardless of Sub being turned on or off). You might use this for tuning across a segment of a band with repeaters to listen to the repeater output and input simultaneously. Finally, a true diversity mode exists so you can listen to signals from two different antennas to enhance the received signal.

Sub Receiver Controls

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Sub and Main Receiver AF Controls

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 Sub and Main Receiver RF and Squelch Controls:

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Sub VFO

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5.10.3 The Sub Receiver Connected to the KAT3 ATU

When you install the Sub receiver you must connect its antennas either to the KAT3 ATU or to the AUX RF BNC. This affects the antenna choices you can make. Table 5‑12 shows the configuration menu choice when the Sub receiver is connected to the ATU.

 

Main Receiver and Sub Receiver Sharing the Same Antenna

Figure 5‑19 shows a K3 equipped with the KAT3 antenna tuner and the KRX3 Sub receiver. The Sub receiver antenna input is connected to the KAT3 ATU (solid line). Two relays control the antenna selections. K11 on the Sub receiver board selects the antenna to be used by the Sub receiver. It may be either the Main receiving antenna or the auxiliary antenna. Relay K11 is switched when the Sub receiver is turned on (tap SUB) and then by either holding RX ANT or entering b SEt mode (hold BSET) and then selecting the Sub antenna by tapping ANT. You must have Sub turned on to switch relay K11.

Relay K18 in the ATU allows you to choose which antenna the main receiver is using by tapping ANT. If the Sub receiver is switched to AUX by relay K11 and is connected to the ATU as shown, the Sub receiver will be the antenna the Main receiver is NOT using.

In Figure 5‑19 the Main and Sub receivers are sharing the same antenna, in this case, ANT1. You will likely use this configuration when chasing DX while operating split, so you can hear both the DX on the Main and the pileup on the Sub receiver. Switch relay K11 so that the Sub antenna display shows MAIN. Both receivers may use ANT 2 by tapping ANT to switch relay K18. When both receivers are using the same antenna, you will notice a 3 dB drop in signal strength due to the splitter used[2]. As we will see in Section 5.12 below, both receivers can share a separate receive-only antenna connected to the RX ANT IN BNC connector on the KXV3A.

 

 

Main and Sub Sharing the Same Antenna

1.       CONFIG:KRX3 Ant=Atu

2.       Tap SUB to turn Sub receiver on.

3.       Either:

Hold RX ANT, which toggles between MAIN and AUX. Set it to MAIN.

or

Hold BSET, and then tap ANT, which toggles between MAIN and AUX. Set it to MAIN. Tap A/B to exit.

4.       Tap ANT to select ANT1 or ANT 2 for Main and Sub antennas.

Figure 519.  K3 with KAT3 ATU and KRX3 Sub Receiver sharing one antenna.

 

 

 

Main Receiver using ANT1 and Sub Receiver using ANT2  

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5.10.4 The Sub Receiver Connected to the AUX RF (BNC)

Your other possible choice for the Sub receiver's antenna connection is the AUX RF (BNC) connector. You are likely to choose this if you frequently use diversity reception. Again, you  must open up your K3 and plug the coax lead from the Sub receiver into the AUX RF BNC jack to make this choice.

 

Main Receiver and Sub Receiver using ANT1 or ANT2

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Main Receiver using ANT 2 and Sub Receiver using AUX RF (BNC)

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5.11Dual-Band Receive Operation

After you set CONFIG:VFO IND yES, the two K3 receivers can operate on different bands. For example, you can monitor six meters for band openings while operating on another band. However, you do not have complete freedom in how you choose which band is on which receiver because of the low pass filter in the antenna path selected for the Main receiver and transmitter. Also, you cannot receive on either receiver while the K3 is transmitting.

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5.12The KXV3 Receive Antenna Interface Option

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5.12.1 KXV3A Receive Antenna Interface, Normal Operation, Main = ANT1

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5.12.2 Using a Receive Antenna on the Main Receiver

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5.13    The K3 with KRX3 and KXV3

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5.13.1 Normal Operation, Main = ANT1, Sub = ANT 2

In Figure 5‑29 there are now three relays that control the antenna selections. In addition to relays K1 and K18, the K11 relay allows you to choose an antenna for the Sub receiver. Successively tapping RX ANT switches K1. As Figure 5‑29 shows, when relay K1 is in the lower position the Main receiver uses ANT1 or ANT 2, depending on relay K18. The Sub receiver uses the other of the ANT1/ANT 2 pair. This is equivalent to Figure 5‑20.

 

Main on ANT1, Sub on ANT2

1.  CONFIG:KRX3 Ant=Atu

2.       Tap SUB to turn Sub receiver on.

3.       Either:

Hold RX ANT, which toggles between MAIN and AUX. Set it to AUX.

or

Hold BSET, and then tap ANT, which toggles between MAIN and AUX. Set it to AUX. Tap A/B to exit.

4.       Tap ANT to select ANT1 for Main antenna and ANT 2 for Sub antenna.

Figure 529.  Main = ANT1, Sub = ANT 2.

 

 

5.13.2 Using a Receive Antenna on both Receivers

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5.14Diversity Reception

Effective diversity reception requires two receivers, each fed by a different antenna. To be effective, the two antennas may have the same or different directivity but be widely separated from each other, or may be close together but have different horizontal and vertical polarization. For example, on 160 meters a good combination is the transmit antenna, say a vertical, plus a Beverage or receiving loop. Diversity reception can improve signal copy by reducing fading. When you use two different antennas, QSB fading generally affects each of them differently. By listening to the two signals from the two antennas we can "smooth" out the fading effects. In Figure 5‑31 we see the Main receiver feeding audio to the left headphone and the Sub to the right. When the two receivers are closely matched with minimal difference in phase delays in each, as they are in the K3, we can have an effective diversity receiver. As the signals received by the two different antennas change, we perceive the signal "moving" around in our head. This can sometimes greatly improve copyability.

Achieving good results with diversity reception depends on many factors, including the combination of antennas used, the propagation at any given time, and current noise conditions including polarization of the noise. What is especially effective at one time may not be so at another.

When diversity reception is activated, the Sub receiver is linked to and controlled by VFO A. VFO B remains independent and can be used to set the transmit frequency for split operation as described below.

 

 

Figure 531. Diversity reception.

 

 

Roofing Filters for Diversity Reception

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Activating Diversity Reception

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5.14.1 Diversity Antenna Selections

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Sub Receiver Connected to the AUX RF BNC

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Using a Receive Antenna on the Main Receiver

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Diversity Reception Activated

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Sub Receiver Connected to ATU

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Operating Split in Diversity Mode

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5.15Spurious Signal or Birdie Removal

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