How to use the FLEX-3000 RF Preamp & Attenuator

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How to use the FLEX-3000 RF Preamp & Attenuator


How to use the FLEX-3000 RF Preamp & Attenuator

  System Dependencies
  Minimum PowerSDR Version:     1.18.3
  Applicable Hardware:     FLEX-3000  

Content provided by: FlexRadio Systems Engineering

Background
(Content used from a National Contest Journal (Nov/Dec 2000) article by Gary Breed (K9AY))

A receiver's dynamic range is an important fundamental performance parameter.  The FLEX-3000's dynamic range (IMD DR3) includes signal levels ranging from the noise floor of its own circuitry to the highest received signal level that causes audible intermodulation distortion (IMD or intermod). The narrow spaced (2 KHz) dynamic range with the preamp off for the FLEX-3000 is somewhere in the mid 90 dB range where the noise floor is ~ -135 dB. This means that the maximum signal strength would be ~ -40 dB that equates to a approximately S9+35. 

The key to optimal reception is to maintain the received signals within the "IMD "free" range of your receivers dynamic range. You achieve  keeping the input signal strength within this range by controlling the incoming signal levels with gain or attenuation.

So when do you need to use the preamp and when do you need to use the attenuator?
On 15 to 6 meters, the background noise level can very near the bottom of the FLEX-3000's dynamic range. At these frequencies, the addition of signal pre-amplification may be necessary to increase received signals that are above the noise level, but below the threshold of the FLEX-3000 sensitivity.

However, when the higher frequency bands are wide open, signal levels can be very strong! If we still have a lot of preamplifier gain turned on when those big signals are present, we will exceed the upper limit of our desired dynamic range and generate signal-covering intermodulation. The challenge is to be aware of the overall range of the received signals. You want to turn *on* the preamp when we need to hear weak signals, but turn it off (or even use some attenuation) to avoid intermodulation products from strong signals. The use of a preamp on these bands is not a "set it and forget it" type of setting.

The low frequency bands (160-40m) require special attention to not using pre-amplification and possibly need to use attenuation in the presence of strong signals and band noise. The 40-meter band is a unique case, since it can be either quiet or noisy. The band has the added factor of megawatt international broadcast stations. Like the other low bands, you will never need a preamp on 40m unless you are using an inefficient receiving antenna. On all the low bands, the question is, "How much attenuation do I need and how do I implement it?"

The easiest way to add attenuation with the FLEX-3000 is to reduce the AGC Threshold gain (AGC-T) on the PowerSDR console.  When the incoming signals are very strong, you should use the -9 dB attenuator available in the RX Gain drop down control on PowerSDR console.

Configuration and Settings for the FLEX-3000

The FLEX-3000 has three (3) states for amplifying or attenuating the input RF gain.  These setting are selectable from the PowerSDR console.  They are:
  • ATTN - This setting inserts a -9 dB attenuator into the RX signal path
  • OFF - This setting provide neither pre-amplification or attenuation in the RX signal path
  • PRE1 - This setting inserts +5 dB @ 20m and +11 dB @6m of pre-amplification into the RX signal path.
  • PRE2 - This setting inserts +14 dB @ 20m and +20 dB @6m of pre-amplification into the RX signal path.
Below are some guidelines for setting the preamp and AGC-T settings for various bands.

 Band (meters) 
 Preamp1  
 Preamp2 
 Attenuator
 Off
 AGC-T Range 
 160-60  N/A  N/A  On or Off  On or Off  60-80
 40-30  Usually Off
 N/A  On or Off  On or Off  65-85
 20-17  On or Off
 Usually Off
 On or Off  On or Off  75-90
 15-12m  On or Off
 On or Off
  Usually Off  On or Off  75-95
 10m  Usually Not Used
 Usually On
  Usually Off  On or Off  90-100
 6m  Usually Not Used
 Usually On
  Usually Off  On or Off  100-120



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Last Modified:Sunday, April 4, 2010
Last Modified By: Administrator
Type: HOWTO
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