IMD and BPSK Quality

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IMD and BPSK Quality
Clint Hurd kk7uq
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Presentation on the Web

• A printable copy of this presentation is
  available at http//KK7UQ.com
• A PowerPoint file of the presentation is also
  available, email Clint Hurd at discobay_at_waypt.com
  for a copy on CD.

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BPSK31 Modulation

• Phase modulates an audio carrier at 31.25 baud
  this audio modulates SSB transmitter
• Reverses phase 180 degrees (BPSK)
• Reduces signal level at phase shift time to
  reduce spurious frequencies using a cosine
  function

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PSK31 Signal Coding

• 31.25 Baud Rate
• One Bit Time 1/31.25 32 ms
• 0 Defined As Phase Change at Beginning of Bit
  Time
• 1 Defined as No Phase Change at Beginning of
  Bit Time

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Contiguous BPSK31 0 Cycles Each Phase Shift
marks the beginning of a data 0 This is the
BPSK Idle pattern IMD -30dB _at_ Offset Freq
700 Hz Note Waveform is a true two tone signal
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Close up of Phase Shift at Reduced Signal Point
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0
1
0
TRANSITION 010 IMD -30dB Signal is NOT
reduced when there is no phase shift to be done,
hence it is flat between the ramp up and ramp
down at the data 0 phase shift points
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IMD Inter Modulation Distortion

• If BPSK signal is run through a linear system in
  the rig, the distortion and effective bandwidth
  of the signal are minimized
• If the signal is run through a non-linear system
  (e.g. clipped or compressed), distortion occurs
  and the effective bandwidth goes up
• IMD (Inter Modulation Distortion) is the
  measurement used to define the level of distortion

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PSK SIGNAL BASICS

• A clean signal has unwanted 3rd order products at
  -24 dB or better
• Overdriving the signal may increase power output,
  but does NOT improve copy.
• In the examples which follow you will see the
  effect of clipping on the transmitted signal,
  both in effective bandwidth and IMD

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One BPSK31 Cycle Overdriven and Clipped IMD -11
dB
Compared to a good signal at -32 dB IMD
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MixW Spectrum Scan of BPSK31 Heavily Clipped
Signal Measured IMD -11 dB Effective Bandwidth
150 Hz
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MixW Spectrum Scan of BPSK31 Good
Signal Measured IMD -32 dB Effective Bandwidth
31 Hz
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MixW Spectrum Scan of BPSK31 Moderately Clipped
Signal Measured IMD -19 dB Effective Bandwidth
100 Hz
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BPSK31 Signals

• In these examples, you will be able to compare
  the
• MixW Waterfall view
• MixW Spectum view
• Oscilloscope view of the transmitted waveform

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IMD -32 dB Good Signal
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IMD -18dB Poor Signal
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IMD -11dB Very Bad Signal
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Stay Within Your Transmit Audio Passband 500
2250 Hz
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Setting the Audio Drive Level

• Adjusting the audio drive level is the proper way
  to set power output.
• You do NOT improve the signal quality of an
  overdriven signal by reducing the power level
  setting on the transceiver - you only reduce the
  power of an improperly adjusted signal

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Setting the Audio Drive Level

• Adjusting the audio drive level can be done at
  the
• PC using the WAVE control slider
• Transceiver Mic. Gain

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PC Output Audio Control
- The WAVE output is the one used to generate the
PSK signal - The WAVE and Volume work together -
Mute all other sources
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Transceiver Mic. Gain

• The transceiver Mic. gain can also be used to
  control audio drive level.
• But normally this is set for proper phone audio
  level and left alone when using PSK.

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How Much Power?

• Protect your transceiver finals - normally run at
  about 50 of rated CW output (peak)
• Turn off Compression
• Run with minimum ALC

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Conservative Method

• Pick a waterfall frequency in the middle of your
  rig output filter - typically 1200 Hz
• Set your rig power control to 100
• Turn off Compression
• Transmit an IDLE signal (no typing, buffer empty)
• Adjust audio drive level to produce an output of
  30 (ave. power) or 50 (peak) of rated CW
  output.
• ALC should be at low or zero level

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Why Derate Power So Much?

• The PSK signal will run at 60 duty cycle when in
  IDLE, and up to 90 when transmitting data.
• The transceiver audio path must stay linear (no
  or little ALC or Compression).
• Other digital modes run at 100 duty cycle

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Aggressive Method

• Use an IMD monitoring device such as the IMD
  Meter by kk7uq while you transmit
• Turn off Compression
• Set the rig output control to desired level such
  as 50
• Transmit an IDLE signal
• Adjust the audio drive level to produce an IMD of
  -24 dB or better - typically run at -30 dB

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IMD Meter
- Stand alone instrument - Picks up signal via
whip antenna - Amplifies signal through wideband
RF Amp - Calculates IMD by analyzing the shape of
the signal envelope
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Giving an IMD Report

• Monitor the IMD measurement at the bottom of the
  MixW screen
• Signal must be in IDLE (no typing, buffer empty)
• Check the S/N of the signal - it should be above
  36 dB to get an accurate report

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Things to look out for

• If S/N is too low, the IMD will be reported low
• If S/N is too high, the IMD may also be reported
  low - because the receiver may clipping the
  signal - adjust the RF gain down if needed

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Spectrum Display
Above MixW spectrum shows S/N about 35 dB, 3rd
order product is about -32dB (IMD in lower right
of display)
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IMD Measurement
The strength of the primary signal compared to
the 3rd harmonic above is -19 dB hence IMD is -19
dB this is a strong signal, and the RF gain
should be checked to be sure that the clipping is
not occurring in the receiver
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Take an IMD measurement with a grain of salt

• If you are given a low IMD (say -19 dB)
• Have them use the spectrum display and check the
  S/N to be sure that there is enough signal to
  properly report your IMD

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Let Your Eye Be Your Guide

• If the signal you are monitoring has S/N about
  -20 dB and IMD is about the same number
• If the signal looks clean i.e. straight sides, no
  apparent side bars, then the real IMD is probably
  -24 or better.

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• End