IMD and BPSK Quality Clint Hurd – kk7uq. Presentation on the Web A printable copy of this presentation is available at

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IMD and BPSK Quality Clint Hurd – kk7uq

Presentation on the Web A printable copy of this presentation is available at http://KK7UQ.comhttp://KK7UQ.com A PowerPoint file of the presentation is also available, email Clint Hurd at discobay@waypt.com for a copy on CD. discobay@waypt.com

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

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

Contiguous BPSK31 “0” Cycles Each Phase Shift marks the beginning of a data “0” This is the BPSK “Idle” pattern IMD = -30dB @ Offset Freq = 700 Hz Note: Waveform is a true “two tone” signal

Close up of Phase Shift at Reduced Signal Point

0 10 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

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

PSK SIGNAL BASICS A clean signal has unwanted 3 rd 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

One BPSK31 Cycle Overdriven and Clipped IMD -11 dB Compared to a good signal at -32 dB IMD

MixW Spectrum Scan of BPSK31 Heavily Clipped Signal Measured IMD -11 dB Effective Bandwidth 150 Hz

MixW Spectrum Scan of BPSK31 Good Signal Measured IMD -32 dB Effective Bandwidth 31 Hz

MixW Spectrum Scan of BPSK31 Moderately Clipped Signal Measured IMD -19 dB Effective Bandwidth 100 Hz

BPSK31 Signals In these examples, you will be able to compare the –MixW Waterfall view –MixW Spectum view –Oscilloscope view of the transmitted waveform

IMD -32 dB Good Signal

IMD -18dB Poor Signal

Stay Within Your Transmit Audio Passband 500 – 2250 Hz

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

Setting the Audio Drive Level Adjusting the audio drive level can be done at the: –PC using the WAVE control slider –Transceiver Mic. Gain

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

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.

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

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

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

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

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

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

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

Spectrum Display Above MixW spectrum shows S/N about 35 dB, 3 rd order product is about -32dB (IMD in lower right of display)

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

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

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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