VK6ROX B

Homebrew DStar Repeater

AUDIO CONNECTIONS

IT IS IMPORTANT THE REPEATER AUDIO IS NOT FILTERED!

T857 EXCITER AUDIO INJECTION POINT AT LIMITER

Audio injection point at exciter limiter

Extract Audio from T855 Receiver at Demodulator IC output

Extract Audio at output of Demodulator IC

Audio Waveform at Demodulator Output of T-855

Audio waveform at demodulator output

Repeater (T-857) Deviation Adjustment Waveforms

ID-80 direct to aux receiver with this demodulator output

This oscilloscope screen capture represents the repeater deviation target (gold standard). This was monitored at the demodulator of an auxiliary receiver listening to an ID-80 directly.

Starting deviation RV6 at 10thirty position works with ID-80 etc

This captures the starting deviation for the repeater which worked 100% with the ID-80, ID-91, ID-92, IC-800, ID-880, IC-2820 ie the transceivers with the AMBE Vocoder installed.

 

Repeater deviation set as close as possible to ID-80 levels

This oscilloscope screen capture shows the T-857 deviation set via RV6 to match the response from the ID-80 as closesly as possible.  RV6 was set to approximately 9 oclock position.  This deviation level is expected to work correctly with the newer ICOM transceivers with the DSP implementation of DSTAR (ID-31, ID-51 7100 etc).

 

Physical Connection to T855 Receiver

Physical connection to T855 receiver

TAIT T2020 connections included for information (first implementation of homebrew DSTAR repeater used a pair of Tait mobile rigs for transmit and receive).Sound Card Connections

Initially I had incorrectly connected the soundcard headphone audio to the T2020 microphone in socket - this resulted in slightly "gargly" audio, extreme sensitivity to audio level and incorrect text messaging on the radio screens.  I was stumped for quite a while and once I realised the error, was very surprised the repeater audio could be decoded at all with this mis-wired condition.

LNA PRE-AMP S PARAMETERS

The +22dB gain was padded down to about +14dBm.  The LNA has a noise figure better than 0.5dB and was put in line with a cavity BPF to improve repeater sensitivity.

LNA Preamp S Parameters

CAVITY BPF PLACED AHEAD OF LNA

A cavity BPF (Band Pass Filter) was placed upstream of the LNA to attenuate unwanted signals.  The cavity filter characteristics were similar to those shown below, but centred on 432.725MHz

cavityfilter

 

"GARGLY" AUDIO IS A SIGN OF INCORRECT AUDIO PATH

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VK6ROX B SENSITIVITY WAS VERY POOR WITHOUT THE LNA PREAMP.  AFTER INSTALLATION THE PERFORMANCE WAS QUITE SYMMETRICAL - IF YOU COULD HEAR IT YOU COULD GET INTO IT.