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PostPosted: Mon Jan 30, 2017 12:48 pm 

Joined: Mon Sep 23, 2013 11:17 am
Posts: 5
Just thought I'd share what I've been using ST for lately. I have on my bench a prototype transmitter I'm designing, the ST test tones have come in very useful for testing modulation, audio response and different varicap bias voltages etc... I only wish the slider to select the test tone was more linear above 15khz. I'm using a trace alpha card also for its flat response which I also confirmed on the scope, it gives a perfect square wave at 30hz.

I'm using two varicaps, one fed with the PLL control voltage to set the frequency and keep it locked, and the other for modulation. Now I've found that the bias voltage to the modulation varicap makes a difference to the audio response above 10khz and also phase angle between the 19khz pilot and RDS. The deviation also changes when the transmitter frequency is changed, requiring the modulation level to be set again for 75khz deviation. To get around all this I'm scaling the bias voltage to the modulation varicaps so the modulation level and audio response is consistent across the band. I have also gone for a longer lock time using a "lead lag" configuration in the PLL loop filter, so a 30hz square wave will modulate at 75khz (or very close to it within 2khz) without the PLL trying to correct it.

The trouble with some transmitters out there is they use a single varicap to tune the whole band and modulate the MPX signal, hence leaves very little room for 75khz deviation, the audio response is not flat as a result, phase difference occurs between pilot and RDS, and depending on the loop filter, low audio frequencies will have a hard time modulating.

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