Re: [Repeater-Builder] Crystal Phase Noise?

[Kevin Custer]

Hi Scott Thanks for the reply Kevin-   -------Subject repeaters will have to remain unnamed as I currently enjoy a good relationship with their point of origin and I want very much to keep the situation that way. I will say however that transmitter circuit wise , they are esentially the same as those produced by Spectrum and I have a couple of them too. I other words, a simple crystal oscillator (with an audio modulated varicap modulating the crystal capacitance) followed by a multiplyer string and final amplifier-----ie, no PLL's. The difference in the modulation (FM) noise of of the subject repeaters and others is noticable via a  monitoring receiver-----some repeaters are quieter than others and I'm not talking about RF quieting----I'm talking about differences in audible background hiss assuming a "full quieting" strong RF signal. Do you have any ideas as to where the "hiss" modulation might be coming from. I think it is random phase noise in the crystal oscillator but I can't point at the problem part or design problem and am wondering if the crystal it's self could be my noise source. Can you take a listen (maybe with a HF receiver) and see if the oscillator is noisy?  Realize that it will be less noisy by the amount of multiplication in the radio, but comparing it with a properly working unit will show you if the problem is at the oscillator level or not.  If not, successive stages can be monitored likewise to see if the problem raises in latter stages. I know in some models of the Spectrum exciter, the varactor is biased to around 4 volts.  Modulation is imposed upon the bias through a capacitor.  If the bias provided is not stable or quiet, the effects you are hearing could be the result of such instability.  Looking closely with a scope around the varactor and its biasing should tell you if this is a problem.  As I remember, quiescent bias was set with a variable resistor, which may have become noisy. Skipp has suggested that in rare cases the crystal itself can be noisy, but his case history suggests a problem in possibly 1 in 10-15 units, which you seem be have a problem more common than that. Let us know how you make out. Kevin Custer Yahoo! Groups Links To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/   To unsubscribe from this group, send an email to: [EMAIL PROTECTED]   Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.