Re: 20/30M Daughterboard output voltage
- At 1:57 pm ((PST)) Sat Dec 29, 2007, in Digest 2083 Bill Dumke wrote:
>4c. Re: 20/30M Daughterboard output voltageAgreed, and everybody should make this check, but that doesn't tell
> Posted by: "Bill Dumke" billd@... billspiro1
> Date: Sat Dec 29, 2007 1:57 pm ((PST))
>In fact, that brings up another way of checking if the voltage is
>adequate to drive the LO quadrature chain. Just take a DC voltmeter and
>measure the voltage at the outputs of the frequency dividers to see if
>ithey are 50% of the IC supply voltage. If it is not, you have a
>problem. A square wave will produce a 50% duty cycle which can be
>measured with the voltmeter. That is what you should be getting.
you whether there is any safety margin over the two schmitt trigger
thresholds range at the clock input of the first divider.
If the voltage swing is only just sufficient then you could get a very
elusive fault as the divider slips in and out of operation with small
changes in temperature or power supply voltage - both of which
might be caused by the divider slipping in and out of operation,
or by switching form receive to transmit.
Another technique which does not require a HF scope would be
to make a diode RF probe for a high-impedance DC voltmeter.
This would give comparative Vpp readings which are about one
diode drop low but could be calibrated against the rail-to-rail
switching CMOS output of an active divider stage.