LF: G0MRF 300 w Tx / DDS input

[G0MRF@aol.com]

In a message dated 1/31/04 1:47:14 PM GMT Standard Time, g4wgt@talkgas.net writes:

> Hi all at LF,
>
> Can anyone advise me regarding using an external vfo on the 300 w
> transmitter external vfo option. Has anyone used this feature? Connecting to
> the external vfo input pin produces no output. I have just built the KG6CYN
> DDS VFO offered by the G-QRP CLUB.
>
> Output: Buffered 1 Vpp into 50 Ohms for 50KHz to 40 MHz - Non Buffered
> Output also provided.
>
> The original can be found at http://www.qsl.net/kg6cyn/DDS_Project.htm
>
> Similar e-mail to David direct.


Hello Gary.

Have just seen your question and Alan's reply.

The external input is designed for CMOS levels. However if you have a DDS with less than 12V P-P then Alan's suggestion of two resistors is a good one. My only thought would be that these should not be of equal value as it would cause false triggering of the 4013.

So, how about 47k and 33k. This would give 4.95 Volts on the input which would allow a 1V margin and would prevent false triggering from noise etc.

However a 1V P-P DDS will still be too low. You need around 3V P-P to be sure.

Assuming you wish to use the DDS and not the internal VXO (permanently) then there's a better choice.

You can use IC2 on the circuit to amplify the DDS.  IC2 has a gain of 10 and would be ideal for a 1V  P-P output DDS

To do this you need to break the connection between L1 C11 and C12.

Feed your DDS into C12, having isolated L1 / C11 and you will have enough signal to drive the CMOS logic.
I also suggest that to avoid the false triggering problem you add a resistor in parallel with either R8 or R9 so that the output of  IC2 is not sitting at 6.0V. If you select a value to give 5 Volts or 7 Volts that would work well.  ( 27k in parallel should be about right)   

Hope that helps.

73

David  G0MRF