If you use a DDS of the AD9850/51, and probably the AD9832 type, there
is a very neat way of amplitude keying them. Place an NPN transistor
in series with the D/A current setting resistor, and a small resistor
in teh emitter to give some neg feedback and key that. Using a low
pass filter on the drive waveform allwos a gentle ramp rather than a
hard on-off. More details are somewhere on the Analog Devices web
site
Not that gentle amplitude ramoing s a particularly good idea when used
for driving switching PAs
Andy
'jnt
On 12 May 2012 20:28, Gary G4WGT <[email protected]> wrote:
> Hi Martin, LF,
>
> I have used relay keying using qrs.exe to key the output from my DDS VFO &
> it was a very simple, stable & reliable method. The DDS VFO remained on all
> the time. seems to me the way to go.
>
> 73, Gary - G4WGT.
>
> ----- Original Message ----- From: "James Moritz"
> <[email protected]>
> To: <[email protected]>
> Sent: Saturday, May 12, 2012 7:14 PM
> Subject: Re: LF: IEEE488
>
>
>> Dear Martin, LF Group,
>>
>> Provided you were happy with simple on/off keying, i.e. QRSS rather than
>> DFCW or the various mFSK modes, there is no need to interface the sig gen to
>> the PC. You could just connect the sig gen output via a relay or other bit
>> of gating circuitry, set the generator to the frequency you want and key the
>> relay with the QRSS software... Actually, for most types of TX, a better
>> idea is to leave the generator going constantly and key the PA stage. I used
>> to use an old Racal generator in this way.
>>
>> Cheers, Jim Moritz
>> 73 de M0BMU
>>
>
>
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