RadioBanter - View Single Post

March 2nd 06, 03:31 AM posted to rec.radio.amateur.homebrew

wrote:
On Wed, 1 Mar 2006 09:04:13 -0500, "xpyttl"
wrote:

wrote in message
. ..

If you building a radio that runs on
batteries then using more power may be bad.

Also keep in mind that more power=more heat

If you are building a simple analog VFO, temperature compensating the VFO
can be the most tedious part of designing a receiver. Depending on how
tight your box is, the difference in heat could be an issue. Keeping the
oscillator stable while delivering more power also means more buffer stages
between the VFO and the mixer.

Actually even without the heat issue you still have to compensate it
or ambient variation will drive you nuts. Granted a few transistors
delivering 5-10mW of power is not a great heat generator when you add
all the surrouding possible sources.

If you are designing with a DDS, of course, all this is pretty much moot.
With a typical DDS chip and a packaged clock oscillator at some high
frequency, the oscillator will draw so much current and generate so much
heat that what the mixer requires is invisible.

Since buffereing the VFO is a good idea anyway the buffer and later
statges can supply the 5 or more milliwatts needed for level 7 rings.
Since those stages can be "remote" the small heat generated is not a
big issue. However between a VFO, buffer and a buffer to deliver
power you can be hitting 30-50mA and on batteries thats a bigger
issue.

If you using DDS, likely power is not an issue and the combined DDS
and control plus display could be surprizingly high or at least has to
be managed.

However you approach the problem a little though to the overall
effects are important. After all what usually seperates a great
reciever from a passable one is attention to the little details.

Allison
Some of the modern DDS chips require little power. Analog Devices
has some DDS chips that draw less than 50ma at 5v, I think there is
one that takes but 15ma. True a vfo will draw even less, but we
are not talking about gobs of power in any case.

DDS vfo's have very low phase noise, and the ones that can be clocked
at 100mhz or higher can deliver quite low spurs. The AD9954 series
have a 14 bit DAC and can make a very good HFO for a single conversion
receiver with no PLL loop filter needed to clean up the output.