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[aprssig] More efficient use of channel capicity through shorter packets

Stephen H. Smith wa8lmf2 at aol.com
Thu Oct 12 16:25:23 UTC 2006

ron.stordahl at digikey.com wrote:
> The Motorola Micor's are crystal controlled and work very well.  I do 
> not know if one could run a modern synthesized mobile radio with TDX 
> as low as 100 milliseconds...my recollection..and it is from many 
> years ago..was that such radios required a longer TXD for their 
> frequency to settle down after key down.  This may no longer be true.

It depends entirely on the synthesizer scheme in a particular radio. In 
some radios, the synthesizer shifts from being the RX local oscillator 
(i.e. offset from the operating frequency by the IF frequency) to being 
on-channel when you transmit. This is the classic version of a 
synthesized design. It has the minimum parts count and complexity and is 
the slowest settling. 

In other designs, the synthesizer always remains on the RX local osc 
frequency.  On TX, a crystal oscillator on the IF frequency is turned on 
and mixed with the LO to generate the TX frequency.   In this scheme, 
the synthesizer doesn't have to un-lock and  re-lock on a new freq each 
time you TX.  The TX settling becomes just the startup time of the 
crystal osc -- just as fast as the classic crystal-controlled radios.   
[Note that this approach is a "true" transceiver just like an HF SSB rig 
where the TX uses the RX IF, local  osc and mixer chain backwards on TX. ]

And then there are the newest designs that use DDS (direct digital 
synthesis).  These designs don't use a PLL system at all; basically they 
clock a D-to-A converter with a waveform derived from a crystal osc and 
variable divide-by-N counter. They can effectively slew instantly from 
one frequency to another, limited only by how fast a controller can load 
the divider with a new divide-by value.

In the early 1980s, I was involved in one of the first public safety 
mobile data projects.  We had a "fast draw" competition between the 
crystal-controlled radios of the day (the Motorola Micor and the GE 
Mastr II) to determine which could key up in the fewest number of 
milliseconds.   Out of the box the Micor keyed up much faster.  We 
determined that the GE "ICOMs" (Integrated Compensated Osc Modules - a 
crystal combined with a temp-compensated dedicated oscillator in a can,  
for each channel) were slow on the startup when powered up on TX.   

These radios used crystals operating at 1/12th or 1/18th of the 
operating frequency with a chain of frequency multipliers to get to the 
operating freq.    We wired the TX ICOMs (oscillators) to be powered ALL 
the time to avoid the startup and settling time.  This of course would 
have produced a continuous dead carrier in the receiver. We got around 
this by modifying the radio to turn the power to the multiplier chain on 
and off while leaving the (1/18th TX freq) osc running all the time.   
We were able to get the time from keyup to full TX down to 7 
milliseconds with this expedient.    

The moral of all this is that crystal-controlled radios aren't 
neccesarily fast on the draw and not all synthesized ones are slow.


Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com

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