Re: Generating phase modulation.



steveu wrote:


steveu wrote:



Even most FSK transmitters were capacitively coupled until the end of

the

80s. Most early paging transmitters, for example. Most paging
receivers

(in

what remains of that industry) are still capacitively coupled. Clean

DC

capable FM transmitters had to wait until DSP based modulators become
economically viable. Its all a horrible fudge, and it does cost in
performance, as the codes make to attempt to approach DC balance.

Early paging transmitters used direct DC coupled modulation of the
crystal, which was multiplied and frequency shifted then. Synthesized
transmitters used to modulate both reference crystal (DC coupled) and
VCO (AC coupled); there was special adjustment to match deviations. The

performance was indeed affected by that adjustment.
If a frequency standard was used as a reference, which was impossible
to


modulate directly, I did forward/backward phase rotation using 7486,
7404 and 7474. PLL smoothed phase jerks :)))) That was before DSPs.
On the receive side, big problem was DC offset due to carrier frequency

mismatch and discriminator zero drift. The receiver had to be AC
coupled


to cancel that offset. Good cure for lost DC was Schmitt comparator at

the input.
In POCSAG/FLEX/ERMES, there was no attempt to make codes DC-balanced.


Some POCSAG transmitters with ovenised references used a variety of
tricks
to get DC modulation, but usually with a variety of side effects.
People
did things like switching the n or m value in an n/m PLL, which had
horrible side effects.

The PLL will be kicked out of lock, followed by nasty reacquisition
process. When I designed paging transmitters, I dropped this idea.

Wow! What did you do to make it that bad? That isn't the real problem. The
real problem is the bias caused by the different loop update time between
n/m having n set x and to x + 1. This makes the thing sensitive to the bit
pattern, with significant wavering of the carrier frequency with specific
patterns.

Most transmitters, including some major brands, made
no attempt at DC modulation at all. The first totally clean modulation
I
ever saw from a paging transmitter was the from the first one with a
DSP
modulator.

Yes, DSP modulation looks very nice; however the problem with DSP are
different digital residuals showing up as out of band emmissions.

Sure. Various intermod related mess is always a pain with digital
modulation.

BTW, AD makes DDS chips which can generate linear frequency ramps by
themselves. I often have a sad feeling that with the technology of today

it is possible to do a lot of things finally in the right way; like,
say, optimal FM receivers. However, modern technologies are mainly used
to breed idiotic monstrous solutions, like 8-VSB, Windows Vista, LTE,
DAB, etc. which are nothing but a waste of resources.

Regards,
Steve
.

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