Clean Transmitters
 

Wes Hayward in _Experimental Methods for RF Design_ bemoans the fact
that our transmitters are so dirty in terms of noise floor, close-in
phase noise and IMD. I see newer transmitters like the Mark V offer a
cleaner class A mode which really seems to help IMD, and read a
previous post regarding a transmitter that filters the noise floor, but
wonder if there are any homebrewers out there who are working this
problem? This seems the type of problem that we hams can and do ignore
completely, unless we are contesters or have a neighbor on the bands.

Regards,
Glenn Dixon AC7ZN

On Tue, 07 Jun 2005 15:02:38 -0700, MadEngineer wrote:

Wes Hayward in _Experimental Methods for RF Design_ bemoans the fact that
our transmitters are so dirty in terms of noise floor, close-in phase
noise and IMD.

This is not such a big issue at HF where band noise
is relatively high. Keeping noise and distortion to
a minimum is a lot more important on VHF where noise
is low, local stations are very strong and everyone
has an ultra low noise preamp. I can operate within
+/- a few KHz of some of the cleaner local VHF stations.
Some of the nastier ones need about 30KHz of elbow
room.

any homebrewers out there who are working this problem?

Many homebrewers put a lot of effort into keeping
phase noise and IMD to an absolute minimum. Some
of the cleanest signals on the 80M band can be
heard on the homebrew net frequency (3.727MHz).

73, Ed. EI9GQ.


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Many homebrewers put a lot of effort into keeping
phase noise and IMD to an absolute minimum. Some
of the cleanest signals on the 80M band can be
heard on the homebrew net frequency (3.727MHz).

73, Ed. EI9GQ.

Hi Ed, now get them all to get on at say 05, 0600Z so we can have the
propagation to do that.

Regards
W4ZCB

"MadEngineer" wrote in message
oups.com...

Wes Hayward in _Experimental Methods for RF Design_ bemoans the fact

but wonder if there are any homebrewers out there who are working this
problem?

You don't suppose Wes may have homebrewed a rig or two in his time?

...

He has indeed.

But what he was writing about would be something like a method that
enables a medium power (tunable, not crystal-controlled) transmitter
that, while transmitting SSB, still allows reception of adjacent weak
signals from a receiver connected to a nearby separate antenna. At the
time of Wes's writing, it seemed clear to this reader that Wes believed
this technology didn't exist, at least in the amateur community. Since
his writing I haven't really seen the problem addressed beyond what I
mentioned in my first post, and I was asking if anyone knew anything
more.

Regards,
Glenn Dixon AC7ZN

But what he was writing about would be something like a method that
enables a medium power (tunable, not crystal-controlled) transmitter
that, while transmitting SSB, still allows reception of adjacent weak
signals from a receiver connected to a nearby separate antenna. At the
time of Wes's writing, it seemed clear to this reader that Wes believed
this technology didn't exist, at least in the amateur community. Since
his writing I haven't really seen the problem addressed beyond what I
mentioned in my first post, and I was asking if anyone knew anything
more.

Regards,
Glenn Dixon AC7ZN

You're describing the oscillator phase noise problem, and since G3SBI's
unveiling of the "H"-mode mixer, THE compromise to a "perfect" radio.

All oscillators have some degree of phase noise, from a poorly designed VFO,
to a well designed one, (One of the better is Gumms class C design featured
in several of Haywards books). The DDS has VERY little phase noise, but has
substituted serious problems with spurious content in it's spectrum, and use
of a PLL to get rid of those results in phase noise again (outside the loop
bandwidth)

Lots of folks working on it, the same G3SBI has come up with a multiple
resonator VCO for his PLL in the CDG2000. While it has had some technical
criticism, Tibor Hajder has established that the concept is theoretically
sound and results in a faster roll-off of phase noise than the single
resonator approach. (Applied Microwave and Wireless, Oct 2002)

That same problem from the interference caused by the "lousy" transmitter,
will be encountered from the phase noise of the local oscillator of the
"receiver connected to a nearby separate antenna". Unless of course, the RX
is a TRF design with no LO.

W4ZCB

On Wed, 08 Jun 2005 00:00:54 +0000, Harold E. Johnson wrote:

Hi Ed, now get them all to get on at say 05, 0600Z so we can have the
propagation to do that.

Regards
W4ZCB


Hi Harold,
The homebrew net gang are never heard during the hours
of darkness. They probably build rigs during the night
and operate during the day :-)

It should be quite easy to make contact on 17M. I would
like to give it a try sometime. I have yet to work a
homebrew SSB rig from the USA.

73, Ed. EI9GQ.


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Hi Ed, now get them all to get on at say 05, 0600Z so we can have the
propagation to do that.

Regards
W4ZCB


Hi Harold,
The homebrew net gang are never heard during the hours
of darkness. They probably build rigs during the night
and operate during the day :-)

It should be quite easy to make contact on 17M. I would
like to give it a try sometime. I have yet to work a
homebrew SSB rig from the USA.

73, Ed. EI9GQ.

GM Ed

Well, I can cure your problem, and give you something to cover that hole in
the wall at the same time if you'd get busy on a STAR offspring. The
alternative frequency to 3.727 is 14.305 or 14.307 when some net is in
operation. I've worked 10 STARS up there and have made up a little WAS
certificate for guys that have done likewise. So far, Peter got number one,
Alessandro number 2 and a really weak contact with DL9 (he was running 20
watts to a field day dipole on a day with REALLY lousy propagation) gave me
number 3. (Peter has an endorsement for working 20 of them!)

I know what you mean, we have way too much money over this way (We keep
spending money we don't have, THAT will change) and homebrew is a dying
artform. I make a lot of contacts where the guy on the other end responds
with "yours is the first homebrew transceiver I've EVER worked." My Signal
One hasn't been turned on in over a year and thinking of getting rid of it.

There IS hope. After Dayton, we have two more under construction, one in NJ
and one in GA.

Regards, County Cork almost got invaded last Spring to see you, but we loved
Goray so much, I only made it down to Waterford. You live in a beautiful
part of the world.

Melt solder!

W4ZCB

Thanks, Harold, I appreciate the info and will try to find the article.

Phase noise is but one of the problems Wes mentions, and on the
transmitter side he makes it clear the IMD particularly in the
transmitter final stage needs work and is perhaps the largest of the
problems on the transmitter side. Wes implies that he can actually
hear such distortion simply by listening to the band (I don't know what
to listen for beyond a particularly wide-frequency signal). Chris
Trask has done excellent work on this, but haven't seen actual IMD
numbers from his techniques.

I'm currently thinking a poor IMD transmitter would only be a problem
in a wideband mode such as SSB or SSTV. Though in theory a poor IMD
transmitter might cause keyclicks in CW (I really haven't thought this
out), I doubt this is much of a problem.

Regards,
Glenn Dixon, AC7ZN

I'm sorry, Harold, I don't know that 'TRF' stands for. Is that
somehting similar to TSDR 'true software defined radio'? That is, an
A/D hooked directly to an antenna?
Glenn

I'm sorry, Harold, I don't know that 'TRF' stands for. Is that
somehting similar to TSDR 'true software defined radio'? That is, an
A/D hooked directly to an antenna?
Glenn

I HATE it when I do that! No, it's an OLD term. Actually, one of the few
that's older than me. Stands for Tuned Radio Frequency, the topology from
before Armstrong came along. Tuned RF amplifiers at the signal frequency,
followed by a detector and audio. No oscillators.

IMD, in particular odd order IMD, is readily identified by tuning to the
opposite sideband of a SSB signal with a selective receiver. If you can hear
the signal weakly but clearly, you're listening to the suppressed sideband
of the transmitting station. If everything is garbled, (AND MUCH stronger
than the suppressed sideband which you probably CAN'T hear due to the
garbage ) You're hearing odd order IMD.

If it's a REALLY strong signal, it may be being generated in your receiver.
If it's not terribly strong, (Like maybe "S"-9 although this is being pretty
subjective) it's likely that the transmitting station is producing the IMD.

We've been sold down the river to some extent, published tests of amateur
equipment in this country, now examine the IMD with respect to a single tone
of two tones used in the test to generate the IMD. This makes the equipment
look like it's 6 dB better than it would be if you tested it against BOTH
tones which is the benchmark our military and most other countries use to
evaluate IMD. Commercial offerings these days, particularly in more modestly
priced gear is really pretty sorry in this respect.

IMD is bad news, but there are cures for it. Phase noise is bad news, and
there's no cure for it other than using very High Q sources that still have
it but at much reduced levels. New technology that uses spur cancellation in
digital synthesis is also helpful, but out of reach for most of us.

Everytime you multiply an RF signal by 2, you increase it's phase noise by
close to 6 dB. Everytime you synchronously divide that signal by 2, you gain
that same 6 dB. Obviously, if you can make a very low phase noise variable
oscillator at a high enough frequency so that you can divide it by a zillion
and still have it high enough to do what you wish to do with it, you wind up
with a low noise source. That's a bit expensive as well.

Sorry for the long post, didn't have time for a short one.

W4ZCB

On Thu, 09 Jun 2005 20:43:06 GMT, "Harold E. Johnson"
wrote:


I'm sorry, Harold, I don't know that 'TRF' stands for. Is that
somehting similar to TSDR 'true software defined radio'? That is, an
A/D hooked directly to an antenna?
Glenn

I HATE it when I do that! No, it's an OLD term. Actually, one of the few
that's older than me. Stands for Tuned Radio Frequency, the topology from
before Armstrong came along. Tuned RF amplifiers at the signal frequency,
followed by a detector and audio. No oscillators.

IMD, in particular odd order IMD, is readily identified by tuning to the
opposite sideband of a SSB signal with a selective receiver. If you can hear
the signal weakly but clearly, you're listening to the suppressed sideband
of the transmitting station. If everything is garbled, (AND MUCH stronger
than the suppressed sideband which you probably CAN'T hear due to the
garbage ) You're hearing odd order IMD.

If it's a REALLY strong signal, it may be being generated in your receiver.
If it's not terribly strong, (Like maybe "S"-9 although this is being pretty
subjective) it's likely that the transmitting station is producing the IMD.

We've been sold down the river to some extent, published tests of amateur
equipment in this country, now examine the IMD with respect to a single tone
of two tones used in the test to generate the IMD. This makes the equipment
look like it's 6 dB better than it would be if you tested it against BOTH
tones which is the benchmark our military and most other countries use to
evaluate IMD. Commercial offerings these days, particularly in more modestly
priced gear is really pretty sorry in this respect.



IMD should be expressed referencing a single tone of a two tone test
signal. That is the way most tube manufacturers do it.

Some of the radio manufacturers reference IMD to PEP which makes it
look like the IMD is 6 db further down!

The PEP of a transmitter is 6 db higher than either tone of a two tone
signal.
If the IMD is 30 db below the level of one tone of a two tone signal
then it will be 36 db below PEP.

73
Gary K4FMX

IMD should be expressed referencing a single tone of a two tone test
signal. That is the way most tube manufacturers do it.

Some of the radio manufacturers reference IMD to PEP which makes it
look like the IMD is 6 db further down!

The PEP of a transmitter is 6 db higher than either tone of a two tone
signal.
If the IMD is 30 db below the level of one tone of a two tone signal
then it will be 36 db below PEP.

73
Gary K4FMX

Poorly expressed on my part. Thanks for the correction.

W4ZCB