[Amps] Designing the Cleanest Linear with RF Negative Feedback
Karl-Arne Markström
sm0aom at telia.com
Sat Dec 17 06:11:15 EST 2005
In theory, you can measure transmitter IMD at the receiver output,
but in the real world this also results in influences of the receiver in-band
IMD characteristics.
Only few HF receivers are specified for in-band IMD performance, and
those who are usually have a rating of - 50 dB or so. AGC characteristics
also have a profound influence on this performance.
This means that transmitter IMD performance better than about - 45 dB
relative to one tone begins to be masked by receiver in-band IMD.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "Bill Fuqua" <wlfuqu00 at uky.edu>
To: <garyschafer at comcast.net>; "Tom Cathey" <K1JJ at comcast.net>
Cc: <amps at contesting.com>
Sent: Saturday, December 17, 2005 12:05 AM
Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
>
> Better yet, if the two tones are only a hundred Hertz or so apart, you can
> take the audio output of the SSB receiver and put it into the sound card of
> your computer and do an FFT. You can use a number of off the internet
> freeware FFT programs. In fact if you use "Spectrum Lab" (see
> <www.qsl.net/dl4yhf> ) to generate the tones and display the received
> spectra at the same time. Naturally you have to have a separate receiver
> and transmitter to do this.
> The receiver must be in SSB mode and with 100 Hz difference in the two
> tones you should be able to display the two tones and their IMD products
> within the 2 or so kHz bandwidth of the receiver. Their relative amplitudes
> will be easy to observe and measure.
> No need for calibrated attenuators or S meter readings. Just don't
> over drive the receiver or sound card input.
>
> 73
> Bill wa4lav
>
>
> At 04:59 PM 12/16/2005 -0500, Gary Schafer wrote:
> >Hi Tom,
> >
> >You do have a spectrum analyzer in your shack! Your receiver.
> >Modulate the transmitter with 2 tones and tune across with your cw
> >filter on another receiver. Note the level of one of the tones on the S
> >meter. Then tune to the 3rd order product and note the level difference.
> >Same thing a spectrum analyzer does.
> >If you want to get real accurate put a step attenuator in front of the
> >receiver so you don't depend on S meter calibration.
> >Now you will have a base to work from.
> >
> >73
> >Gary K4FMX
> >
> >
> >Tom Cathey wrote:
> > > That's more valuable info, Marv - Tnx again!
> > >
> > > A few follow up questions:
> > >
> > > If I tap off the FT-1000D 10mW low level point, can I run a long ~ 20'
> > coax
> > > cable to the amplifier CA2XX module's input, or will this cause problems?
> > > This is for 75M only. Maybe there is a way to do this.
> > >
> > > I looked at the FT-1000D's circuit that puts out 10mW, just before it goes
> > > into the power amp board. I'm trying to figure if it's possibly as clean
> > > as -55db there. I don't have a spec analyzer. It uses all pnp
> > transistors -
> > > a 2SC2026 base driven, driving another 2CS2026 in emitter follower,
> > driving
> > > a 2CS1973 in emitter follower at 10mW out. There's much more stuff
> > involved,
> > > like the balanced modulator, etc. Is this enough info to make a guess
> > from
> > > your experience of what kind of IMD we are dealing with at this
> > point? This
> > > will have a big effect on what direction I take, of course. Or maybe I
> > > could sample it into a receiver and get an idea using the same relative
> > IMD
> > > procedures I use for a big amp.
> > >
> > > OK on the sample amplifier using a 6146 input running reduced voltages and
> > > 1W, low power for cleanliness. Guess NFB will not help there.
> > >
> > > Last question: Let's say I did put two 4CX-350's in cascade, so had
> > lots of
> > > gain to work with. What is the practical limit for conventional negative
> > > feedback? Is it a matter of running into instability, perhaps? And I
> > take
> > > it from your comments, that you would run feedback from the final to the
> > > predriver, [two stages at a time only] and then where would you run the
> > > second loop to cover the pre-pre driver 4CX-350 and the input 6146, for
> > > example?
> > >
> > > 73,
> > > Tom, K1JJ
> > >
> > > ----------------------------------------
> > >
> > > Hi Tom,
> > > That '5106 is probably for Cable TV use and likely cuts off around
> > 40 MHz
> > > (on the low end!).
> > >
> > > If I recall correctly, some of the general purpose units that go
> > down to
> > > a MHz use numbers from CA28XX group. They come in both single ended and
> > > push-pull versions.
> > >
> > > I've seen a few of the 350J's for sale on the web over the years. I
> > > don't believe the demand is very high for those as they have a 26V heater
> > > and as such can't be dropped into a 4CX250B socket. There is also the
> > > 4CX600J/JA/JB.
> > >
> > > The 6146 shown on the Hughes schematic is run deep into Class
> > A. Note the
> > > low screen & plate voltages. I bet the numbers were just fine running all
> > > of a watt output.
> > >
> > > For feed forward, an error amp with perhaps another 4CX350FJ would be
> > > necessary. It is not a difficult scheme to implement but, it would double
> > > the parts count for the project. If you read about the technique on the
> > > web, keep in mind that "they" are typically working with transistor amps
> > > that start with distortion numbers 20dB worse than tubes. Therefore, in
> > > this case, less correction power will be required, the output combiner
> > ratio
> > > will optimally be a bit higher, and power lost from the main amp will be
> > > lower.
> > >
> > > The subject line said you wanted "the Cleanest Linear". To avoid
> > > degrading the system, the driver should exhibit distortion specs at least
> > > 10dB better than the amplifier. Your FT-1K will still be useful for the
> > > receiver.
> > >
> > > 73 & Good morning,
> > > Marv WC6W
> > >
> > >
> > >
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> > >
> > >
> >
> >
> >
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