EMRFD Message Archive 1819

Message Date From Subject
1819 2008-07-19 09:32:19 bkopski A HB 100 mHz RF Sweep Generator
Hi All -

I have been trying "off and on" for several years to build a good-
working wide band RF sweep generator and only recently have achieved
reasonable success. Much of this is traceable to lessons learned
from "the book". Several posted photos show some assembly and
performance details of this HB instrument. It is built with readily
available parts and supplies and housed in a standard 2 1/8" X 3" X 5
1/4" minibox.

The assembly shown is the "RF part" of my sweeper-system. It is
based on the classic heterodyne technique. A separate utility
voltage ramp generator – which I use for a variety of purposes on the
bench – provides the VCO sweep drive signal input. A ramp of about
2.5 to 9.5 volts is needed to cover the full frequency range.

Power to the RF assy. is 15 VDC at less than 200 mA. There are two
50 Ohm outputs with one being a nominal 0 dBm source and the second
being a -6 dBm "monitor" port for use with a counter or similar. A
manual control provides > +/- 3dB level adjustment for both. Another
manual control allows a CW output setting over the < 1 mHz to > 100
mHz range in the absence of a ramp generator input.

The block diagram is built over both sides of a pc chassis.
The "bottom" side holds an SBL-1 mixer. The LO port is driven by a
similarly located POS-400 VCO, and the rf port is driven by a 210 mHz
free running oscillator. There are pads and a 220 mHz LP filter
between the oscillator and the mixer.

The pins of the two MiniCircuits components emerge on the "top side"
of the board for wiring. The mixer i.f. output drives 4 stages of
wide band amplification wherein the last two are "parallel sourced"
by stage two to provide the two outputs. There is a 120 mHz LP
between stages 1 and 2.

The sweeper has a leveling loop that detects the level at the main
output amplifier and then controls the 210 mHz oscillator bias
current to adjust its power level so as to maintain the desired
output level. This scheme works very well with only minimal
frequency pulling. The manual level control establishes the
reference power level. There is a discrete 51 ohm resistor between
the leveled amplifier output and the output connector which sets the
output impedance.

Two posted photos show some performance aspects. One shows output
signal purity within the nominal 100 mHz span of a 0 dBm level with
any other "stuff" at better than -50 dBc. The other photo shows
output flatness over the amplitude adjustment range and over the full
frequency span as being < +/- 0.5 dB. In actuality, the output slope
shown is largely in the measurement system and in fact as best I can
determine the sweeper output is less than half the motion shown.

There is one application photo which is an overlayed two level sweep
display of a nominal 20+ mHz 5 element 0.2 dB Cheby LP filter as
designed with the aid of Wes' "LowHi" program – check out the
displayed ripple! The detector on the filter output is my (QEX)
version of the QST 8307 power meter as described elsewhere in earlier
postings. (Other recent postings detail the wide band amplifier stage
(s) as they were being developed.)

This generator works so well that I'm reconsidering repackaging in a
larger enclosure with a self contained ramp drive circuit. On the
other hand, so much was learned in this very long pursuit that I'm
now considering trying for a much-harder 200 mHz sweeper design
instead – for the challenging fun of it.

So much to do, so little time to do it!

1821 2008-07-20 14:01:34 victorkoren Re: A HB 100 mHz RF Sweep Generator
Bob, Today isn't it much easier just to use a DDS? it has almost
perfect amplitude, sweep linearity, spurs level (at least for last
generations of DDS like AD9912) and you can even generate the sweep
voltage with a DAC.
1822 2008-07-21 07:15:19 rotfunkblau Re: A HB 100 mHz RF Sweep Generator
... hmm at least for european buyers there are two affordable DDS
based network analyzer/signal generator/power meter:

a) www.dl1alt.de up to 500Mhz price 240.- Euro
b) www.funkamateur.de up to 160Mhz, price 165.- Euro

sorry that I don't know any US sources for comparable prices. Very,
very useful tools.

73 de Guenter

1823 2008-07-21 09:48:32 Kenneth Stringham Re: A HB 100 mHz RF Sweep Generator
I would like to point out that the DDS does not have perfectly flat amplitude response over its entire range of useful output. It has a SIN(x)/x type response. This very predictable, but not perfect. Once easily compensate for this fact because of the predictability of the amplitude, or one could use it for the high level drive to a mixer where the amplitude change will be minimized.
The comment about the DAC is  a bit strange unless you are referring to use the DAC to generate the ramp. Then this would be a perfect application of the DAC. You would only need a counter with a variable input rate to change the rate of the ramp.
In addition, I believe there is a version of the DDS that has the counter built in of just this type of application. All you would need to input the start and end frequencies and sent the execute command.
One, also, must remember that the designer clearly indicated what his intent was with this project. It seems to me that he has achieved his design goal using his initial conditions. He should be commended for his achievement and we should realize that this is just what this forum is about. One should endeavor to develop the type of project he/she wants using whatever technology is at hand. If what this designer has done generates some ideas in your own mind, then get to designing your own version and share it with this group. Please include those steps that worked and those that didn't so we can all benefit from your work.
Ken - AE1X

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1824 2008-07-21 12:39:48 Gary Johnson Re: A HB 100 mHz RF Sweep Generator
Congratulations to Bob on a great addition to the EMRFD collection of useful tools. This
oscillator design is something almost anyone can assemble at low cost, using building blocks
that are easy to understand and modify. Low parts count, nothing very critical, no software
required, and... no spurious output. I like it. By the way, Bob, that's some of the neatest
"ugly" construction I've seen.

Gary, WB9JPS
1827 2008-07-22 12:08:52 john sakellakis Re: A HB 100 mHz RF Sweep Generator
Where can we find details of your sweep generator..schematic etc ?

Regards & 73

John����� W8GXU

1828 2008-07-22 12:12:40 bob bailey Re: A HB 100 mHz RF Sweep Generator
yes Bob, lest get this info posted so we all can build one. and a etching
pattern for a pc board would be nice too, (just kidding).