EMRFD Message Archive 3493

Message Date From Subject
3493 2009-08-22 19:30:04 iq_rx A Good Old VFO
For several critical receiver applications in my lab I've used old Collins PTOs converted to solid state (I just replace the triode in the classic Hartley circuit with a J310 and run the circuit from a 9 volt regulator). I have half a dozen of them in dedicated propagation study receivers, and one SSB exciter I occasionally use on UHF. The other day I was changing something else in one of my receivers and connected the solid-state PTO to the frequency counter on my bench. The PTO was set to 3.100000 MHz. From a cold start (it hadn't been turned on for years) it drifted three Hz over the first ten minutes, and then a total of 10 Hz over the next few hours. When I calibrated one of my 144 MHz propagation study receivers 25 years ago, total frequency drift was <18Hz/hour. I expect most of that was aging of the overtone crystal oscillator in the premix circuit.

Old Collins PTOs are common (someone at Dayton this year had a box of unknown ones in decent shape for $10 each, and there are R390 PTOs in the current Fair Radio flyer). I've never had one fail, tuning resolution is infinite, phase noise is low, digital noise is zero, and once I build one into a receiver, that part of the project is done--no improvements, software upgrades, needed.

My research receivers are connected to a baseband Fourier analyzer (yes...even 25 years ago). The finest resolution I've used for serious experiments is 10 milliHertz, but more often I use 1 Hz resolution, with 1024 channels in the output spectrum. I often average spectra for more than a minute, so frequency drift needs to be less than 1 Hz per minute. The solid-state Collins PTO is much more stable than needed even for those critical experiments.

This is not a fluke. Every Collins PTO I've converted to solid state using a U310 or J310 has had similar performance.

Sometimes it is useful to remember that the major benefit of digital frequency synthesis is that it is quick, cheap, and frequency agile. No commercial manufacturer could afford to build a transceiver with a Collins Mil-Spec PTO in it these days. But for an amateur with mechanical skills or access to surplus hardware who needs just one good oscillator, the venerable Hartley with a temperature compensated tuned circuit and a JFET can provide outstanding performance.

In music, art, architecture, automobiles, motorcycles... there are recognized "golden eras" where some combination of factors resulted in technical hardware that is widely recognized as being superior to what is being produced today. Often the difference is directly related to the amount of skilled labor needed during production. As technical hobbyists, we automatically assume that new is better, but as experimenters, we should be open to the idea that sometimes the technology, ideas, and block diagrams of an earlier era are superior to the cost-driven disposable technology coming off fully automatic assembly lines in some out-of-the-way place with inexpensive labor and attractive business tax codes.

The idea that old technology designed decades ago by retired guys might be better than new technology is a radical concept in electronics. But NASA is using a brand new, hand built, Traveling Wave vacuum tube in the current Moon exploration mission. After 100 years of radio experiments--it is fun to look back and find old technology that might actually work better than some of the new things we've been inventing recently.

Best Regards,

Rick KK7B
3494 2009-08-23 09:15:30 Ashhar Farhan Re: A Good Old VFO
wow. an incredibly insightful post. i guess that sufficiently advanced
technology becomes art at some point. at that time, individual
craftsmanship and sheer personal effort leads to far better
performance.

i am an enthusiastic student of free running oscillators. and my PTO
results have been far from satisfactory. can some one explain what
goes into building a stable PTO like this? there is always the
WA6OTP's famous kit to play with. But what are the things to consider
in building a super stable oscillator? Surely, they do use some
capacitors which can also be the limiting case for temperature
stability.

- farhan


3496 2009-08-24 06:45:13 timshoppa Re: A Good Old VFO
3497 2009-08-25 05:06:28 bubnikv Re: A Good Old VFO
Dan Tayloe designed nice PTO for his NorCal2030.

http://www.qslnet.de/member/df7tv/nc2030_pres_2004_10.pdf
page 24:

Nylon Kevlon exhibited linear frequency vs. temp curve
• Easy to frequency compensate using N220 type caps
• After compensation, test PTO drifted 73 Hz over a 63 F
temperature change

Full manual for NorCal2030:
http://www.norcalqrp.org/files/NC2030/NC2030_v5.pdf

My father OK1HBD built a PTO using micrometer like on the top picture from Wikipedia some 30 years ago in cold era Eastern Europe. Apart from WWII military radios there was nothing to buy, one had to build his rig.
http://en.wikipedia.org/wiki/File:Micrometers.jpg

I think he filed away the horseshoe shaped part and epoxied a piece of ferrite stick on the screw, but a piece of brass would work as well.

73, Vojtech OK1IAK, AB2ZA
3499 2009-08-27 17:39:52 w4zcb Re: A Good Old VFO
> But what are the things to consider
> in building a super stable oscillator? Surely, they do use some
> capacitors which can also be the limiting case for temperature
> stability.

Temperature stability is somewhat but not completely independent of
the mechanics for a linear-tuning broad-range PTO. EMRFD has some good
examples of temperature compensation in the real world. These
techniques have to be applied whether it's a
brass-screw-in-a-plastic-spacer $1 VFO or a PTO with hundreds and
hundreds of precise machined and sheet metal pieces.

Tim N3QE

A little tardy, been off to FL for a week visiting an old friend.

We built the Signal One with a 1 MHz PTO. (3.1-4.1 MHz). Rolled
threads on a phosphor bronze shaft and log spaced turns on the ceramic
coil form. I was the congenital idiot who mandated the 1 MHz coverage,
it was to serve as a tuneable IF for VHF/UHF converters. I doubt we
ever built one for under $100 in 1970 dollars and the radio demanded
TWO of the things to accomodate the second receiver. All this AFTER
having purchased an old Drake VFO to reverse engineer it. (It had such
niceties as a 1/4 inch shaft coming out a 3/8 inch hole in the
aluminum wall with a spring to pull the shaft up tight against one
side of the hole) I suspect Bob Drake had assigned something like $10
to it's construction.

At any rate, the 1 MHz bandwidth was a killer. That and incorporation
of the first frequency counter in an amateur transceiver, (Excuse me,
an "integrated station") so the new owner could measure the drift. The
problem was that if you temperature compensated the PTO at midrange,
it was overcompensated at one end of the range and undercompensated at
the other. A remarkable place to stuff a good Huff n puff stabilizer
and forget the temperature compensation. MUCH nicer than a PLL, they
add almost zero noise to the LC PTO.

W4ZCB
3500 2009-08-28 08:03:29 Roelof Bakker Multiple output power splitter
Hello all,

I should like to feed 4 receivers from one antenna.
To present a proper 50 ohm source to feed the receivers, a power splitter
seems to be a good idea. It will be precedid by a broad band amplifier to
compensate the losses in the power splitter.

Minicircuits sells 1 to 4 port power splitters and I happen to have a
PSC-4-5. The PSC-4-5 is specified for a frequency range of 1 - 800 MHz.
However, I need a splitter for 50 kHz to 30 MHz.
Minicircuits carries the PSC-4-6 , 10 kHz to 40 MHz, which should be ideal
for this application. However I have not been able to source one in
Europe.

I have succesfully been building the hybrid combiner / power splitter from
EMRFD/SSD in the past, so it might be sensible to roll my own 1 to 4 port
device. Unfortunaly information on the topology seems hard to find.

Can anyone tell me how to build a passive 1 to 4 port splitter or guide me
to the right information?

If someone knows a source for the PSC-4-6 that should be nice, in case the
project is less succesfull as I hope.

Thank you in advance.

Roelof Bakker, pa0rdt
Middelburg, Netherlands
3501 2009-08-28 08:28:34 w4zcb Re: Multiple output power splitter
I have succesfully been building the hybrid combiner / power splitter
from
EMRFD/SSD in the past, so it might be sensible to roll my own 1 to 4
port
device. Unfortunaly information on the topology seems hard to find.

Can anyone tell me how to build a passive 1 to 4 port splitter or
guide me
to the right information?

If someone knows a source for the PSC-4-6 that should be nice, in case
the
project is less succesfull as I hope.

Thank you in advance.

Roelof Bakker, pa0rdt
Middelburg, Netherlands

Ah Roel, today is your unlucky day. Instead of having the pleasure of
building your own, I just happen to have a North Hills 4 way splitter
good for 0.2-110 MHz. BNC's and about 1 x 2 x 2 inches. Yours for an
address. Good on QRZ.com?

Regards
W4ZCB
3502 2009-08-28 08:50:12 Roelof Bakker Re: Multiple output power splitter
Thanks a lot for your kind offer.
It is much appreciated.

The address is ok on QRZ.com.

I will still try to roll my own, but now I have something to compare!

Best regards,

Roelof, pa0rdt
3503 2009-08-28 08:53:31 w4zcb Re: Multiple output power splitter
The address is ok on QRZ.com.

I will still try to roll my own, but now I have something to compare!

Best regards,

Roelof, pa0rdt

Out of here today Roelof

W4ZCB
3504 2009-08-28 09:42:30 Steve Ratzlaff Re: Multiple output power splitter
A good homebrew article at
http://www.dxing.info/equipment/rolling_your_own_bryant.dx
Steve
----- Original Message -----
3505 2009-08-28 11:02:39 Roelof Bakker Re: Multiple output power splitter
Thank you very much indeed.

I will let you know when the splitter has arrived.

73,
Roelof
3507 2009-08-30 08:52:20 fjch100 Re: Multiple output power splitter
if you can read German, DG0SA has a web site, he same some power splitters ( 100W) and also has a PDF with some Theory....

http://www.wolfgang-wippermann.de/





3508 2009-08-30 10:25:20 Roelof Bakker Re: Multiple output power splitter
Yes, I can.

Our country is so small that to please the neighbours, I had to learn
German, French and English at school.

Thank you for the link!

73,
Roelof Bakker, pa0rdt