EMRFD Message Archive 1695
Message Date From Subject 1695 2008-06-05 21:33:19 ve7ca2 Analog VFO Freq. Stablization In EMRFD, Wes wrote about the subject of analog VFO temperature
Last year I rebuilt the VFO used in my homebrew HBR2000 to include two
500 khz ranges selectable with a latching relay. The latching relay
failed (?) so I decided to try to improve the VFO drift situation
while I had the VFO apart. Not that the drift was bad, +or- 20Hz once
the VFO warmed up, but I was sure it could be improved. With no
excuse not to address the drift situation I built a temp. sensor with
a LM335 and brought out an old Coleman picnic cooler for the heat
chamber in which I built a shelf, included a bulb with an on/off
switch, ( I used a 40 Watt bulb for a slower temp. rise) and a fan.
Wes describes the use of a temperature chamber (or warming oven) on
page 4.5-4.6 and also 7.42 Also QST Dec 1993 page 37
After several runs with the VFO in the chamber and running the
formula's in EMRFD I determined that if I replaced an existing 50PF
N150 with a 68pf N150 cap I should be able to nail the drift right on.
I couldn't find one. No one stocks polystrene caps around here
either. I tried combining two caps in series like a 100pf N150 and a
220pf NPO and a couple other combinations but to no avail. I then
remembered that I was given a box of parts when my friend VE7YQ passed
away several years ago. I went to the attic and found the box and
began sorting though the stuff and found a box of ceramic caps. It
took a while but I found a 68pf N150 in perfect condition, never used
before and I suppose 50 years old at least. I popped it in the VFO,
let it stabilize over night and the next morning I measured a +3.2
ppmC. Wow, I couldn't believe it so I ran another test later that
same day and got a similar result. I have since put the VFO back into
the rig. Monitoring the VFO ouput using my HP8640 counter, the VFO is
stable to +or- 2.5 Hz. Not bad for an analog VFO using a WWII tuning
cap. and other assorted old parts.
I was so encouraged by the results that I decided to attack a used
Kenwood VFO model 820 unit that I had purchased on eBay for $20.
Beautiful reduction gear and dial. (it didn't work but was easily
fixed). I am using it for a 6 meter stand alone transceiver that I am
building however the warm up drift was excessive and it kept drifting
even after warming up for 30 mins. In the chamber with an 10 degree C
increase in temp it drifted 947 Hz. After a couple of runs in the
chamber and replacing a couple of key capacitors, I was able to reduce
the drift, after 20 minute warm up in the chamber then increasing the
temp. 10 deg C to 46 Hz.
So my friends, don't be afraid to be aggressive with a drifting VFO.
If I can tame one, you can without doubt.
Web Site: http://www.shelbrook.com/~ve7ca/
1696 2008-06-07 10:58:20 Rick Re: Analog VFO Freq. Stablization Hi Markus,
Thank you for the description of those 2 projects. I have also used Wes's technique to
study the drift of my own analog VFOs and then improve them, with similar results. I was
pleased to see that you were able to improve a commercial VFO as well. Hams often
assume that commercial gear is already optimized--but such things can always be
improved by someone with the right test equipment who understands the procedure.
In my case the right test equipment was already available. I used my little remote
thermometer with a sensor on the end of a wire inside a styrofoam cooler, and stuck the
clip-on lamp from over the bench in the cooler to supply the heat. I also tried one of
those blue freezer packs from the freezer compartment to cool it down. So I didn't build
or buy anything to put together the test equipment. Then I spent a few evenings
measuring and keeping notes--while scrounging through the parts drawers for an
assortment of NP0 and N150 capacitors.
My results were similar to yours--I started with a few tens of Hz drift and was able to
reduce it to a few Hz per hour. That is less than the drift of many synthesized radios that
lock to an uncompensated crystal oscillator.
Thanks for writing this up--it's a great technique that should be used more often.
1701 2008-06-12 07:02:08 ve7ca2 Re: Analog VFO Freq. Stablization and other thoughs Hi Rick. I am surprised how much "lore" there is bouncing around on
the WEB about analogue VFO"s and how bad they are. Most of what I
have read is just a bunch of bunk. Doug, W1FB (now a SK) and Wes,
W7ZOI used high quality test equipment to quantify VFO drift and then
came up with some very good recommendations as to how to select
components build a stable a VFO. Then Wes, showed us how to measure
drift with very simple test equipment and a warming oven, then how to
employee different temp. compensations to control drift. And
following these principals you can make, as you agree, a very stable
VFO, with the added benefit that they typically have very low phase noise!
These days, I am measuring and making more notes than I ever did
before as I build different kinds of ham equipment. Being I am not
an electrical engineer, I learn from others who have already gone down
the road I am heading. What I have learned is, 1. build in modules,
2. use tested circuits that have been built by someone you can trust,
3, first, build the circuit exactly as the author built it, 4. then
test and measure it. 4. make really good notes, and 5, last but not
least if you think you can improve the circuit, go ahead but at least
you know where you are coming from. Works for me.
This EMRFD Yahoo group is fantastic. I really appreciate all the
input from the different individuals who are willing to take a few
moments and share their experiments, their success and failures too.
We learn so much from each other.
Enough rambling. Happy homebrewing fellows,
1722 2008-06-18 04:16:13 Glen Leinweber Analog VFO Freq. Stablization Have programmed a PIC to do the frequency measurement and temperature
measurement part of EMRFD's "oven".
Temperature runs can take awhile.
You want to change temperature slowly to let all parts of the VFO stabilize.
So jotting down frequency counter readings over a long time span is a pain.
This PIC sends frequency, temperature over a serial port to a PC, which
collects readings into a text file. The text file is suitable to import
With no display, the frequency counter is dead simple, less than $10
And for temperature, a 20-cent surface-mount thermistor works very well.
The PIC's own analog-to-digital converter measures its voltage, using a
series dropping resistor from the +5v supply. A linearizing equation
voltage to degrees Celcius, with 0.1 degree resolution.
Frequency is measured over a ten-second period, giving resolution of
0.1 Hz. These PICs can count up to at least 100 MHz., if you set up the
hardware correctly. A head-end preamp inside the oven is a good idea,
so that VFO loading is light - this also allows the actual PIC counter to
be external so that oven temperature doesn't cause counter drift.
Haven't finished doc's completely , but have schematics at:
The PIC code isn't there, but intend to make it public domain, so e-mail
me if you want a quick copy of code to play with.
look for "EMRFD oven temperature, frequency instrument"
under the "TEST & MEASUREMENT EQUIPMENT" heading.
1724 2008-06-18 07:05:22 Johan H. Bodin Efficient amp's (was: re:Analog VFO Freq. Stablization) Glen Leinweber wrote:
> Haven't finished doc's completely , but have schematics at:Hi Glen,
I just read about "waveform sculpting" on your page
http://epic.mcmaster.ca/~elmer101/effamp/effamp.html and it
reminded me of this interesting paper:
maybe you have seen it before...