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AF7V
11-08-2006, 09:18 PM
Can a signal generator be used as a VFO in transmitters and receivers? What would be the caveats in an application like this?

The particular rig I'm wondering about is a Heathkit DX-40, which primarily used crystal control, but has connections for an external VFO.

Bert

WB2WIK
11-08-2006, 09:21 PM
Assuming it's very stable like a real amateur VFO should be and can provide the correct frequency and signal amplitude required by the transmitter, of course this will work.

One "caveat" is you will likely need a way to key the VFO (signal generator) "on" when transmitting and "off" when receiving, otherwise radiation from the connecting cable and wiring inside the DX-40 is likely to be very audible in your receiver, and mask signals you're trying to hear.

WB2WIK/6

VK2TIL
11-08-2006, 11:18 PM
My generators serve very well as the LO in direct-conversion experiments.

These use diode mixers and usually require 7-10dBm power levels.

Someone who knows the DX-40 might be able to tell you what level of VFO injection is required. Possibly 10-20 dBm but I'm guessing.

Most "good" generators output about 10 dBm; sometimes 13.

The cheaper kinds have much lower outputs; a Leader that I have only puts-out minus 2 to minus 5 dBm.

Its output is also not very "clean" and its shielding is poor.

W8ZNX
11-08-2006, 11:48 PM
most sig gens
will not have enought output
to excite a DX 40
will need to put in a buffer/amp stage

90 % of the older sig gen
will have a very crude dial / freq read out
have a nice older sig gen
great for working on old gear
BUT it drifts like crazy

# # # # # # # # # # # # AND

40 meter band on the dial covers about this much space

# # # # # # # # # # # #< # # # >


some high end lab sig gen
are a diffrent world
but you may still need to
build a amp/buffer stage to get enought drive
for a tube style older transmitter

for some older tube xmitters
have used a beat up old
Ten Tec 509 Argonaut as a vfo
depending on the transmitter
capacitive couple it to osc or buffer stage

can turn the drive up and down
right on the argo

good luck

Mac

W9GB
11-09-2006, 02:40 AM
Bert -

The Heathkit HG-10 often appears on eBay and is the matching VFO. #I just ran a quick search and came up with 78 hits on VFO -- some could be used. #You desire 80 through 10 meters? #or just a couple of bands?

IF you only wanted one band, like 80 or 40 meters -- you could easily rework this cheap Siltronix Model 80-1 unit from K5LW on eBay -- item number: 250046313606

This unit is solid-state, has a nice dial (a computer graphics program could customize for your frequency range) and the price is right !
You could use Doug DeMaw's (sk) W1FB Design Notebook or W1FB QRP Notes ...
to rework the coil(s) to your desired frequency. You might change the variable capacitor (~ 100 ) - depending on frequency spread you desired. #Nice winter project.

w9gb

This is the type of project that disappeared from QST in the 1990s, largely since Doug's departure. #There are hundreds of these units being trashed -- that can easily be reworked with a nice DIY article for newcomers to the hobby and novice builders.

K4KYV
11-09-2006, 02:56 AM
Quote[/b] (w9gb @ Nov. 08 2006,19:40)]This is the type of project that disappeared from QST in the 1990s, largely since Doug's departure.
They moved real technical articles from QST to QEX. Now you have to pay extra for a subscription even if you are a full paid member. QST has pretty much become a worthless rag; takes me less time to go through a newly arrived issue of QST than it takes me to go through our local small town newspaper every morning.

W9GB
11-09-2006, 03:02 AM
Quote[/b] ]They moved real technical articles from QST to QEX.
I read Ham Radio through the 1970s and consider Jim Fisk, Doug DeMaw, Irwin Math and Joe Carr to be my monthly DIY instructors throughout that decade.

An article of this type would be considered "too basic" for QEX (which I also read). #
The Internet and DIY TV are the real competitors for QST. #I am waiting for another company to take a run at producing a TV series on cable focused on electronics (robotics, car/mobile audio & devices, antique radio/boatanchors, basic electric, RF/radio, computer/network/WiFi/wireless) -- SciFi and CNet played around with the idea in the 1990s -- but not advertised/ business focused -- remember Michael "Mike" Jerrick (now in NY on Fox's Dayside) and SciFi Buzz in the mid-1990s -- it will happen eventually.

N0WVA
11-09-2006, 03:06 AM
If you use it for AM work, Id say yes......you would fit right in with the rest of the drifty AM rigs.

WB2WIK
11-09-2006, 04:31 PM
Quote[/b] (N0WVA @ Nov. 08 2006,20:06)]If you use it for AM work, Id say yes......you would fit right in with the rest of the drifty AM rigs.
Depends on the signal generator.

My standard lab generator at home is a 30 year-old HP 8640B with a TCXO time base. Its "drift" is less than modern ham gear vintage 2006, at about 0.3 ppm/H, and of course it is a phase-locking generator so its output signal has the same stability as its PLL reference oscillator.

These are available today for less than $350, usually -- simply because they're "old."

"Amateur" or hobbyist generators might well drift, and also have hum or lots of other problems...

WB2WIK/6

SM0AOM
11-09-2006, 04:54 PM
This works beautifully if a few precautions are taken.

First, some tube transmitters use a crystal oscillator circuit that may "take off" on an unrelated frequency if the crystal is replaced with an external signal source. The self-oscillation may destroy delicate output stages in i.a. the HP8640.

Therefore, always use a buffer stage between a solid-state signal generator and a tube transmitter.

Second, some signal generators may pick up RF by the power cord and
generate a keying "chirp" when the power supply voltages become unstable with keying.

I have used this approach several times to "excite" tube AM and CW transmitters, using the HP606B + 8708 synchronizer, Schomandl, Rohde & Schwarz and Adret decade oscillators and of course the HP8640B.

If you look closely on pictures of broadcast AM HF transmitters, you
sometimes can spot a decade oscillator or signal generator used as its carrier source.

73/

Karl-Arne
SM0AOM

AG3Y
11-09-2006, 05:05 PM
The broadcast transmitters I was familiar with all used a crystal-based design, with the crystal timebase installed in a temperature controlled oven.

I would NOT trust a variable frequency oscillator, no matter how good it was, in a broadcast situation. It would be too easy to bump a knob !

I'm betting that the oscillators or signal generators were used to produce test waveforms, etc. but NOT in the main timebase. But of course, I could be wrong !

73, Jim

AF7V
11-09-2006, 05:12 PM
The signal generator was given to me by a local ham. Someone gave it to him and so he was passing it on to me. I don't know what the drift is like yet, since I haven't had a chance to use it. #I haven't been able to find much information on the Bay State Electronics SG-132 online either.

My question was mostly academic, since right now I'm waiting on parts to recap the transmitter and also studying for the General exam in two weeks. After that I still have to get grounding and antennas up before I can get on the air.

I was pretty sure that it could be used. Still, I had the nagging question of, if they could be used, why isn't it more commonly used. Some of the replies to the thread have put this into better perspective for me. Thanks.

Bert

AF7V
11-09-2006, 05:31 PM
Quote[/b] (w9gb @ Nov. 08 2006,19:40)]The Heathkit HG-10 often appears on eBay and is the matching VFO. #I just ran a quick search and came up with 78 hits on VFO -- some could be used. #You desire 80 through 10 meters? #or just a couple of bands?
I though the right VFO, was the VF-1, and that was going to be a bit more difficult to aquire. As I mentioned above, the signal generator was a gift, so I was wondering about using it.


Quote[/b] (w9gb @ Nov. 08 2006,19:40)]This is the type of project that disappeared from QST in the 1990s, largely since Doug's departure. #There are hundreds of these units being trashed -- that can easily be reworked with a nice DIY article for newcomers to the hobby and novice builders.
You're right on here. I got into ham radio to learn to build and understand the gear. The idea of being able to rework equipment really appeals to me. If Ham radio were to completely degrade to the plug and play for everyday consumers mentality, I honestly think I would have to devote my time to other hobbies.

WA2ZDY
11-09-2006, 05:43 PM
You're right. The VF-1 was the matching VFO for the DX20-35-40. The HG10 came along with the HW16 and DX60. VF-1's can be pricey as can most boatanchors. My first VFO was a VF-1 that I bought at a hamfest in April 1975 for $12. I still remember very clearly handing over my $$$ to the seller and being excited about being freed from my crystal on 7123.

I'm with Jim, I don't see a VFO/sig generator being used for BC purposes. But as I read Karl-Arne's post I assumed he meant hams using old BC transmitters. There are plenty of hams using BC rigs on 160, 75 and even 40 AM, and I've read that signal generators are commonly used as such.

I had a nice Boonton generator years ago. It finally died and the owner wanted it back. That thing had a digital readout, was accurate, clean and did not drift. And it ccould be amplitude and frequency modulated and sounded pretty fair.

WB2WIK
11-09-2006, 05:59 PM
Quote[/b] (WA2ZDY @ Nov. 09 2006,10:43)]I had a nice Boonton generator years ago. It finally died and the owner wanted it back. That thing had a digital readout, was accurate, clean and did not drift. And it ccould be amplitude and frequency modulated and sounded pretty fair.
That was probably the Boonton 102A, which was my project when I worked there in '73-'77.

I have one, too, and mine works fine. Of course the one I have is an engineering prototype and I mostly built it, myself, so I know everything that's in it. http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif

WB2WIK/6

WA2ZDY
11-09-2006, 06:52 PM
It was my ex-brother-in-law's, who got it from MY ex's ex-boyfriend who worked with you. Damned if I can remember his name, but my ex had mentioned having met you. That was sure a lot of years ago.

And yes, 102A sounds right. Sure wouldn't mind having it or an equivalent now. I actually used that thing to work the local 2m repeater before the days of CTCSS.

WB2WIK
11-09-2006, 07:49 PM
Chris, yeah I remember you telling me this before.

Wonder who it was? I don't know.

I worked with a lot of people there and had many friends because we used to socialize a lot after work...we had a traveling volleyball team and played other companies, etc.

We built about ten prototype 102As as I recall, and I have one...don't know where the other nine went.

73

Steve WB2WIK/6

W9GB
11-09-2006, 11:31 PM
Quote[/b] ]The right VFO for the DX-40, was the VF-1. It was going to be a bit more difficult to acquire.
As I mentioned above, the signal generator was a gift, so I was wondering about using it.

I did see a Heath VF-1 on eBay in past 2 weeks that would have needed a total rebuild (rebulid the kit from scratch - with new components - where required). It was very cheap - when I last saw it, I did not check to see its final sale price.

w9gb

K9STH
11-10-2006, 01:00 AM
The Heath VF-1 was definitely the most manufactured VFO ever made. Then came the HG-10 / HG-10B (only difference was the paint job). I still see VF-1 VFOs going for under $30 with the average being in the $40 to $45 range. Of course on eBay prices vary all over the place, especially if 2 or 3 people get into a "bidding war".

There were actually two versions of the domestic VF-1. Electrically they are identical but the front panels are different. Then there was the VF-1 that was manufactured by Daystrom in England. The tube line up is different and the cabinet is definitely different. However, the basic circuit is the same as the stateside VF-1.

I have both versions of the VF-1 that was made by Heath in St. Joseph, Michigan (mailing address was Benton Harbor but the actual plant was in St. Joseph). Somewhere between 10 percent and 15 percent were the earlier version (originally sold for use with the Heath AT-1 transmitter). Those have more "lettering" and graphics on the front panel and have a bronze "tint" to the paint on the panel. The later version have a "silver" front panel and considerably less graphics and somewhat less lettering.

A photo showing the different front panels is attached.

Glen, K9STH

N9LCD
11-10-2006, 01:16 AM
Another problem with older tube-type signal generators may be, dirty word, harmonics! Especially at high output levels.

With an old URM-25 generator cranked all the way up, I could tune the 20th and 25th harmonics, about 1 GHz, on an APR-4!

The reason: A lot of signal generators don't have tuned output stages or tuned output filters; EVERYTHING gets out!!!

Jerry
N9LCD

SM0AOM
11-10-2006, 02:40 PM
Quote[/b] (AG3Y @ Nov. 09 2006,18:05)]I'm betting that the oscillators or signal generators were used to produce test waveforms, etc. but NOT in the main timebase. #But of course, I could be wrong !
Well, in fact you are...

Take a look on the picture at the top of:
http://www.transmitter.be/bbc-sk55.html
at the top right hand corner.
There you can spot a Schomandl ND60M remote controllable decade synthesizer.

This is common practice in AM HF broadcasting, where one transmitter usually is required to cover many frequencies in different bands, using "time-table" control.

For decades "Radio Sweden International" shared the transmitter site used by the air-ground HF services that I consulted for.

Their transmitter was a 500 kW Thomson-CSF unit that used a Telefunken-Deutsche Aerospace decade signal generator as its RF source. It was controlled by a mechanical clock-timer arrangement that selected the proper frequency via a plug-board matrix.

73/

Karl-Arne
SM0AOM

WA7KKP
11-13-2006, 10:54 PM
Well, you need stability far in excess of most cheap sig generators, except the digitally programmed kind. VFO's are usually keyed with the transmitter, to prevent backwave being transmitted. This is when the VFO/signal source is radiated during key-up.

Now I remember articles in the old CQ's and QST's that used the BC221 frequency meter as a VFO. On the lower frequencies, they were quite accurate and stable, but they're not too common on the market today.

if I were you, I'd see about home-brewing a VFO, from one of the older Handbooks from the 60's or 70's. The VF-1 was the matching VFO for the DX40, but they were marginal in operation, and could be chirpy if not powered from a separate power supply, not the transmitter itself.

Gary WA7KKP

K9STH
11-14-2006, 12:15 AM
Actually, with the older equipment it is generally MUCH better to let the VFO run constantly during the transmit period rather than trying to key it. When you key the VFO most of the time a definite "chirp" happens.

As far as a "backwave" goes, this doesn't happen if the transmitter is cathode keyed or if the stages are properly biased to cutoff. It will happen if the final amplifier is not properly biased to cutoff when the key is in the "open" position (when running Class C). It can also happen when the final is used as a linear amplifier and the earlier stages are not properly biased to cut off. However, those are definitely problems which need to be addressed.

The fact that the VFO is running constantly when the transmitter is in the "transmit" position can make "off the air" monitoring difficult. That is when one's receiver is used to monitor the actual keying of the transmitter. This can be reduced or even eliminated by turning down the r.f. gain of the receiver during transmit. With some receivers a bias voltage can be applied to reduce the gain "automatically" when the transmitter is activated.

But, especially when an external VFO is used, the vast majority of the time the VFO is going to have to run constantly during the transmit period to avoid a definite "chirp". Of course as the VFO frequency is multiplied to a higher frequency band this "chirp" is going to become more and more a problem.

In the case of the VF-1 when used on 160 meters the "chirp" is often not that bad. But, when multiplied to 80 meters it becomes more pronounced. The same thing goes when operated on 40 meters using the 7 MHz output. The "chirp" on 40 meters is often not that bad. However, as the frequency is multiplied for 20, 15, and 10 meters the "chirp" gets worse and worse. The same thing as for other external VFOs, the higher the frequency the worse the "chirp".

The WRL 755 and 755A VFOs are among the best in avoiding "chirp" even when keyed. These employ a built-in buffer/amplifier which helps isolate the oscillator. However, even with those VFOs it is still better to let the oscillator run during transmit unless something is used to key the VFO before the transmitter and to allow it to run for a few microseconds after the transmitter stops transmitting.

Back when I first upgraded to General Class in 1959 I used a WRL Globe Chief 90A with a WRL 755 VFO. I bent the contacts on a 12 volt coil relay so that one set of contacts made before the other and those contacts stayed longer than the other. Those contacts were used to key the VFO and the other contacts keyed the transmitter. This allowed me to operate "break in" (I used a tube-type "TR switch" to electronically switch the antenna between transmit and receive). The 12 volts to the relay was actually keyed by my Vibroplex "bug". However, the "clacking" of the relay was annoying to anyone else in the room. Fortunately, my shack was in the basement away from everyone else.

If I keyed the VFO directly with the transmitter it definitely had a "chirp". Again, on 160 meters it wasn't that bad and the same with 40 meters. But, when the VFO was multiplied the "chirp" started to be really noticeable.

These days I have several different VFOs including Heath VF-1s and a WRL 755. When using transmitters that do not have a built-in VFO I definitely let the VFO run constantly to avoid any "chirp".

As for the VF-1, the stability, "chirp", etc., can be vastly improved by replacing the 6AU6 oscillator with a 6AH6 (direct replacement, no modifications needed). Also, replacing the 0A2 regulator with an 0B2 (which drops the screen voltage to 108 volts instead of 150 volts) and running the VFO from a regulated 150 volt source helps immensely. Neither tube change requires any rewiring. By using a regulated power supply the plate voltage on the oscillator tube doesn't change and the lower regulated voltage on the screen helps with the stability.

Heath changed the VFO tube in the TX-1 Apache from a 6AU6 to a 6AH6 to improve stability. This helped very much.

Glen, K9STH

WB7DMX
11-14-2006, 03:21 AM
then again, there is always the DDS type of vfo that would be much better, and a whole lot more accurate too.

I just finished building one that covers 0 to 20 mhz, and starting another one that covers 0 to 60 mhz.

AG3Y
11-14-2006, 03:38 AM
Quote[/b] (SM0AOM @ Nov. 10 2006,10:40)]
Quote[/b] (AG3Y @ Nov. 09 2006,18:05)]I'm betting that the oscillators or signal generators were used to produce test waveforms, etc. but NOT in the main timebase. But of course, I could be wrong !
Well, in fact you are...

Take a look on the picture at the top of:
http://www.transmitter.be/bbc-sk55.html
at the top right hand corner.
There you can spot a Schomandl ND60M remote controllable decade synthesizer.

This is common practice in AM HF broadcasting, where one transmitter usually is required to cover many frequencies in different bands, using "time-table" control.

For decades "Radio Sweden International" shared the transmitter site used by the air-ground HF services that I consulted for.

Their transmitter was a 500 kW Thomson-CSF unit that used a Telefunken-Deutsche Aerospace decade signal generator as its RF source. It was controlled by a mechanical clock-timer arrangement that selected the proper frequency via a plug-board matrix.

73/

Karl-Arne
SM0AOM
Very interesting web site Karl, but I was not thinking of international, frequency agile SWBC transmitters, but rather of just a fixed-tuned single channel rig as licensed by our Federal Communications Comission for use on one of our Broadcast Bands!

And you must admit that the Frequency synthesized frequency generator in that picture is a far step above your average VFO, even your average piece of test equipment that is liable to show up in a ham shack! http://www.qrz.com/iB_html/non-cgi/emoticons/wink.gif

73, Jim

KA5PIU
11-15-2006, 11:54 AM
Hello.

Some function generators can do cover very broad ranges and are very stable and clean, people just do not think of them as
signal generators.

SM0AOM
11-18-2006, 06:15 PM
Quote[/b] (AG3Y @ Nov. 14 2006,04:38)]Very interesting web site Karl, but I was not thinking of international, frequency agile SWBC transmitters, but rather of just a fixed-tuned single channel rig as licensed by our Federal Communications Comission for use on one of our Broadcast Bands!

And you must admit that the Frequency synthesized frequency generator in that picture is a far step above your average VFO, even your average piece of test equipment that is liable to show up in a ham shack! #http://www.qrz.com/iB_html/non-cgi/emoticons/wink.gif
I may have to remind the readers that there exist other countries in the world, that are not under FCC jurisdiction.

In these countries,even for fixed tuned MF broadcast transmitters, it has become common practice to use a synthesized oscillator as the carrier source.

One of the reasons is that the buyers of MF transmitters want to be prepared for coming digital broadcast modes, and the exciters that use frequency synthesis usually
are "upgradeable" to i.a. DRM or other digital emissions.

With respect to my "ham shack" it may not be average #http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif , but not only one, but several "decade oscillators" have followed me home over the years.

If one is seriously interested in weak-signal or narrow band communications, accurate frequency control or measurement, a "decade oscillator" controlled from an atomic or GPS frequency standard is almost indispensable.

As a side effect, a sub-Hertz accurate and drift-free frequency source for the "boatanchor" CW or AM rig is available.

A mention was made of the "function generators".

Many of these (HP3325 comes to my mind) have very good long-term stability and high resolution, but some may have noise sidebands that are considerably worse than signal generators.


73/

Karl-Arne
SM0AOM

VK6ZGO
11-21-2006, 02:06 PM
As someone said,it depends on the generator. Some have extremely good specs as well as accurately controlled output levels & as such may be #better than a homebrew VFO.
The question of #using a sig gen as a source for a Broadcast Tx is another question,( Synthesisers normally trace their accuracy back to a Crystal or better source.)
I have seen it done in an emergency with an ordinary Signal generator. (Existing Tx destroyed by fire, spare sent in a rush had wrong xtals) --Gen was an old Advance valve unit,but was quite stable -only used for about a day,then xtals arrived.--I don't think I'll get anyone into trouble, it was a long time ago --back in the 1960's.
I had occasion to use a Marconi unit back in 1967 which looked great,but was in fact ,just a VHF gen with HF added on as an afterthought. (They mixed another VHF osc with the main gen o/p & took the difference sig ) .problem was ,the second VHF osc wasn't ovened or well shielded,or basically stable,so the thing was useless for HF
73 vk6zgo

VK6ZGO
11-21-2006, 02:07 PM
As someone said,it depends on the generator. Some have extremely good specs as well as accurately controlled output levels & as such may be #better than a homebrew VFO.
The question of #using a sig gen as a source for a Broadcast Tx is another question, Synthesisers normally trace their accuracy back to a Crystal or better source.
I have seen it done in an emergency with an ordinary Signal generator. (Existing Tx destroyed by fire, spare sent in a rush had wrong xtals) --Gen was an old Advance valve unit,but was quite stable -only used for about a day,then xtals arrived.
I had occasion to use a Marconi unit back in 1967 which looked great,but was in fact ,just a VHF gen with HF added on as an afterthought. (They mixed another VHF osc with the main gen o/p & took the difference sig ) .problem was ,the second VHF osc wasn't ovened or well shielded,or basically stable,so the thing was useless for HF
73 vk6zgo

VK6ZGO
11-21-2006, 02:08 PM
Sorry about the double post
vk6zgo

WA4ILH
11-21-2006, 02:52 PM
I once tried using the ouput of my Cushman CE-6A signal generator output as a test source for a 2 meter transmitter. It worked and I got full output BUT... almost as an afterthought, Before I put this on an antenna, I checked the output on a spectrum scope and was shocked to see spurs every 4 Mhz or so. Down 10-20 db but this was NOT GOOD. The signal generator output of my Cushman shop monitor is NOT CLEAN! Good for testing but that's all. A better quality HP generator most probably would not have this problem.
Tom WA4ILH

VK2TIL
01-03-2007, 05:54 AM
I resurrected an old(ish) thread on power supply metering to add something; I'm doing the same here.

This thread was about using a signal generator as a VFO (although it rambled-off into some very interesting side issues).

I was reminded that, a couple of years ago, I bought a close-to-mint Leader LSG-15; these are the "hobbyist" type of instrument, it's no HP but it has a nice slow-motion dial and it was going for only a few $$$$.

I thought it might be useful for driving bridges and other measuring devices; my other "lab" generators (Rohde & Schwartz SMSs) are digital with "step" buttons and it's sometimes easier to "sweep" or "tune" with an old-fashioned knob instead of a pair of buttons.

I got it out a while ago to use as a VFO for some DC receiver experiments (SBL & TUF mixers).

The problems I encountered were two-fold; it doesn't have a 50-ohm output impedance and its output level into 50-ohms was about -2 dBm. Neither characteristic is helpful to 50-ohm/level 7 (dBm) mixers. http://www.qrz.com/iB_html/non-cgi/emoticons/sad.gif

But its "tuning" ability was very useful.

Ever since I bought it I'd had in mind an output amplifier for it; this thread made me move the project up in the Gunnadoo list and I did it over Christmas/New Year.

The amp design was "borrowed" from Solid State Design (Chapter 8, figure 15) with a different Vcc resistor to accommodate the available 18V supply and to reduce the power output.

The circuit was built "ugly" (very http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif ) and attached to the output BNC (this was originally a "funny" version of a UHF socket and I changed it to a BNC while I was in "improvement" mode.

http://img120.imageshack.us/img120/4200/ampphoto2gr9.jpg (http://imageshack.us)

Max output is now 12 to 13 dBm; this can be reduced to the 7 dBm required by the mixers with a 5 or 6 dB pad which will do the 50-ohm match no harm either. http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif

This output should be adequate to drive sensitive bridges, too.

The output is pretty clean; harmonics are a minimum of -30 dBc and most are 40 or 50 dB down. That's certainly good-enough for most purposes.

Another mod I want to make is to make it useable from 12V DC; that would make it portable for antenna measurements. Its mains (240V here) supply delivers 18V DC for its circuitry and I would have to use a "chopper" (ICL7660 perhaps) to boost a 12V battery; that might cause some RF interference so some experimenting is required.

That part of the job has been retained on the Gunnadoo list. http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif

So, I now have a signal generator that can be used as a VFO; that was the subject of this thread.

KC4YLV
01-03-2007, 10:19 AM
<a href="http://cgi.ebay.com/USB-DDS-HF-Generator_W0QQitemZ290067965429QQihZ019QQcategoryZ 1502QQssPageNameZWDVWQQrdZ1QQ
cmdZViewItem" target="_blank">usb dds synth on ebay</a>

&#036;75

dc to 50 mhz in 1 hz steps

usb controllable

should nail this project down quite nicely http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif
bring that old rig into the next age, old school circuitry using new school tuning stability and control.

VK2TIL
01-03-2007, 11:25 AM
These look quite good; and from VK, too&#33;&#33;

I think Mark is a VK amateur; can&#39;t remember his call at the moment. He is producing some interesting stuff.

I bought one of his first devices several years ago; a 6-channel PC-based voltmeter. It never worked despite a few &#39;phone calls and him sending me new parts although I thought it was in the software; I eventually gave-up and wrote it off to experience.

But I&#39;m sure he has things under control these days.

It&#39;s curious that no spec is given for harmonics of the oscillator itself although the attenuator is specified at -40dBc.

My digital SMSs (100 kHz to 512 MHz) are spec&#39;d at -30dBc or better for harmonics but this is highly conservative; they actually measure at better than -40.

Digital oscillators are still behind analog ones in spectral purity and attenuators made from good resistors don&#39;t create harmonics at all.

Some years ago I met an audiophile-type electronics bloke; he had built an LC oscillator, using a 4-gang tuning capacitor, for audio frequencies to get spectral purity.

So I wouldn&#39;t replace the analog &quot;innards&quot; of the Leader with digital ones. It serves its purpose admirably now that it has a bit more horsepower and is something like 50 ohms impedance.

And it has a real tuning knob&#33;&#33; http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif

The DDS-type oscillators would, however, be useful for many experimenters.

WA7KKP
01-04-2007, 09:36 PM
The venerable BC221 frequency meter and its cousins was commonly used as a VFO for many post WWII ops. Reasonable stability, and definitely accurate (if you had the calibration book).

It was essentially a VFO with a hetrodyne mixer, driving a pair of headphones so you could beat it against the output of another transmitter.

Yep, it&#39;s been done before . . .

Gary WA7KKP

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