Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Nov Wed 13, 2019 11:47 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 40 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: Circuit question
PostPosted: Mar Tue 20, 2018 2:35 pm 
Member
User avatar

Joined: Jan Fri 02, 2009 11:32 pm
Posts: 618
Location: Equinunk PA 18417
Quote:
I want to get mine to where it is the same output level so that I am sure I have enough signal level for any alignments I need to do. Also the output impedance needs to be 50 ohms if possible.
You said it earlier, "Think I'm trying to make a silk purse out of a sow's ear". The difference from 14.6 volts to 110 volts may look huge on paper but is only about 17db, which is not a big deal at all. You'll see big voltage swings with any generator unless every single point of the circuit under test and your scope probes are all operating at the same impedance. That won't happen for most of us mere mortals. RF is nothing like audio because everything becomes part of a tuned/mis-tuned circuit.

I'd restore the original circuit, change the value of C10, and swap in a 1k resistor instead of the output attenuator, temporarily. Do you still see such a large difference in output voltage across bands?

edit: fixed typo

_________________
-Richard


Last edited by richfair on Mar Tue 20, 2018 3:21 pm, edited 2 times in total.

Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Tue 20, 2018 2:51 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34393
Location: Maryland 20709, USA
Tube Radio wrote:
What is the average maximum output voltage of good quality RF generators?
About 1 volt.

This is a signal source, not an AC power generator.

Tube Radio wrote:
Also the output impedance needs to be 50 ohms if possible.
Why? Are you aligning high-quality communication receivers?

Typical consumer radios have no defined input impedance, so the generator source impedance is irrelevant.

- Leigh

_________________
73 de W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Tue 20, 2018 4:27 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20331
Location: Warner Robins, GA
Not sure the purpose of the two caps, but they were in mine when I got it and judging by the other caps were original to the unit.

Suppose they might be to adjust the oscillator some on those two bands perhaps.

Yes R14 was indeed added as it orivuded a convienent way to properly bias the cathode follower.

I agree I need to get the impedance low.

I am thinking perhaps building the LT1252 preamp and having it fed from a wire stuck in band E coil might be the best solution to the problem as it will both amplify the signal and also provide a low impedance output. I could then either find a small 50 ohm attenuator that can fit inside the RF generator or ditch the internal attenuator entirely and use an external attenuator. far as the signal level varying with frequency maybe I could add some sort of filter in the feedback network that will alter the gain based on frequency which would reduce the output voltage variations.

Will I be able to make this perfect? No

Wiill it work better than stock and be better than anything I currently have and can afford? Yes


Bob, I did try a 20K resistor and while it did reduce the clipping of the - part of the waveform the counter also would only work on a limited range of frequencies where the output was its strongest.

The 50K impedance of the counter must be too low for that circuit.



At work I have a function generator that can go to 20Vpp unloaded and 10Vpp with a 50 ohm load.

So to get 20Vpp out of the preamp unloaded I would need it to have a gain of 12.61

Also it looks like the voltyage decreases as I go lower in frequency so I could perhaps use a capacitor in the feedback circuit to increase gain as I go lower in frequency.


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Tue 20, 2018 4:38 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20331
Location: Warner Robins, GA
richfair wrote:
Quote:
I want to get mine to where it is the same output level so that I am sure I have enough signal level for any alignments I need to do. Also the output impedance needs to be 50 ohms if possible.
You said it earlier, "Think I'm trying to make a silk purse out of a sow's ear". The difference from 14.6 volts to 110 volts may look huge on paper but is only about 17db, which is not a big deal at all. You'll see big voltage swings with any generator unless every single point of the circuit under test and your scope probes are all operating at the same impedance. That won't happen for most of us mere mortals. RF is nothing like audio because everything becomes part of a tuned/mis-tuned circuit.

I'd restore the original circuit, change the value of C10, and swap in a 1k resistor instead of the output attenuator, temporarily. Do you still see such a large difference in output voltage across bands?

edit: fixed typo


I've messed with the value of C10 before and if I mess with it too much one of the bands does not oscillate on its lower end plus if I go lower in value it causes a phase shift as well.

I don't think there's as large of a change in output voltage, but that's because on the lower two bands I'm only getting the positive peak of the sinewave due to the cathode follower not being biased right.


Leigh wrote:
Tube Radio wrote:
What is the average maximum output voltage of good quality RF generators?
About 1 volt.

This is a signal source, not an AC power generator.

Tube Radio wrote:
Also the output impedance needs to be 50 ohms if possible.
Why? Are you aligning high-quality communication receivers?

Typical consumer radios have no defined input impedance, so the generator source impedance is irrelevant.

- Leigh


I do have a Hammarlund SP-600 JX-17 and a Collins R-390 plus I might decide to get other communications receivers at some point and it is nice to have an RF generator with a 50 ohm impedance as I can then use it with a proper attenuator instead of a variable resistor.


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Tue 20, 2018 5:04 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 26776
Location: Annapolis, MD
Disclaimer: I have not followed the whole discussion.

First, the fact that an RF generator does not produce a sine wave is a non-issue. Many of them purposely use distortion to get the harmonics for the higher bands.

Output impedance? The main reason for worrying about this might be ringing in the cable---I solved such an issue with my Paco E200C---just put a 50-ohm resistor at the end of the cable. If I wanted a better attenuator, It would simply connect there.

Attenuator? Waste of time/$ if the generator does not have good shielding. Much easier to use "loose coupling"---eg just put the RF output close to the right place in the circuit.

Modify the frequency response of an amplifier by adding feedback---good way to make an oscillator....


As already discussed, pig parts pass poorly for pretty purses.....
The one mod that my E200C might get someday is a counter output on the back. Further "improvements" to this classic might actually reduce its value.....

YMWACV

_________________
-Mark
"Measure voltage, but THINK current." --anon.


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Tue 20, 2018 6:15 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20331
Location: Warner Robins, GA
I reinstalled the 1K pot without the load resistors and with R14 still in circuit.

On band A at the lowest frequency the counter will not read which will be solved by adding that preamp to the counter. Also the counter still somewhat loads down the output plus adding the 22K resistor causes the counter not to read at any frequency.

Also there's more of the negative part of the waveform being output until I go to band C where the sinewave looks like a proper sinewave which means the cathode follower stage is either not biased right or the oscillator output on bands A and B is just too high.


Measurements to follow soon as I take them.

I'm still thinking I might have better results if I stick a wire in band E coil and use that preamp circuit as when sticking the scope probe there it provided the best looking signal with the least variation in output across bands A-D.

That said perhaps I could even ditch the tube oscillator and substitute it with a JFET using lower B+ voltage.


EDIT:

Frequency measurements are only as accurate as the dial tracking is.
Voltage measurements are only as accurate as the voltage display in the Tektronix 2246 oscope is.

Attachment:
Measurement 2.jpg
Measurement 2.jpg [ 24.03 KiB | Viewed 1976 times ]


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Tue 20, 2018 6:57 pm 
Member
User avatar

Joined: Jan Fri 02, 2009 11:32 pm
Posts: 618
Location: Equinunk PA 18417
Tube Radio wrote:
Also it looks like the voltyage decreases as I go lower in frequency so I could perhaps use a capacitor in the feedback circuit to increase gain as I go lower in frequency.
I don't understand. I think you mean to say that within any single band, the output voltage is smaller at the lower frequency end of the dial than at the other end. Or do you mean something else?

Quote:
I've messed with the value of C10 before and if I mess with it too much one of the bands does not oscillate on its lower end plus if I go lower in value it causes a phase shift as well.


So, C10, are you saying that a value smaller than 30pf will prevent the oscillator from oscillating? (If true I don't understand how.) What do you mean by "phase shift". Phase shift is the result of a time delay, which means the shift is relative between two circuit points. Are you actually talking about the shape of the waveform, perhaps you are seeing the oscillator's fundamental frequency being reduced relative to its harmonics, and thus the waveform looks less like a perfect sine?

_________________
-Richard


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Tue 20, 2018 7:01 pm 
Member
User avatar

Joined: Jan Fri 02, 2009 11:32 pm
Posts: 618
Location: Equinunk PA 18417
Doggonit, I didn't see the added measurements until after posting.

Hey, those voltage measurements look GREAT! Very consistent across the bands for this quality of generator, remember that a double of voltage, like from 2v to 4v, is only 6db

_________________
-Richard


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Tue 20, 2018 8:08 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20331
Location: Warner Robins, GA
I forget exactly but either a smaller or larger value affected the oscillator on one of the bands.

I mean that with the 10K pot the voltage would decrease as I went up in frequency and the low end of each band would be lower than the high end of the previous band.

By phase shift I mean something like in this diagram of a high pass filter.

This is with the load C10 sees. I calculated the resistances to get the load. Did not count R14 as I figured the cap adequately bypassed it.

Attachment:
Phase.jpg
Phase.jpg [ 86.18 KiB | Viewed 1967 times ]


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Tue 20, 2018 9:46 pm 
Member
User avatar

Joined: Jan Fri 02, 2009 11:32 pm
Posts: 618
Location: Equinunk PA 18417
Phase shift at any one frequency is relative to two different circuit points. Are you looking at the phase (delay) from one side of C10 to the other side? 'Cause I don't see what the problem is unless your OCD has flared up! C10 was originally 3pf. I can understand it would cause phase shift of lower frequencies, but it would also reduce the output voltage of the lower bands vs. higher bands, which sounded like a good thing until you posted your latest measurements which differ in a huge way from an earlier posting. So, anyway, 30pf is working well for you. A smaller value should not kill the oscillator whereas a larger value might.

_________________
-Richard


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Wed 21, 2018 12:05 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20331
Location: Warner Robins, GA
I can tend to be OCD about some things, but not this unless a phase shift will affect the alignment of a radio or communications receiver.

I am thinking perhaps mine had a 30pF cap stock, but I'm not too sure.

Looks like perhaps Stark may have tried to correct some of the circuit deficiencies in a later revision which might be what I have.

I've got to order one of those chips for the counter so I can finish its preamp circuit so I'll order two and duplicate the preamp just to see how well it works connected to a wire stuck inside the band E coil.

If it works well enough like that I might just go with that circuit and ditch the cathode follower completely.

Tomorrow I can take measurements of the scope probe stuck in band E coil and see if the voltage differences are any less.

I agree this will never be perfect, but I do love to experiment and this is a great way to learn even if I only learn what doesn't work.


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Sat 24, 2018 2:41 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20331
Location: Warner Robins, GA
I thought about it and decided maybe a transistor would do good enough at amplifying the signal from a wire stuck in band E coil so I thought about the general purpose transistors 2N2222 and 2N3904.

Looked up the data for the 2N3904 and it is suitable for RF use to over 100MHz so it is perfect for this.

Started looking up circuits and found these two.

The first one looks like it is more suited for what I am wanting to do plus it uses parts I can easily get.

Attachment:
1 images.png
1 images.png [ 5.92 KiB | Viewed 1941 times ]


Attachment:
1 Emitter-Follower-Circuit-Diagram..png
1 Emitter-Follower-Circuit-Diagram..png [ 18.38 KiB | Viewed 1941 times ]


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Sat 24, 2018 3:37 pm 
Member
User avatar

Joined: Jan Fri 02, 2009 11:32 pm
Posts: 618
Location: Equinunk PA 18417
First one won't work, input impedance too low and low gain. Lower circuit has more gain but you'd have to see if there is enough pickup from coil E especially at low frequencies. Try it and see what happens. Cap values not critical, resistors within 20% should be fine.

_________________
-Richard


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Sat 24, 2018 5:08 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20331
Location: Warner Robins, GA
I tried the 2N3904 to sub for the oscillator tube using a 10K collector resistor and running on 12Vdc.

It did work after a fashion, but it only produced a positive going pulse waveform and the frequency didn't jive with the dial.

I might play around with that as well. Perhaps some bias is needed.

Now if I can get the oscillator running on a transistor I can make the whole thing run on 12Vdc and if the dial tracking is not right I could maybe even ditch that and install the frequency display where the dial was.

Would be quite interesting and I would then have a pretty good RF generator. I could even add a proper audio oscillator and proper modulation to further enhance the RF generator's functionality.

I don't really have any issue modding this as I know it already is not high end and at this point it is more along the lines of improving the generator.

I had thought about using the preamp I am installing on the frequency display to pick up the signal from a wire stuck inside band E coil, but I don't knowq if that preamp is set to take the output to the rail voltage with any input or if it will produce a sinewave.


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Sun 25, 2018 3:18 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2321
Location: Saskatoon
http://theradioboard.com/rb/viewtopic.php?t=5422


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Sun 25, 2018 4:07 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20331
Location: Warner Robins, GA
Can't remember but I think I ordered two of those chips used for the preamp for the frequency display. I may try that circuit to pick up the RF given it was designed for another frequency display I use with my Hamarlund SP-600 JX-17 and does a good job of picking up the LO without affecting the LO at all without any direct connection to the LO circuit.


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Mar Wed 28, 2018 1:28 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20331
Location: Warner Robins, GA
I got the chip yesterday and installed it on the counter board in the spare chip slot.

The counter works great at all frequencies and provides minimal loading to the circuit.

it does pick up the signal quite well, although I would need to run the preamp at a higher B+ voltage in order for it to properly handle the signal level of band A and B without distortion.



EDIT:

So the circuit can be powered off the 6.3Vac winding of the RF generator provided the first filter cap on the counter board is at least 1000uF.

I did notice that at two spots (around 260KHz band A and around 320KHz band B) the counter displays a higher frequency and it doesn't seem to be related to the waveform or the preamp output.

Any ideas?

I've figured a way to mount the counter to the RF generator.

I've got a small 5 sided box which I will put the counter in then I will remove the handle and drill two holes in the box to match the holes in the cabinet for the handle. Two nuts and bolts will secure it fine.

Not sure how I'd route power and signal though.


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Apr Tue 03, 2018 12:25 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20331
Location: Warner Robins, GA
Here's an update.

I've figured out how to mount the display to the generator.

Attachment:
20180402_184005-800x600.jpg
20180402_184005-800x600.jpg [ 65.44 KiB | Viewed 1865 times ]


The extra hole is where the wiring will pass through.

Attachment:
20180402_184043-800x600.jpg
20180402_184043-800x600.jpg [ 84.49 KiB | Viewed 1865 times ]


Will mount a couple buttons on the front.

I plan on repurposing the HIGH-LOW RF level switch on the generator to switch the counter input from the generator output to a jack (had mounted it for the counter to plug into when it was external) so that it can also be used as a frequency counter and I'll switch it to a BNC jack so that I can use a 10X scope probe for measuring the frequency of circuits with higher output levels which would load the circuit less than a direct connection.

Power to the circuit will come from the 6.3Vac winding of the power transformer.

To do that requires the audio oscillator tube be removed, but the modulation scheme didn't work so good and I never used it anyways.

That will also lighten the load on the power transformer and it will run a little cooler.

I am tempted to figure a way to use the same preamp chip I used with the counter in place of the cathode follower which would provide a low output impedance.

Any thoughts on that?

Thinking perhaps I can make it a simple buffer and use a higher B+ voltage.

I will leave it as is with the cathode follower and try that first to see how well it works.

My only concern is the counter loading down the circuit some when connected then if I need to use the counter to measure something else I will have to readjust the output level of the generator.


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Apr Tue 03, 2018 5:30 am 
Member

Joined: Apr Thu 14, 2016 8:25 pm
Posts: 464
Location: pensacola fl
Have you tried increasing c8 and c10 to lower the reactance and allow a higher drive to the cathode follower. Also take your output for the counter follower from the pin 3 cathode to get the highest output. this could be enough level for your counter. I have a heathkit IG102 and it can drive my frequency counter on all bands.


Top
 Profile  
 
 Post subject: Re: Circuit question
PostPosted: Apr Tue 03, 2018 5:25 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20331
Location: Warner Robins, GA
C8 looks like it starts to gradually bypass R7 as the frequency increases.

Increasing C10 I doubt will help much.

The main issue is too much drive on bands A and B.

That said after adding the preamp circuit to the counter it works fine with the RF generator so much so that I can switch the repurposed HIGH-LOW RF switch to external counter input and the counter will still display the correct frequency for several bands.

That said maybe I didn't even need to connect the counter to the RF generator output after all and could have just ran a wire close to the oscillator and picked up the frequencies properly without loading the output any.

Here's some measurements taken with a HP 3586C selective level meter with the RF generator connected to the BNC jack and the impedance set to as high as it will go which I think is 1 meg.

Would have liked to test it with the 50 ohm load of the SLM, but that reduced the output so much that it wasn't very practical.

Attachment:
Measurement.png
Measurement.png [ 7.78 KiB | Viewed 1849 times ]


Don't know how meaningful those measurements are given the high input impedance the SLM is set to.

Now if I wanted more outpuit I could replace the RF attenuator pot with a 2K pot and ditch the 1K series resistor.

EDIT:

Here's the measurements with the 50 ohm load of the SLM.

Sinewave was greatly reduced in level on all bands and was still a sinewave except for the higher end of bands A and B where the over drive of the cathode follower was still an issue.

Attachment:
Measurement 2.png
Measurement 2.png [ 6.5 KiB | Viewed 1845 times ]


EDIT 2:

Here's the completed RF generator.

The switch on the right side is a SPDT momentary switch and takes the place of the two buttons.

The buttons are still on the PC board as both have to be pressed upon powerup to enter the menus.

Attachment:
20180403_231205-800x600.jpg
20180403_231205-800x600.jpg [ 86.12 KiB | Viewed 1839 times ]


Did my first alignment with the RF generator and it went quite well.

These frequency displays area good way to make service grade RF generators more accurate.


Top
 Profile  
 
Post New Topic Post Reply  [ 40 posts ]  Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 6 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB