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 Post subject: Modulator build
PostPosted: Jul Wed 06, 2016 10:59 pm 
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I am looking at getting a VFO http://www.pongrance.com/DDS2016.html and since it does not have internal amplitude modulation I will need a modulator circuit designed so that I can modulate the carrier for use with aligning radios along with stepping up the 200mVpp output to a larger voltage. Will also need an attenuator circuit. I will need an internal 400Hz modulation if possible and will also want external modulation.

The VFO can operate from 1Hz through 34MHz.

I will not be using this below 100KHz as that would require a capacitor to be increased in value on the VFO in order for a flat response down to 1Hz and will also require changing the step resolution to 1Hz then changing it back higher when I'm outputting anything other than audio frequencies.

Suppose a transistor or FET that can go to 34MHz or higher will work for the RF amp and the same possibly for the modulation amp.

I will have a B+ of 12 Vdc, but can go as high as necessary and use a 7812 for the VFO B+.


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 Post subject: Re: Modulator build
PostPosted: Jul Thu 07, 2016 12:11 pm 
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Adding an external modulator can be very complicated. As a rule an AM modulated signal generator is modulated at a low level and the resulting signal amplified by a class A RF amplifier. A principal reason for that is that the higher the carrier level the more difficult it is to obtain a clean modulation envelope. If you were modulating a 10 kilowatt carrier it would be very hairy (been there, done that multiple times in the last 35+ years).

Since you are already starting with a fairly low level that should keep things relatively simple. Have you had a chance to review any existing examples of modulation schemes which might work at that level? I just got home from the hospital yesterday, and it may be some time before I will have a chance to review the modulation circuits used in some solid-state broadcast transmitters of the last few decades to see if I can suggest something applicable to your needs.

An attenuator should not be difficult. Amateur radio literature is replete with examples of home built RF step attenuators which, if built with reasonable care, will provide results entirely satisfactory to your needs. In addition there are commercial attenuators such as the HP 355 series which you might find at a reasonable price at a hamfest (they tend to be overpriced in online auctions). Many of those attenuators have been damaged by careless use but it is easy to perform a basic field test with a 50 Ohm termination and a small DMM.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: Modulator build
PostPosted: Jul Thu 07, 2016 12:33 pm 
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I had initially thought of doing the modulation like it is done in the LM-386 part 15 AM transmitter kit, but I don't know how well it would work substituting a transistor for the TCXO used in that transmitter kit.

EDIT:

Here's how the transmitter does its modulation

Attachment:
LM-386 ARF post.jpg
LM-386 ARF post.jpg [ 73.9 KiB | Viewed 3142 times ]


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 Post subject: Re: Modulator build
PostPosted: Jul Thu 07, 2016 12:50 pm 
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That is the modulation scheme that comes first to my mind - collector modulation of a bipolar transistor. Matching the input and output to the modulation transistor should not be difficult. You should be able to simplify the output network.

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Dale H. Cook, Antique Radios and Test Equipment (GR/HP)
http://plymouthcolony.net/starcity/radios/


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 Post subject: Re: Modulator build
PostPosted: Jul Thu 07, 2016 12:57 pm 
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Here's the simplified version

Attachment:
LM-386 2.jpg
LM-386 2.jpg [ 67.78 KiB | Viewed 3140 times ]


Given this circuit was made for the AM band I don't know if the 1mH inductor would be proper or if it will need to be a different value for higher frequencies.

Should I add any sort of resistor across the RF input? Seems to me that the base of the transistor is floating given the VFO I'm going to buy has an output coupling cap.

Here's a 400Hz sinewave oscillator I found from here http://www.mymorninglight.org/ham/400Hz.htm

Attachment:
400Hz oscillator.jpg
400Hz oscillator.jpg [ 55.21 KiB | Viewed 3137 times ]


That way I will have a built in source of modulation. I will either switch it in before the 100K audio level resistor or more than likely I will switch it after the resistor using a variable resistor on the output of the oscillator so I can set its level properly regardless of where the audio level control is set. I'm thinking the easiest way would be to replace the 1K collector resistor with a 1K trimpot leaving the collector to ground cap where it is at then using a smaller value coupling cap after the variable resistor.

The only thing I don't know are the transistors used in that circuit.


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 Post subject: Re: Modulator build
PostPosted: Jul Thu 07, 2016 6:51 pm 
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Location: pensacola fl
look at heathkit ig102 diagram or the much more powerful ig42 wich has am and a large output pad as its output stage has a large output tube. they can be had on ebay and elsewhere at decent prices from time to time.


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 Post subject: Re: Modulator build
PostPosted: Jul Thu 07, 2016 9:21 pm 
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Looks like you don't need anywhere near 100% modulation.

In that case, how about connecting a CdS opto-isolator from the output to ground, and drive it's LED with your 400 HZ generator? Every cycle it will attenuate the RF output depending on how bright the opto-isolator's LED is driven. The CdS cell has max resistance in the megohms, and lowest resistance in the hundreds of ohms, so it doesn't get near shorting the output to ground.


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 Post subject: Re: Modulator build
PostPosted: Jul Thu 07, 2016 10:46 pm 
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True, although I also do need to step up the signal a little larger than the 200mVpp it is coming from the VFO.

Thing is I don't know how much gain that transistor stage will have so I don't know what the output voltage will be.

Just looking to get the average output voltage of a typical RF generator.


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 Post subject: Re: Modulator build
PostPosted: Jul Thu 07, 2016 11:19 pm 
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that output stage of the heathkit ig42 a sweep tube and your eico 315 could get you as much as a volt output and they used attenuation after the stage of gain so you get all you need and more. just about and of the sweep tubes will work and it is broad banded meaning no tuned circuits on the output.


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 Post subject: Re: Modulator build
PostPosted: Jul Thu 07, 2016 11:29 pm 
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True, although I'm not using this for the EICO.


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 Post subject: Re: Modulator build
PostPosted: Jul Thu 07, 2016 11:57 pm 
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that stage will work with a vfo. why is the output of that vfo so low?


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 Post subject: Re: Modulator build
PostPosted: Jul Fri 08, 2016 1:29 am 
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Think it is that low because it is meant to replace the VFO in some ham radio gear.


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 Post subject: Re: Modulator build
PostPosted: Jul Fri 08, 2016 2:51 am 
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does it make a square wave or sine wave? if it is a sine wave then a linear biased stage tube copy a black and white tv video amp as they are high gain and have enough to drive the 6AV5 to more than enough output before the attenuator. if the output is square then the stage can be non linear. the heathkit stage is also the modulator stage so you have what you need then. if you copy the heathkit 1g102 it uses a 6AN8 with much less padding but a health output can be gotten to align receivers.the circuit is very similar just the output is different and the power suply for the 42 has a screen regulator tube to make it perform better. it will even drive an output level meter.


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 Post subject: Re: Modulator build
PostPosted: Jul Fri 08, 2016 11:53 am 
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The output is a sinewave.

In keeping with this being small and easily portable I would prefer not to use tubes if at all possible.

Here's the circuit I plan on using.

Image

Two things I have a question about.

1. Given the 2N3904 stage was designed to be fed with the square wave output of a TCXO, do I need to change anything about that stage?

2. I saw this box of over 300 TO-92 semiconductors http://www.goldmine-elec-products.com/p ... ber=G21402 Is it possible to use transistors or FETs instead of the OP-AMP? If so what changes would I need to make?


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 Post subject: Re: Modulator build
PostPosted: Jul Fri 08, 2016 5:36 pm 
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What you have there should work however the output will be frequency dependent as to amplitude. weather there is enough drive for a wide range attenuator will depend on load impedance and amount of attenuation.


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 Post subject: Re: Modulator build
PostPosted: Jul Fri 08, 2016 6:50 pm 
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I just noticed something. I left off a .1uF cap from the output of the LM-386 to ground.

EDIT:

Here's an updated schematic.

Image


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 Post subject: Re: Modulator build
PostPosted: Jul Mon 11, 2016 4:10 pm 
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Location: Lexington, KY USA
For "plate" modulation to work, the modulated stage must have enough RF drive to make the stage non-linear. The nice sinewave gets all squared-up.

In a transmitter, a low pass filter follows the modulated stage to restore the sinewave. But the filter is frequency dependent, and has to be tuned. Not what you want on a bench signal generator.

Service grade signal generators usually have output waveforms that are nothing like a sinewave, anyway.

Some of those synth ICs have modulation input pins. You might look at the data sheet.

Ted


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 Post subject: Re: Modulator build
PostPosted: Jul Mon 11, 2016 5:03 pm 
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I'll have to look at that and see if it does have a modulation input pin.

If so that would greatly simplify things and I would only then need a suitable amp stage.


Last edited by Tube Radio on Jul Mon 11, 2016 5:41 pm, edited 2 times in total.

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 Post subject: Re: Modulator build
PostPosted: Jul Mon 11, 2016 5:27 pm 
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Location: Lexington, KY USA
I couldn't resist looking into this.

The AD9834 DDS IC used on your VFO module does have a pin that can be used to provide amplitude modulation of the RF output.

The VFO module manual & schematic is here: http://www.pongrance.com/dds-2016manual.pdf

The AD9834 data sheet is here:
http://www.analog.com/media/en/technica ... AD9834.pdf

The FS Control pin17 on the AD9834 is an analog input that determines the RF output amplitude.

An app note showing how to produce AM with the AD9834 is here:
http://www.analog.com/media/en/referenc ... CN0156.pdf

This app note describes sourcing the audio with a DAC, but an analog source will work as well.

The easiest way to do this would be to apply the audio to the FS Control, Pin17, through an added series resistor. You can leave the existing 6.8k R1 and 0.1uF C10 in place.

For best results with high modulation percentages, the value of R1 should be increased, to perhaps 15k or 16k. A smaller value for C10 would accommodate higher modulation frequencies.

The added resistor could be another 6.8k, or perhaps 4.7k or 10k. Provide AC coupling for the audio. Put a cap in series with the added resistor.

The audio voltage required is pretty small, and the impedance fairly high, so you don't need a big power amp like the LM386. Your phase shift oscillator can drive it directly.

If you want to get fancy, you might consider using a dual op-amp IC to make an audio oscillator and buffer amplifier. The old HP circuit with the lightbulb can provide pretty low distortion.

Ted


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 Post subject: Re: Modulator build
PostPosted: Jul Mon 11, 2016 6:34 pm 
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You meant pin 1 not pin 17, right?

It looks like the FS adjust wants to see a max of 1.2 volts for minimum RF output and 0 volts for max RF output.

It would then seem like there is a dc voltage on FS adjust pin 1.

If so would an audio source be able to drive the voltage from 0-1.2 volts?

I would then only need a transistor on the output to step up the RF some.

The attenuator I am not sure how best to do that, but I'm wondering if it would be possible to have a higher B+ on the output stage and just adjust B+ voltage to control the output signal level?


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