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 Post subject: HP8640 Modification COMPLETE!
PostPosted: Mar Thu 01, 2018 6:16 pm 
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Location: Greenville, TX
My 8640 low end is about 450kHz. I need a suggestion for an external unit which would divide the output frequency by 2 and provide a modulation input for lower IF frequencies (i.e. 275kHz etc.).

Thank you,

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Last edited by radiodale1 on Mar Wed 14, 2018 2:34 am, edited 1 time in total.

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Thu 01, 2018 10:30 pm 
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Many audio generators will cover frequencies up to 455 kHz and beyond. I use an HP 651B "Test Oscillator", set the frequency with a counter, then use a scope to look at the signal during alignment. No modulation is necessary.

Image

Dave


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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Thu 01, 2018 10:36 pm 
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There are a few oscillators like Dave mentioned, which have an output attenuator to provide usable signal levels. You can also find an HP11710 A or B converter unit. The "A" converts from 5.005 to 5.5MC down to 5KC to 500KC. The "B" converts 50.01 to 61MC down to 10KC to 11MC. I don't know about the output level correspondence compared to the generator's output levels.

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Fri 02, 2018 12:20 am 
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Thank you both.

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Fri 02, 2018 2:48 am 
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HP Application Note 171-2 describes how to extend the frequency range of the 8640B down to near DC.


Download a copy from here:
http://www.hpmemoryproject.org/an/pdf/an_171-2.pdf

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Fri 02, 2018 3:48 am 
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Elegant solution Chuck. I think I'll build it.

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Fri 02, 2018 4:29 pm 
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Location: Rochester, MI
Thanks for posting that; I've added it to my 8640 file.

Edit: If anyone is interested, there's a seller on ebay with an hp 10514a mixer listed for $25 and I think he'll take $20 if you try the make offer. Disclaimer: I'm not associated with the seller other than getting one for $20.

The attenuator is easy and it seems like you could replace the hp461a with a minicircuits module and build the whole thing into a flat chassis sitting on top or below the 8640.


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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Sat 03, 2018 3:49 am 
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I've been tempted to build one since I originally found that Appnote, but I usually just hoist my HP 8656A up on the bench for low frequency IF's.

If you do build one post your results.

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Tue 06, 2018 12:43 am 
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Chuck,
Here are the results at 150kHz with 75% AM. Low pass filter on output. I'll generate other frequencies if you like.


Attachments:
150kHz.jpg
150kHz.jpg [ 186.92 KiB | Viewed 605 times ]
Not Overdriven.jpg
Not Overdriven.jpg [ 188.77 KiB | Viewed 600 times ]

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Tue 06, 2018 1:37 am 
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Dale

Looks good on the scope, at least the second picture does. Any harmonics generated by the setup?

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Tue 06, 2018 2:03 am 
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Harmonics of what frequency?

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Tue 06, 2018 2:16 am 
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radiodale1 wrote:
Harmonics of what frequency?



The fundamental, any 300KHz or multiples of the 150KHz in the output?

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Tue 06, 2018 4:49 am 
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Location: Greenville, TX
Would I turn off the modulation and use my HP distortion analyzer? It covers 600kHz.

Also, check the +7 level in the app note. The output level (aux) is -10dBm and the signal after the attenuator is -16dBm. The output of the amplifier then is -16 +20 = +4dBm not +7 as shown in the note. Should the fixed attenuator be reduced to 3dB from 6dB?

Thank you,

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Wed 07, 2018 1:00 am 
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Location: Greenville, TX
Here's what the 150 kHz looks like without AM.


Attachments:
Clean 150 kHz.jpg
Clean 150 kHz.jpg [ 186.47 KiB | Viewed 548 times ]

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Wed 07, 2018 3:01 am 
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If just for alignment of an IF circuit, I would think that some harmonics would be greatly reduced by the IF itself and the desired frequency would be selected and peaked pretty naturally. This is probably correct as most IF alignments start out somewhere near the optimum and most IF circuits do not have the range to accidentally peak on a twice the freq. harmonic? IT is true that alot of oscillators and function gens like an HP 200CD can tune as high as 600 KHz. but they are not modulated if that concerns you, and the output levelsare usually much greater than an RF generator, thus getting a low enough signal level might be just at the bottom of the pot range, unless you make or use an attenuator. And some of those like the HP are not super accurate dials so a counter check might be in order after a good warmup.


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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Fri 09, 2018 1:29 pm 
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Dale,

What are you using to amplify the signal from the aux. output? The app note recommends an HP unit--are you using that, or a similar amplifier, or did you build something specific for this purpose?

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 Post subject: Re: HP8640 Modification Needed
PostPosted: Mar Fri 09, 2018 3:07 pm 
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Location: Greenville, TX
Using the specified HP amplifier.

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 Post subject: Re: HP8640 Modification COMPLETE!
PostPosted: Mar Wed 14, 2018 2:39 am 
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Joined: Jun Mon 01, 2015 9:34 pm
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Location: Greenville, TX
Here is the final setup with the correct amplifier and home brew low pass filter. Waveform is 262kHz with 1kHz modulation.


Attachments:
Down Converter.jpg
Down Converter.jpg [ 193.18 KiB | Viewed 356 times ]
262 kHz.jpg
262 kHz.jpg [ 177.54 KiB | Viewed 356 times ]

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 Post subject: Re: HP8640 Modification COMPLETE!
PostPosted: Mar Wed 14, 2018 4:44 am 
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MIGHTY nice looking Dale.

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