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Old 3rd Mar 2019, 2:40 am   #21
John KC0G
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Default Re: Racal RA17C?

You can read the entire text of the Rafuse paper, ie Symmetric MOSFET mixers of high dynamic range", which David mentioned in post #17 at:
https://ieeexplore.ieee.org/document/1154637
The scan is mediocre. I opened it in a separate window and enlarged it.
You need full access to see the figures on the second page (there were only two), but the text paints a good picture.

73 John
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Old 3rd Mar 2019, 10:47 am   #22
Radio Wrangler
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Default Re: Racal RA17C?

Rafuse missed a trick - something which could have been done then and improved his mixer significantly.

He substituted DMOS FETs into the ring mixer circuit which had been invented to cover the difficulties of using a 1-port active device (the diode). He overlooked the opportunities offered by the MOSFETs being 2-port devices.

A nice circuit that made the most of them was suggested to me by Ian Buckner, over lunch on the trad napkin, in about 1990/1. A year or so later Colin Horrabin submitted a similar arrangement to Pat Hawker which went public in 'Tech Topics'. Look up 'H-mode mixer'

Kudos to both Ian and Colin, it isn't often that there's solid documented evidence of MIT missing a trick for nearly a quarter of a century. There was no connection between Colin and Ian, they had the same idea independently.

David
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Old 6th Mar 2019, 2:39 am   #23
Synchrodyne
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Default Re: Racal RA17C?

Quote:
Originally Posted by Radio Wrangler View Post
A nice circuit that made the most of them was suggested to me by Ian Buckner, over lunch on the trad napkin, in about 1990/1. A year or so later Colin Horrabin submitted a similar arrangement to Pat Hawker which went public in 'Tech Topics'. Look up 'H-mode mixer'
The said TT issue is in RadCom 1993 October, pp.55,56. There were follow-up articles in RadCom 1998 July, pp.58,59, 1998 August pp.60,61 and 1998 September pp.58,59.


Cheers,
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Old 11th Mar 2019, 12:32 am   #24
Dugfirtree
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Default Re: Racal RA17C?

Greetings,
after rebuilding circuits in V9 &V10 now I get bad audio distortion when I turn audio above 1/4 max, so I need to align the 37.5 MHz filter & Amp. The alignment directions say Inject an accurate 37.5 MHz to TP1. How accurate should I be? I can connect an external OCXO ocs. to my HP8640B and get very accurate. However, should I just inject signal at TP1 and rock my freq back and forth on Sig Gen for peak VTVM reading and use that freq?

Also, on the next adjustment (step 6.7) 1 MHz xtal adjustment, do I adjust to 1 Hz accuracy or should I leave alone?

My next rebuild step will be to remove 2nd VFO and rebuild, then remove & rebuild 100KHz IF deck, doing alignments for each after each rebuild. I'm leaving the 1 st VFO for last as it looks like the 40 MHz filter will need a sweep generator to align and the manual does not list the procedure.
Any recommendations for the sequence of the rest of the rebuild? and will I need to adjust the 40 MHz filter?

Kurk
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Old 11th Mar 2019, 2:54 am   #25
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Default Re: Racal RA17C?

With the B version of an 8640, its counter should be plenty accurate enough, and you can always press the 'counter lock' button.

The Racal manual was written in the days of free-running valve oscillators in most sig gens.

The 40MHz filter does need a sweep generator and scope. Don't attempt it without something accurate as a marker.

David
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Old 12th Mar 2019, 1:03 am   #26
Dugfirtree
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Default Re: Racal RA17C?

Greetings,
I agree David retuning the antenna tune all the time is irritating, I did not realize I did not have to do that on the R390 until you mentioned.
I rebuilt the circuits around V9 & V10 and now I get bad audio distortion with audio at ¼ max, so I need to align the 37.5 GHz filter. How accurate does my Sig Gen need to be? With an ext 10 MHz ref I can get within about 20 Hz of exact freq. The 8640B is hard to adjust to better than 20 Hz. Also, in the next step of the alignment it says to adjust the 1 MHz oscillator. Should I adjust this as close as I can to 1 MHz or should I leave it where it is?
I plan on doing total rebuild, next is 2nd VFO then the IF deck. I plan on doing alignments after each section rebuild. I am saving the 1st Lo for last. Will I need to align the 40 MHz filter after the VFO1 rebuild? My manual does not have a procedure for that, it says need sweep generator and factory jigs.
Does anyone have any recommendations for the rest of the rebuild, maybe sequence.
Kurk
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Old 12th Mar 2019, 1:05 am   #27
Dugfirtree
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Default Re: Racal RA17C?

Greetings,
I agree David retunning the antenna tune all the time is irritating, I did not realize I did not have to do that on the R390 until you mentioned.
On RA17C I rebuilt the circuits around V9 & V10 and now I get bad audio distortion with audio at ¼ max, so I need to align the 37.5 GHz filter. How accurate does my Sig Gen need to be? With an ext 10 MHz ref I can get within about 20 Hz of exact freq. The 8640B is hard to adjust to better than 20 Hz. Also, in the next step of the alignment it says to adjust the 1 MHz oscillator. Should I adjust this as close as I can to 1 MHz or should I leave it where it is?
I plan on doing total rebuild, next is 2nd VFO then the IF deck. I plan on doing alignments after each section rebuild. I am saving the 1st Lo for last. Will I need to align the 40 MHz filter after the VFO1 rebuild? My manual does not have a procedure for that, it says need sweep generator and factory jigs.
Does anyone have any recommendations for the rest of the rebuild, maybe sequence.
Kurk
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