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Old 2nd February 2018, 03:13 PM   #191
w5jag is offline w5jag  United States
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Stupid DBM Question:

I don't have a DC blocking cap at the output of the MMIC. On the MMIC side of that 27 ohm resistor, I have 0.6 VDC. On the DBM input pin, I have 2.4 mVDC.

I put the 27 ohm resistor there as a precaution against oscillation in the MMIC. The MMIC is an NEC uPC1651.

Do I need a blocking cap here anyway? When the second LO is hooked up and switches the diodes on, will that look like a short to the MMIC, or will that 27 ohm resistor be enough to isolate the two?

The voltage at the power pin of the MMIC is 5.15 VDC, and if the TL-317L goes south, all of the positive voltage rail could eventually wind up at that pin, and that 27 ohm resistor won't limit the current much. Do I need one as a safety measure, if for no other reason?

Or is this worrying about nothing?

Win W5JAG

tentative preliminary observation, don't hold me to it:

Comparing the 45 MHz output of each board, the MOSFET board is quieter / more sensitive than the DBM board.
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Old 2nd February 2018, 04:04 PM   #192
VenusFly is offline VenusFly  Australia
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No RF gear here?
Quote:
Originally Posted by JBL 4645 View Post
I have no idea what the signal wave form would have looked like to blow up a TV set what ever it must have been a demon RF signal from hell.

I mean what are the odds of that happening? Sure TVI was common thing but blowing a TV set?
And people tell me that an EMP from a nuke can't damage modern electronics. hah.

Then again he could've just blamed you for the faulty tv set and wanted a freebie.
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Old 3rd February 2018, 01:58 PM   #193
w5jag is offline w5jag  United States
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So, I found in my library, "Taking the Mystery Out of Diode Double Balanced Mixers", Joshi, Shankar*, QST, December, 1993, which is a good primer for dolts on these things.

That RF input pin is at DC ground potential, so, in many circumstances, a block is probably needed there. This MMIC does not use its output pin as a dc power input, which is why I omitted the block in the first instance. I really didn't even think about it until the next morning; then I looked at the schematic for the MMIC, and decided to check the voltages.

As it's applied with this MMIC, omitting it may still be an okay decision. I'm at the lake, so may be a bit before I can work on it again.

Win W5JAG

* Chief Engineer, Synergy Microwave
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Old 3rd February 2018, 03:14 PM   #194
gerhard is offline gerhard  Germany
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The same thing happened to me some weeks ago. I was driving a RF
transformer from a CMOS gate to produce a floating clock for my
analog switches in a chopper. I was thinking "That transformer looks
like a few 100 Ohms, that won't be much of a short." But it was. :-)
The CMOS output averages at 1.8Vdc.
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Old 3rd February 2018, 03:21 PM   #195
w5jag is offline w5jag  United States
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Gerhard, that Red Pitaya has caught my interest.

I am really cheap when it comes to spending money on hobbies, but that looks like a really versatile device.

When you get it up and running, please let us know how it works for you.

Win W5JAG
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Old 12th February 2018, 12:14 AM   #196
TheGimp is offline TheGimp  United States
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I am considering trying to build a tube based theremin, and am looking at forms for the inductors.

It occurred to me that it might be possible to impregnate cardboard with polystyrene by dissolving it in Toluene and soaking the tube in it the solution, then let it dry.

Thoughts?
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Old 20th February 2018, 03:13 PM   #197
w5jag is offline w5jag  United States
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I think I have used up the usefulness of the FT-950 as an IF, so the next step is probably to pull the SG-9 board and get it into the chassis, or onto a breadboard, and test the front ends on it.

The SG-9 doesn't give an RX sensitivity spec. The examples shown in its manual depict it paired with a simple front end, consisting of an RF amplifier, and an active mixer, so I'm guesstimating it wants about 20-25 dB RF gain ahead of it.

The unbalanced MOSFET mixer board looks like it has about the right gain, with a good distribution. I'll change out the bipolar IF amplifier to a dual gate MOSFET to make it compatible with the AGC line on the SG-9.

The double balanced mixer board looks problematic. It lacks gain, and it looks to be badly distributed. It needs an RF amplifier.

Since everything is at 50 ohms, is it feasible to use one of these MMIC's as an RF amplifier between the low pass filter and the first mixer? Most of them seem to run around 3.5 to 5 dB on noise, which should be fine on HF. It would seem to be a vast improvement over - 10 dB or more.

Win W5JAG
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Old 28th February 2018, 03:02 PM   #198
w5jag is offline w5jag  United States
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Quote:
.... is it feasible to use one of these MMIC's as an RF amplifier between the low pass filter and the first mixer?
The answer looks to be .... maybe.

Can't tell much from the lousy cell phone picture, but I was trying to get a pic of another upc1651 stuck in between the low pass filter and the DBM module. These are nominally 19 dB gain at 500 MHz, and 5dB noise, regardless of frequency. I thought sure it would oscillate - the input and output are only about 5/16 inch apart, but it's not.

It's definitely an improvement, how much is hard to say, I've not played with it much. Both of the boards will eventually be tested against the other.

I'm not crazy about this synthesizer, so I ordered a new Si5351 synthesizer from QRP Labs. I've pulled the modules from the junktron, so I can start integrating, for lack of a better word, the new transverter(s) to the Mizuho SG-9 sideband generator.

I opened up the brick, and it looks like it uses MRF458 finals. They look pristine perfect, so, with a bit of luck, possibly the point of failure is one of the low level stages. I'm still quite a ways from worrying much about that.

Unfortunately, it is now warm and rainy ( floody ) here, which means mowing season starts again, probably as early as next week. This will make the heretofore glacial progress even slower.

Win W5JAG
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Old 4th March 2018, 08:32 PM   #199
w5jag is offline w5jag  United States
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The MOSFET board, again. I was going to change the IF amp to a MOSFET, but just wired it as a second mixer, with the idea that I may be able to get enough conversion gain out of it to eliminate the need for an IF amp at 45 MHz.

And that may be feasible. The transformer on the output increases the gain. Unlike the first mixer, the second mixer does need LO attenuation. I don't know if this is due to output level variation between the variable and fixed output, variation due to frequency, transistor to transistor variation, or what. Getting the LO levels right has been frustrating, to put it mildly. I'm certain I'm not there yet, but I'm learning more about it, and getting closer.

The two crystals after the transformer are an attempt at a simple 9 MHz filter. I can't seem to find any monolithic 9 MHz filters, like are available for 45, 10.7, etc., so this is an attempt to fabricate one, from a batch of twenty crystals bought off eBay for a couple of bucks. This may have merit, probably does, but will be tedious trial and error to get it right.

Been listening to a French contest station on 20 most of the afternoon with this board.

Win W5JAG
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Old 16th March 2018, 02:48 PM   #200
gpauk is offline gpauk  Scotland
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Just in case anyone needs to know, Broadcom / Avago have end-of-lifed over 1000 RF parts... A shame...

Amplifiers
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