I was asked to post some pictures of how the input from the VFO (from the J-310) to the diode ring mixer looks. The picture above is the VFO output across a 47 ohm resistor to ground through the .1 uf cap to the drain of the J-310. The mixer is NOT connected. The question is: Is this enough VFO signal?
Realize that the VFO is just turning the diodes on and off at the VFO rate. See this page for more details:
Look carefully at the scope pattern and at the diagram. Also look at Alan Wolke W2AEW's excellent video (URL in the above post). You will see the importance of the VFO turning on and off the diodes. This is how the diode ring multiplies by 1 and -1. This is how mixing happens. This is how audio is produced.
Now, back to the question: Is the output we see above "enough." We can check to see if the VFO is turning on and off the diodes by reconnecting the mixer to the VFO and looking again at the mixer's VFO input port (with no resistor connected). This is what I see when I do this:
Here you can see the diodes being switched on and off on the peaks of the VFO voltage. That is the flat topping you see. It looks to me as if this is enough. And indeed I have no problem hearing down to the band noise (I can hear an increase in the hiss when I connect the antenna) and I can hear plenty of CW, FT-8 and LSB signals. I am using ordinary 1N4148 silicon diodes.
Often we hear manufacturers say that their mixer (like the SBL-1) needs 7 dbm (about 1.4519 V peak to peak across 50 ohms) input from the VFO. But I think that is just for the SBL-1. Ours is a homebrew diode ring. It is, I think, different. So it might not NEED 7 dbm. In fact, look at the voltage level differences: Across the 50 ohm resistor we see 504 mV p-p. But with the diode ring connected we see 1.5 V p-p. This implies that the LO port input impedance is not 50 ohm, but is probably around 150 ohms. Indeed when I put a 150 ohm resistor across the output of the VFO (no mixer connected) I measured aroung 1.4 V p-p
Bottom line: Just make sure your VFO is turning the diodes on and off.
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Perfect is the thief of good enough.
ReplyDeleteGreat points Bill.
ReplyDeleteThe diodes are switching on and off – job one ; check. The power measured from a LO into a resistor load is termed available power. The actual power going into the mixer will be determined by dynamic factors including impedance mismatch as you stated.
In pro mixers, the recommended LO power usually get determined by the desired input 1 dB compression point of the mixer. The 1-dB compression point of a mixer is defined as the input signal amplitude required to boost conversion loss by 1 dB. It’s the highest signal amplitude a mixer can handle.
Typically , perhaps, the 1 dB compression point occurs at an amplitude ~~ 5 to 10 dB below the available LO power when accounting for the huge variety of mixers.
Lower LO power may incur more conversion losses and IMD. However --- this is for people who seek to maximize receiver dynamic range and performance.
In my opinion, moving from basic, fun, weekend home brew receivers to emphasizing high-performance receiver designs is now , in retrospect, 1 of the many reasons analog Ham home brew has declined.
Other than on contest weekends, do we really need high performance? Newcomers and casual builders just want to have fun with something they home brewed. They maybe don’t need high performance as high performance and fun aren’t always directly related. Simple rigs often prove very enjoyable from my experience.
Your project addresses this concern perfectly. You’re part of the the Renaissance of people returning analog home brew back to its roots established the 50-70s --- emphasizing fun over high performance and a builder experience that is not costly, nor over complicated.
There is nothing wrong with making high performance gear – its awesome --- however, it might not be as effective for attracting new people ( whom we sorely need) and prompting luke-warm Hams back into home brew.
Kudos - keep up the good work Bill and Team!
Popcorn Todd
What's the trick to displaying the p2p voltage and frequency in nice big characters? My Rigol scope has them in tiny text.
ReplyDeleteFound the trick! Measure button, page down, font size extra large. Yay!
ReplyDeleteDean had to teach me how to teach me how to enlarge the fonts!
ReplyDeleteA diode ring mixer built with 1N4148 diodes falls in the same category as a 10 dBm level mixer. To take full advantage of the mixer properties you would have to drive it with at least 10 dBm from a 50 Ohm source.
ReplyDeleteThe mixer also works at lower LO power levels. The Si diodes have lower series resistance than Schottky diodes which means the mixer is able to keep low insertion loss over a wide LO power range.
IM products are generated during the time it takes to switch the diodes on/off. The second image shows that, due to lack of sufficient LO power respectively voltage, the diodes spend a lot of time between the on and off states, compromising the linearity of the mixer. This is to some degree acceptable in a receiver designed for simplicity.