Podcasting since 2005! Listen to Latest SolderSmoke
Sunday, October 24, 2021
WA9WFA's Mate for the Mighty Midget 1966 QST Receiver
Thursday, November 19, 2020
Diode Ring Magic
Friday, January 1, 2021
Glowing Numerals for the Lafayette HA-600A (With Jeweled Movements)
Monday, February 1, 2021
The VFO that I am looking for: The Globe VFO deluxe
Friday, November 13, 2020
How Does My Singly Balanced, Two-Diode, Single Transformer Product Detector Really Work?
As young James Clerk Maxwell used to say, "What's the go of it?" and "What's the particular go of it?"
I studied this circuit carefully when I was using it as a balanced modulator in my DSB rigs. I wrote up my conclusions in my book "SolderSmoke -- Global Adventures in Wireless Electronics."
BALANCED MODULATOR CONFIGURATION:
When I was using it as a balanced modulator, I had the RF "carrier" signal going into L1. This RF signal was 7 dbm, enough to switch the diodes on at voltage peaks. With the "center tap" of L2/L3 grounded for RF, this meant that when the "top" of L2 is negative, the "bottom" of L3 is positive. In this situation BOTH D1 and D2 will turn on and conduct.
When the top of L2 is positive, the bottom of L3 is negative and neither of the diodes is on. Neither conducts.
So we have the RF signal turning the diodes on and off at the frequency of the RF signal.
Audio from the microphone and mic amplifier is sent into the center tap connecting L2 and L3. The level of this audio is kept low, below the point where is could turn on the diodes. The center tap IS grounded for RF by the .1uF capacitor, but it is NOT grounded for AF. That is key to understanding this circuit.
In essence by turning the two diodes on and off at the rate of the RF signal, the audio signal is facing severe non-linearity through the diodes. We could say it is alternately being multiplied by 1 and 0. This non-linearity is what is required for mixing. We therefor get sum and difference products: Sidebands. At this point, Double Sideband.
The way the transformer is set up means the RF carrier signal is balanced out: Even when the two diodes conduct, the top of R1 and the bottom of R2 are of equal and opposite polarity, so there is no carrier signal at the junction of R1 and R2 (they are actually a 100 ohm variable resistor that can be adjusted to make SURE they balance out). So the carrier is suppressed and all that remains are the sidebands: Suppressed Carrier Double Sideband.
PRODUCT DETECTOR CONFIGURATION:
What happens when we use this circuit as a product detector in a receiver? Let's assume we are working with a 455 kc IF. If you run a 454 kc 7 dbm BFO signal into L1, it will turn the diodes on and off as described above. But you will NOT be able to put the 455 kc IF signal into the center tap of L2/L3 -- that center tap is GROUNDED for 455 kc. So you will have to run your IF signal into the resistors, and take the audio output from the center tap of L2/L3. This works. I tried it in my HA-600A. But there is a problem: Envelope detection.
In this arrangement, we are balancing out NOT the 455 kc IF signal, but instead we are balancing out the BFO. We don't really NEED to balance out the BFO -- it can easily be knocked down in the audio amplifiers, and IT is not responsible for the problematic envelope detection. We DO need to balance out the IF signal, because if that gets through we can get simultaneous "envelope detection" and product detection. And believe me, that does not sound good.
So I tried putting the IF signal into L1, and the BFO signal into the resistors (as shown above). I took the audio from the junction of L2/L3. This seemed work better, with envelope detection greatly reduced.
BUT WHAT'S THE GO OF IT?
But how is this circuit mixing in this configuration? The strong BFO signal is still controlling the diodes, BUT, with the BFO signal coming in through the resistors, when the top of R1 is positive the bottom of R2 is ALSO positive. In this situation D1 will conduct but D2 will not. The IF signal is facing a big non-linearity. This will result in sum and difference frequencies. The difference frequency will be audio. But with D1 and D2 turning on and off in a very different way than we saw in the balanced modulator, how does the mixing happen?
I think the answer comes from the summer 1999 issue of SPRAT, the amazing journal of the G-QRP club. Leon Williams, VK2DOB wrote an article entitled "CMOS Mixer Experiments."
Here is Leon's 74HC4066 circuit:
I think those two gates (3,4,5 and 1,2, 13) are the functional equivalent equivalent of the two diodes in our product detector. In Leon's scheme the VFO is supplying signals of opposite polarity. Ours is providing only one signal, but the fact that the diodes are reversed means that they act just like the gates in Leon's circuit. The transformer is almost identical to the one we use in the product detector.
Let's look at the output from Leon's circuit:
Tuesday, January 26, 2021
SolderSmoke Podcast #228
Of course, no travel.
But vaccines are here so maybe soon we can leave our shacks.
In the meantime:
I’ve been playing chess against AI bots on chess.com.
Netflix recommendation: The Bureau. From France. A review from NPR:
A reading from "Conquering the Electron." Germanium vs. Silicon.
Bill’s Bench:
The KLH Model Twenty-one II.
Acoustical Suspension. First receiver
WITH A PILLOW! Bad speaker? Blown AF amp
finals. Hot heat sink. VBE Multiplier. Desitin.
Tony Fishpool’s recommended LM386 boards. 10 for 11 bucks. Nice. They
work. Pictured in the Amazon ad at the
upper right of the SolderSmoke blog page.
Putting a digital display on the Lafayette HA-600A
Test gear trouble. My
Radio Shack multimeter getting flaky. I
many need something better. Auto
ranging? My beloved Maplin AF generator died – will have to fix. I need that
thing. Probably a bad chip. Good thing they are socketed.
I almost forgot about SKN!
But I remembered and I made one contact with the HT-37 and Drake 2-B.
Pete’s Bench:
Presentation to RSGB on Homebrew.
TenTek Troubleshoot.
Swan 240? Looking
nice.
SDR adventures.
MAILBAG
Bill N8ET sent me some really nice Showa 9 MHz 8 pole
crystal filters.
Kevin AA7YQ Smoke jumper!
Building a hybrid SDR.HDR rig.
Launched blog. FB
Nick M0NTV working on similar HDR/SDR project. Great video.
Grayson KJ7UM Hollow State Design – Launched a new
blog. Very FB!
Thomas K4SWL of SWL Post blog. Kearsarge Mountain Transmission system. And recent events.
Peter VK2EMU Poetry.
CW poetry.
Pete WB9FLW looking at DSB rigs…
Drew N7DA Feels not
like a real ham because he hasn’t built a quad from bamboo. Which type of
landscape bamboo is best for antennas?
Ryan Flowers of MiscDotGeek.Com blog is also watching the
Tally Ho YouTube videos of Leo Sampson. Wants to put a WSPR beacon on the Tally
Ho.
Joe KF5OWY Working
with diode ring mixers, trying to see the mixer action on his ‘scope. 1 and -1!
Jim AB9CN sent a cool idea about how to do a 20/17 Moxon.
Roy GM4VKI – I thanked him for his article in SPRAT about
putting a 2n3904 on the output of an NE602 10P mod. Brilliant.
Roger Hayward Told him that I really liked his Dad’s recent
web site updates.
Farhan – Jokingly cursed me for showing him the Oscillodyne
regen of Hugo Gernsback and Jean Shepherd. “Now I will have to build this!”