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Since I liked the sound of the direct conversion receiver so much I thought maybe it would be interesting to try out one of Rick Campbell's complete phasing receivers instead of just the audio signal processor. So, I built an R2Pro. It worked out pretty well. One of the hurdles to overcome when building this receiver was to come up with a VFO that can supply two signals, 90 degrees out of phase with each other - an in phase "I" signal and a 90 degree out of phase, "Q" signal. The point of which is eliminate one of the sidebands. I already had a an IQ VFO, the early version by Craig Johnson. Although it works very well, it draws a lot of current, it is big and would necessate using another box for a transmitter. This is not completly out of the question and I have used it this way but prefer something more compact like what I'm showing here. I made up the transmitter with an FCC-1/2 as the VFO. The VFO is programmed with an offset of minus 800 KHz for receiver and no offset during transmit. The FCC-1/2 has a key input that when brought low will switch between TX and RX. I built up another board using an RF switch to switch the DDS output to a transmit port and a receive port. The Transmit port goes directly into the 5 Watt CW amplifier. In fact, the amplifier is never keyed - it goes on the air with the presence of the TX output of the DDS. The receive output goes to an I/Q splitter taken from another of Rick Campbells designs. It is basically a transformer that gives up an I and Q signal necessary for the downconverter board of the r2pro.
Here is the r2pro:
The down converter is top left. Below it is the Audio Signal Processor. To the right of the ASP is a relay switched audio filter. To the right of the filters is the audio amp. The LNA is top right. I still have some small hum issues to work out but the receiver is very good even at this point. Actually, this receiver is excellent.
...and here are both units. I know, I'm a little out of band.
The vertical buttons on the left are for the FCC-1/2 and the horizontal buttons are mem1 and mem2 of the AA0ZZ keyer.
The left toggle switch (RX on the bottom) is on/off and the other is a switch for the two RC audio filters (SSB narrow and CW)
How does a transformer generate a 90 degree phase split? (It must be a frequency dependent delay).
How does a transformer generate a 90 degree phase split? (It must be a frequency dependent delay).
KK7B uses this method in most of his designs that I've seen. It IS frequency dependant. Compared to the I and Q outputs of my IQ-VFO it comes up short. I can only get about 20db of opposite side band supression with it. I can get about 45db or so with the near perfect level and phase shift of the IQ-VFO plus the IQ-VFO output is NOT frequency dependant . I got about 38db with Dan Tayloe's FireFly - which I will get back to eventually.There are a lot of things to worry about with this rx, shielding and grounding being the biggest. When everything is just right, i.e., nearby computer off, late at night, and running on batteries, the radio sounds fantastic - just like a DC receiver should, minus one sideband .
I made another of the above circuits and carefully matched the capacitors. Wow, what a differnce! So long IQ-VFO. I now get about 40db or so of opposite side band suppression with the r2pro. I no longer need the IQ-VFO .
The T/R is from W7ZOI's Universal Transmitter QRP Transmitter with some mods. I shortened the key-up delay by changing C10 to 5 mfd from 22 because you could take a quick nap before you would hear receive audio again. It still is not breake-in but it does allow for an easy interface with any other receiver that I might want to try with my version of the universal transmitter. The T/R uses relay control for antenna changeover while switching audio output between the receiver input and the sidetone output. The side tone is horrible so I decided to use the square wave from the keyer - which isn't a whole lot better - but, slightly better. I have a twin T oscillator to solve that problem but want to square away the r2pro first before dealing with nuances of T/R control.
I've never seen one with variable coils but on page 12.40 of EMRFD there is one using a phase trimmer cap. KK7B uses a LPF in that version as well as the Universal VFO on page 12.46 of EMRFD. I just wired the output of mine directly into the down converter.
I did not measure the one I just did other than to look at the I and Q signal with my scope (almost a perfect circle using a lissajous plot). While listening to the opposite sideband of a very loud signal I could hear nothing. I was so pleased that I didn't bother to actually do a proper test . I will, though...eventually.
KK7B says that the R2pro, as well as his previous designs, work at their best when you have the VFO well isolated from the front end of the receiver. There are other construction techniques, that I'm learning, as I go along, that also must be adhered to . The R2pro is not an easy build! Stuffing the boards was easy enough but making everything work together to avoid hum and feedback is another issue. At this point I've boxed up the LNA, down converter and the audio amp/filters. I now need to box up the DDS-VFO/transmitter chassis (shown above with Diz's 5 Watt Amplifier).
With the setup shown above the audio is on the verge of feeding back if I move the volume control just a tad too much. If I use my IQ-VFO the receiver sounds noticeably better and there is no audio feedback no matter where the volume control is. But, like I said, taking the IQ-VFO route would mean a bulky setup and necessitate a third box for the transmitter. Besides, I like using the FCC-1/2 with the Diz amp and the AA0ZZ keyer Ė it just means a bit more work.
So, I have to isolate the DDS, RF switch and the I/Q transformer into a shielded enclosure. Then I have to figure out why Iím getting a T/R thump .
I will post some up-to-date pictures later.
Iíve made several DX contacts so far including a half hour qso with Hawaii when 20 meters was supposed to be dead.
I also bought a spectrum analyzer. I have it for 30 days to figure out whether it will do what I need. So far, it is just fine. I wanted to buy a spectrum analyzer and thought about eBay but spending that much money on something you are not sure of the condition, was just too much of a gamble for me. Plus, those old analyzers are huge - which is also an important consideration.
Here is what the R2pro looks like now.
Upper left is the down converter with the I/Q VFO input at the top.
upper right is the LNA with power and mute feedthru caps
below the down converter is the audio signal processer
to the right of the audio signal processor is the cw/ssb relay switched RC audio filters.
In the same box is the audio amplifier. On the left of that box are feedthru caps for DC to switch the relay and audio in. Power for the audio amp comes in via the feedthru on the top. There is another feedthrough for audio amp mute but it is not connectted at the moment.
This is the IQ-VFO designed by Craig Johnson, AA0ZZ. I built it a few years back and put it in a Ten-Tec box. It puts out two signals exactly 90 degrees out of phase and at this point it is the gold standard for what I what I would like to achieve with my FCC-1/2 5Watt amp box.
The current version of this kit is the IQ-Pro.
Here is what it sounds like (recorded 10PM local (2:00Z) on 20M)
SSB watch you volume at the beginning of this = there is no agc
I have found that the article most referenced for this circuit is from Reed Fisher (W2CQH), "Twisted-Wire Quadrature Hybrid Directional Couplers", QST, January 1978. Unfortunatly, I could'nt find it anywhere other than the ARRL. If you are thinking about building this phasing network for a specific band then the QST article is a must read. Otherwise, I found it for 20 meters just in the course of Googling KK7B. You can also find a 40 meter version in the MicroR2.
It's been interesting, the more I get into this project to see other uses for some of KK7B's other receivers like the one used by SETI.
I modified the FCC-1/2/DIZ amplifier for more isolation of the VFO and phasing output (I/Q) from the rest of the circuits. I will upload pictures soon.
This picture shows an attempt to isolate the DDS signal. The FCC-1/2 is on the left front. To the right is the I/Q splitter and on the left is the EZkeyer with the connectors removed. This keyer has an outstanding quality in that there is a separate mute output (see picture below) which can be set on or off. This mutes the receiver slightly ahead of keydown and keeps muting slightly after key up.
In the back of the box, on the left wall is the code practice oscillator that I lifted directly from Jason, NT7S. This sine wave sidetone is switched via relay to the headphone output (upper left) during TX. The W7ZOI T/R control is the board to the right of the sidetone board. It relay switches the receive audio and sidetone as well as the antenna between the output of the 5 Watt CW Amplifier and the R2pro. To the right is the 5W amp and the low pass filter. Although you can't see it, there is an 8db pi attenuator just to the left of the amp. I also changed the 10K power adjustment pot to a circular pot so that I can adjust the power output form the top. You can just barely see it under the right most SMA connector.
There are 3 feedthru caps running along the bottom of the PC board barrier, hidden from view by the FCC-1/2.
2. keyed +5V from the mute line output of the keyer IC for driving 2 2N3904 for muting the r2pro
3. key line to ground from the EZkeyer Key output to the W7ZOI T/R board
All this effort did make a noticable improvement. I now can turn the volume control up without any trace of feedback, the same as when using the I/Q VFO.
Now I have to figure out why I get a noticable T/R thump on keyup even though the EZkeyer is muting as it should. I also want to do full break-in which I think I can do once I solve the thump problem.
Here is a Field Day 2011 recording. When the cw filter is turned on it will be obvious since there is drop in audio. The filters are quite good despite this. You will also here the key up thump.
This is pretty much the final version of my R2pro.
The radio sounds fantastic.
It is very susceptible to hum from power supplies or wall warts in close proximity. It also doesnít like computer LCD's.
I think I went as far as I want to in terms of shielding. Frankly, I canít think of much more to do. When used with one power supply that I have, you hardly hear any hum at all. Also, of course, if I use a battery, I just barely notice a tiny bit (you wouldn't notice in normal use unless you were really looking for it).
I finally got rid of the transmit thump by using the rest of the T/R circuitry in W7ZOIís
Universal QRP Transmitter MK-II. I was using all of Figure 4 but later dropped the sidetone oscillator in favor of NT7Sís cpo (see above). It is relay switched between the audio jacks the same as the other oscillator was.
Today I finally read the whole article and realized that the top part of Figure 1 might contain some key components necessary in removing the key down thump. Mainly Q6, Q7, Q8 and Q4 - used to time the keying of the driver transistor Q2. I replaced R13 with R4 of Dizís 5 Watt CW amp. It never ceases to amaze me, the good things that come from reading instructions .
I also use a thumbwheel type pot for R6 in the amplifier so I can easily turn the power up and down since I sometimes use it with an external amplifier that only need 2 watts of drive.
So, at this point, I have a complete transceiver (thumpless). The transmit part is pretty much the Universal TX, W7ZOI design, with a few substitutions.
Here is a short thumpless recording. There is some QRM, as you can hear and some recording artifacts on key-up....hmmm. The T/R is not QSK so the intial key-down is the important one. This is not perfect, but an improvement. Eventually I'd like to get QSK.